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C. elegans can self fertilize, or they can mate with males. But are they able to mate with females? Or is there some kind of morphological barrier that prevents that?
Since only the male tail is equipped with various specialized sensory and copulatory structures that enable him to locate the vulva and successfully inseminate the hermaphrodite. Hermaphrodites can self-fertilize, but only males can cross-fertilize a hermaphrodite.
sources: Wormbook: Male development
C. elegans II (2nd edition): Sexual Dimorphism
And now for something completely different—a worm with three sexes
Diane Shakes is one of the authors on a paper examining the odd genetics of a nematode species, knowledge that has important human-health implications. Credit: Joseph McClain
Diane Shakes shakes her head. A trisexual arrangement is really not so different. Three sexes—male, female and hermaphrodite—are "part of the plan" for many organisms. There's even a word for it: trioecious.
And Shakes points out that hermaphroditism has a rich, varied and distinguished place in natural history.
"It's pretty common among invertebrates," she said. "What's not quite so common are self-fertile hermaphrodites. Think about earthworms: They're hermaphrodites, but it still takes two, because of the way the sex works, they're not self-fertile."
Other organisms display sequential hermaphroditism, she said. Oysters and other shellfish change from male to female as they age. "And in some organisms, when the 'leader of the pack' dies, another changes sex to be the new leader," she explained.
Shakes is a professor in William & Mary's Department of Biology. She and her collaborators have been examining Auanema rhodensis, a species of nematode that brings a completely different take to hermaphroditism. She is co-author on a paper in Current Biology that examines the genetics behind the worm's curious trisexual reproductive strategy.
"We're talking about three sexes, here," she explained. "There's males and females—and also hermaphrodites. Their bodies look like a female, but they make both eggs and sperm."
Like most animals, human reproduction relies on sex chromosomes labeled X and Y. Human females are XX and males are XY. Males produce a 50:50 mix of X and Y sperm, and the winning sperm ultimately determines the sex of the children.
Shakes explained that consistent with Mendelian genetics, individual families may not have a 50:50 ratio of boys and girls—but the overall human population stays right at this mark. In A. rhodensis, both the females and hermaphrodites are XX, whereas the males have a single X and no Y. This species not only manages three sexes, but its inheritance patterns confound the predictions of Mendelian genetics.
"What we've figured out is that A. rhodensis has developed ways to stray from the genetics rulebook—specifically in regards to how it handles its X chromosome," she said.
Shakes is one of the authors of "Sex- and Gamete-Specific Patterns of X Chromosome Segregation in a Trioecious Nematode," published in the journal Current Biology.
In previous studies, Shakes and colleagues discovered that the sperm-producing cells in A. rhodensis males have hijacked a cellular program normally used to streamline sperm in order to produce exclusively X-bearing sperm. Thus, when males cross with females, they produce only female offspring.
"If that wasn't crazy enough, in this new study, we've found that the hermaphrodites are also manipulating the genetic dice," she said.
Shakes explained that standard genetic rules predict that XX hermaphrodites should produce 1X eggs and 1X sperm. They found that A. rhodensis hermaphrodites break these rules by producing sperm with two X chromosomes and eggs with none.
"We are still figuring how exactly they do this, but this setup yields pretty interesting genetics," she said.
Shakes explained that A. rhodensis is fully and truly a trisexual species. The hermaphrodites may be self-fertile, but they also are sexually versatile, happy to breed with males and females of the species.
"When hermaphrodites produce offspring through self-fertilization, they produce mostly XX females and XX hermaphrodites," she said. "However, when hermaphrodite cross with males, the joining of 1X male sperm with no-X eggs yields a jackpot of male offspring!"
The trisexual A. rhodensis is an interesting animal in its own right, but the scientists' exploration of oddities in nematode chromosome segregation has important human-health implications.
"Chromosome segregation" refers to the tango of DNA strands in both meiosis (which produces egg and sperm cells) as well as mitosis (which produces new body cells). When the dance of chromosome segregation gets out of step, bad things can happen.
"Abnormalities in chromosome segregation, in most cases, cause an embryo to be spontaneously aborted in the first couple of weeks," Shakes explained. "The ones we hear about are the ones that survive to term. The most famous, of course, is Down's syndrome, where there's one extra copy of one tiny chromosome."
In addition to missteps in embryo development, Shakes points out that chromosome segregation errors raise their ugly heads in the development of many cancers.
"If you look at a cancer cell, there's going to be weird numbers of chromosomes. You will have broken and reattached chromosomes, those sort of things," she said. "When you go from a benign tumor into malignant cancer, it's really common to find defects in chromosome segregation."
Nematodes offer an excellent proxy for the study of human genetics. Both humans and the model nematode C. elegans have roughly the same number of genes, and many of these genes are functionally identical.
However, Shakes noted that the C. elegans genome is much more compact, with less DNA per cell. Nematodes have only six or seven chromosomes, whereas humans have 23. The relative compactness of the nematode genome led to C. elegans being the first multicellular organism to have its genome sequenced.
"It became the working model for the Human Genome Project," she said. "C. elegans has about 20,000 genes. Learning that was a sort of humbling experience because it's got about the same number of genes as a human."
But there is a difference in the two nematode species. C. elegans has two, not three, sexes—males and hermaphrodites. For A. rhodensis, having three sexes gives it additional reproductive tactics. In A. rhodensis, being a hermaphrodite with the ability to make both eggs and sperm is coupled with passage through a specialized, developmentally rugged larval stage.
"These hermaphrodites are the explorers," Shakes said. "They go find a new food patch and when they get there, they can reproduce all by themselves. During their rugged larval stage, they have features that help them resist environmental stressors, and they exhibit behaviors that maximize their dispersal."
The adaptive advantages of being a self-fertilizing hermaphrodite are obvious. There's none of that tedious mate-selection and courtship nonsense. If you're a hermaphrodite A. rhodensis all alone on a desert island, you won't be for long. And the DNA of the offspring is all pure you, of course.
But there is a price to pay for those advantages. Self-fertilizing hermaphrodites don't endow their offspring with the genetic diversity that come from male-female breeding and that benefits progeny in the long run.
Another disadvantage of their self-service sex life is that for hermaphrodites, passage through that rugged larval stage delays the onset of sexual maturity. Whereas males and females grow from embryo to sexual maturity in three to four days, the larval program of a hermaphrodite adds a whole day to the nematode's maturation, Shakes explained.
"One day, in the context of three or four days, is significant," she said. "So if you make males and females, they're going to produce grandchildren for you faster."
Shakes says her hypothesis on the reproductive strategy of the three-sexed A. rhodensis can be summed up as canny evolutionary hedge-betting.
"I think of the species as a gambler," she said. "It's always placing bets on if life is going to get bad or if it's going to get good."
Those bets take the form of the mix of offspring sexes, Shakes explained: "The ultimate goal is to make as many great-grandchildren as possible."
A good life for a colony of nematodes is pretty basic. Shakes sums it up: "I've got plenty of bacteria to chew on and my progeny will have the same—but the environment could change very quickly."
When life is good, a male-female mix will pay off. When things begin to change for the worse, a worm will want to breed a lot of tough little hermaphrodites who will go out into the wider world to explore new food patches and start new colonies.
"The curious thing is that we don't actually know that much about what they are doing in the wild," she said. "There's not a lot of research on that, but there are a lot of features that suggests that in the wild, there are a lots of hermaphrodites."
A. rhodensis was found only twice in the wild, Shakes said. One was in Connecticut and another was in Appalachian Virginia. Both specimens were found associated with other animals, a dead tick and a beetle. She says the tough, exploring nematodes were probably interested in transportation.
"They're probably not infecting those beetles or ticks, but they'll actually sit there and flail in the air. They're trying to get picked up by an insect. They want to ride. they're only a millimeter long—they can only crawl so far," Shakes explained. "If they can get picked up by a beetle or a tick or something, they can move much further."
Spatial Transcriptomics of C. elegans Males and Hermaphrodites Identifies Sex-Specific Differences in Gene Expression Patterns
To advance our understanding of the genetic programs that drive cell and tissue specialization, it is necessary to obtain a comprehensive overview of gene expression patterns. Here, we have used spatial transcriptomics to generate high-resolution, anteroposterior gene expression maps of C. elegans males and hermaphrodites. To explore these maps, we have developed computational methods for discovering region- and tissue-specific genes. We have found extensive sex-specific gene expression differences in the germline and sperm and discovered genes that are specifically expressed in the male reproductive tract. These include a group of uncharacterized genes that encode small secreted proteins that are required for male fertility. We conclude that spatial gene expression maps provide a powerful resource for identifying tissue-specific gene functions in C. elegans. Importantly, we found that expression maps from different animals can be precisely aligned, enabling transcriptome-wide comparisons of gene expression patterns.
Keywords: C. elegans CEL-seq germline mRNA sequencing male fertility spatial transcriptomics sperm.
Why are there males in the hermaphroditic species Caenorhabditis elegans?
The free-living nematode worm Caenorhabditis elegans reproduces primarily as a self-fertilizing hermaphrodite, yet males are maintained in wild-type populations at low frequency. To determine the role of males in C. elegans, we develop a mathematical model for the genetic system of hermaphrodites that can either self-fertilize or be fertilized by males and we perform laboratory observations and experiments on both C. elegans and a related dioecious species C. remanei. We show that the mating efficiency of C. elegans is poor compared to a dioecious species and that C. elegans males are more attracted to C. remanei females than they are to their conspecific hermaphrodites. We postulate that a genetic mutation occurred during the evolution of C. elegans hermaphrodites, resulting in the loss of an attracting sex pheromone present in the ancestor of both C. elegans and C. remanei. Our findings suggest that males are maintained in C. elegans because of the particular genetic system inherited from its dioecious ancestor and because of nonadaptive spontaneous nondisjunction of sex chromosomes, which occurs during meiosis in the hermaphrodite. A theoretical argument shows that the low frequency of male mating observed in C. elegans can support male-specific genes against mutational degeneration. This results in the continuing presence of functional males in a 99.9% hermaphroditic species in which outcrossing is disadvantageous to hermaphrodites.
It takes two to tango
C. elegans, as a species, appears to be far along the path toward complete self-reproduction, as evidenced by a suite of traits related to the degeneration of outcrossing, termed the “selfing syndrome,” such as reduced fitness of hybrid genotypes, reduced pheromonal attraction from hermaphrodites, and diminished male function in mating tests (Garcia et al. 2007 Chasnov et al. 2007 Cutter 2008). However, males have not gone extinct in this species, implicating the periodic or context-dependent importance of outcrossing in the field. Our results highlight the coexistence of self-reproduction and outcrossing in C. elegans as a strategic game and identify hermaphrodite behavior as an important axis of variation regulating this trade-off.
How do C. elegans hermaphrodites regulate whether they mate with males or self-reproduce? We provide evidence for a multifaceted role of the sensory system in regulating this decision. Because mechanosensory and ciliated sensory (osm-6) neurons are required for hermaphrodites to resist mating (Figure 1, E and F), we propose that hermaphrodites perceive physical and/or pheromonal cues from the male that trigger the expression of resistance behavior. Intriguingly, we found opposite effects on mating frequency for two sets of sensory neurons expressing TAX channels (Figure 1, G and H). This result demonstrates that the C. elegans nervous system is capable of both up-regulation and down-regulation of hermaphrodite mating frequency, a prerequisite for the expression of a decision. The cellular basis of these competing signaling interactions, or whether they represent one or more behavioral outputs, remains to be investigated further.
It is largely unknown how behaviors evolve after a major shift in life history. Recent work (Nayak et al. 2005 Baldi et al. 2009) has implicated self-sperm specification and activation as two traits responsible for the origin of hermaphroditism in C. elegans. Specifically, fog-2 appears to be a recently derived C. elegans lineage-specific gene required for hermaphrodites to produce sperm (Nayak et al. 2005). In addition, sperm maturation in C. elegans hermaphrodites requires spe-8 and spe-27 signal transduction (L’Hernault et al. 1988 Minniti et al. 1996). By comparison, C. elegans males do not require fog-2 to produce sperm and use two redundant signaling pathways (involving spe-8/spe-27 or try-5) to activate sperm (Smith and Stanfield 2011). The gain of self-reproduction through self-sperm production is expected to select for reduction in the mating drive of these newly evolved hermaphrodites. Consistently, we found that N2 hermaphrodites naturally depleted of self-sperm (Figure 2A) or carrying mutations that disrupt germline sperm specification or sperm activation (Figure 2C) expressed increased mating. Thus, we speculate that an additional step in the evolution of hermaphroditism is to reinforce self-reproduction through behavioral expression of reduced mating. Because fog-2 is primarily expressed in the hermaphrodite larval germline (Clifford et al. 2000), it is unlikely that fog-2 mediates behavior directly. Rather, we speculate that an internal cue, representing self-reproductive status in mature adults, informs mating behavior generated by the nervous system.
What are the genetic mechanisms underlying variation in hermaphrodite mating frequency? We identified two QTL, mate-1 and mate-2, that account for a large portion of the variation between two strains, N2 and HW (Figure 3B). However, it is unlikely that the mate-1 locus is solely responsible for the observed continuous, quantitative phenotypic variation in the mating frequency among other wild-type isolates (Figure 4A), because the middle of chromosome V that spans the mate-1 locus exhibits almost no variation among these strains and nearly all strains carry an N2 allele (Rockman and Kruglyak 2009). This result indicates that additional genes are likely responsible for variation in mating frequency.
We speculate that behaviors favoring mating are more likely to be the ancestral reproductive state of C. elegans, based on our results with sperm mutants that developmentally phenocopy the hypothesized female ancestral state of C. elegans. Further, the observation that females of C. remanei, a closely related obligate outcrossing species, are much more attractive to heterospecific C. elegans males than are conspecific C. elegans hermaphrodites (Chasnov et al. 2007) suggests that early C. elegans hermaphrodites may have been more attractive or willing to mate with males than they are today.
If hermaphrodite C. elegans can reproduce with females? - Biology
The hermaphrodite germ line produces both male and female gametes&mdashsperm and oocytes, respectively (see Germline Section in WormBook). Oocytes are produced throughout adult life sperm (spermatozoa) are generated during L4 and then used in adulthood to fertilize oocytes. The adult germ line exhibits distal&ndashproximal polarity, with a mitotic cell population located at the distalmost end of the gonad and meiotic cells, extending proximally. Among the meiotic cells is also a gradient of meiotic progression with successive stages of meiosis I prophase extending from the distal arm, around the loop into the proximal arm of the gonad. Gametogenesis occurs in the proximal part of the gonad arm (GermFIG 1).
The distal germ line is a syncytium. Germ cells have incomplete borders and are connected to one another via a central canal called the rachis (GermFIG 1 and GermFIG 2) (Hirsh et al., 1976). Part of the distal gonad is not covered by the somatic tissues (the &ldquobare region&rdquo) and is instead ensheathed only by the gonadal basal lamina (GBL) that covers the rest of the gonad (SomaticFIG 2C) (see Reproductive System - Somatic Gonad Hall et al., 1999). At the base of each germ cell, and covering the rachis, is a thickened extracellular matrix. This matrix contains hemicentin and is thought to reinforce and stabilize the opening of the germ cells to the rachis (GermFIG 2B Pericellular Structures) (Vogel and Hedgecock, 2001). The end point of the rachis may differ as the animal ages. For example, in young adults, the rachis terminates within the proximal gonad, just past the loop in older adults, the rachis terminates in the distal gonad, although still near the loop (McCarter et al., 1997). Oocytes may retain a vestigial connection to the rachis even after moving well past its apparent end point (J. White, pers. comm.), so that a maturing oocyte in the proximal arm might retain a thin, cryptic arm reaching through the loop to the distal rachis.
The distalmost end of the adult gonad, referred to as the mitotic zone, contains a stem-cell population. As germ cells move away from the influence of the DTC (see Reprodutive System - Somatic Gonad), they enter meiosis I and then proceed through prophase I to diakinesis (GermFIG 3A&ndashE) (Hirsh et al., 1976 reviewed in Hubbard and Greenstein, 2000 Hansen et al., 2004).
The mitotic zone of the adult is approximately 20 cell diameters in length, extending from the DTC to the transition zone (GermFIG 3A) (described below Crittenden et al., 1994). With DAPI staining at the level of light microscopy, M-phase nuclei can be distinguished from the rest of the cell cycle: Nuclei are relatively uniform and, at any given time, have a hazy fluorescence in the center and brighter circumferential staining. Condensed chromatin appears as nuclei enter prometaphase. The average number of M-phase nuclei visible in any given mitotic zone in the adult is low, about two nuclei per arm (J. Maciejowski and E.J. Hubbard, pers. comm.). At the level of electron microscopy, mitotic germ cells are uniform in size and appearance (GermFIG 2). Each cell is roughly cuboidal, with a large nucleus. The cytoplasm contains a few mitochondria, limited rough endoplasmic reticulum (RER), and few free ribosomes. The rachis itself is also filled with RER and ribosomes, but contains more mitochondria. The transition zone is characterized by germ cells entering the early phases of meiotic prophase (leptotene and zygotene) and is defined as the area between the distalmost transition nucleus and the proximalmost transition nucleus (Hansen et al., 2004). A change in the nuclear morphology can be shown with DAPI: Nuclei are condensed and crescent-shaped (GermFIG 3D).
After moving through the transition zone, germ cells progress into pachytene and gradually grow. Pachytene nuclei are characterized by a distinctive &ldquobowl of spaghetti&rdquo morphology as homologous chromosomes start to align side by side (GermFIG 3C). Exit from pachytene requires activation of a MAPK (MAP kinase) pathway, thought to be triggered by a signal from the overlying gonadal sheath (Church et al., 1995 McCarter et al., 1997). Progression of nuclei into diplotene occurs in the loop and cells become organized in single file as they enter the proximal arm (GermFIG 3).
In addition to oocyte and sperm fate, programmed cell death (PCD) represents a major cell fate among adult germ cells. It is estimated that approximately one half of all potential oocytes are eliminated in the adult hermaphrodite during progression through prophase of meiosis I. Most cell deaths occur near the loop region of the gonad arm, the region containing pachytene-stage germ cells (see GermFIG 3A,C). It has been proposed that these excess germ cells may serve as a nurse cell population, providing proteins and other cytoplasmic components to surviving germ cells (Hengartner, 1997). Electron and light microscopy analyses of dying cells reveal that cell deaths occur by apoptosis (GermFIG 4A&ndashC) (Gumienny et al., 1999). As in somatic tissues, cell death execution depends on ced-3, ced-4, and ced-9 function. However, genetic evidence also suggests that somatic and germ cell death mechanisms may not be entirely identical (Hengartner et al., 1992 Gumienny et al., 1999). Overlying gonadal sheath cells (likely sheath-cell pair 2) engulf cell death corpses (Gumienny et al., 1999).
Oocyte maturation takes place in the oocyte closest to the spermatheca, just before ovulation, and is stimulated by sperm-derived major sperm protein (MSP) (GermFIG 5) (Miller et al., 2001). During maturation, nuclear envelope breakdown (NEBD also known as germinal vesicle breakdown) occurs, the nucleus becomes less obvious, and cortical rearrangements cause the oocyte to become more spherical. Chromosome arrangement changes as bivalent chromosomes leave diakinesis and begin to align onto the metaphase plate (Ward and Carrel, 1979 McCarter et al., 1999).
Ovulation follows oocyte maturation. Signals from the maturing oocyte and MSP stimulate the rate and intensity of sheath contraction from a basal rate of 10&ndash13 contractions/minute to approximately 19 contractions/minute (McCarter et al., 1999 Miller et al., 2001, 2003). Oocyte maturation also stimulates distal spermathecal dilation through LIN-3/LET-23 RTK pathway activation and IP3 signaling (Clandinin et al., 1998 McCarter et al., 1999 Bui and Sternberg, 2002). The dilated spermatheca is pulled over the oocyte by the contracting sheath, and the spermatheca then closes. The oocyte is immediately penetrated by a sperm and fertilized. Cell&ndashcell recognition between gametes during this process is mediated by SPE-9, a sperm-specific, epidermal growth factor (EGF)-repeat-containing transmembrane protein (Singson et al., 1998). Cytoplasmic streaming in the oocyte accompanies fertilization, meiosis is completed, and eggshell secretion commences (Ward and Carrel, 1979 Singson, 2001). The newly formed embryo then passes from the spermatheca to the uterus via the spermathecal-uterine valve.
Germ-line development spans L1 to early adulthood (GermFIG 6 GermMOVIE 1). All germ cells are descended from either Z2 or Z3 (Schedl, 1997 Hubbard and Greenstein, 2000). In contrast to somatic lineage development, germ-line cell divisions appear to be variable with respect to their timing and planes of division, and hence the precise lineal relationships between these cells are not known (Kimble and Hirsh, 1979). In L4, approximately 37 meiotic cells per arm at the most proximal end of the germ line commit to sperm development. Subsequently, the germ line switches from making sperm to making oocytes for the remainder of development and throughout adulthood. This switch between male and female cell fate results from germ-line modulation of sex determination pathway activity (Kuwabara and Perry, 2001). Germ-line development depends on interactions with the overlying somatic gonad. Somatic gonad cells, or their precursors, affect the timing and position of the germ-line mitosis/meiosis decision, and they exit from pachytene, gametogenesis, and male gamete fate during germ-line sex determination (Kimble and White, 1981 Seydoux et al., 1990 McCarter et al., 1997 Rose et al., 1997 Pepper et al., 2003 Killian and Hubbard, 2004).
The germ line of each gonad arm produces about 150 sperm during L4 (GermFIG 6 and GermFIG 7A) (L&rsquoHernault, 1997). Approximately 37 diploid germ cells per gonad arm form primary spermatocytes while still attached to the rachis. After pachytene, spermatocytes detach from the rachis and complete meiosis, generating haploid spermatids. This process of spermatid formation is called spermatogenesis (GermFIG 7B, C&D) (Ward et al., 1981).
Developing spermatocytes contain a large number of specialized vesicles called fibrous body-membranous organelles (FB-MOs) (GermFIG8 A&B and GermFIG 9A). These organelles contain proteins required in the future spermatids and spermatozoa, including MSP (Ward and Klass, 1982). During development, the FB-MOs partition with the portion of the spermatocyte destined to become the future spermatid (Ward, 1986). The residual body (GermFIG 7B&7D) acts as a deposit area for proteins and organelles no longer required by the developing spermatid (L&rsquoHernault, 1997 Arduengo et al., 1998 Kelleher et al., 2000).
Spermatogenesis takes place within the proximal gonad (GermFIG 7B). The formed spermatids are pushed into the spermatheca by the first oocyte during the first ovulation. In the spermatheca, an unknown signal induces these sessile spermatids to undergo morphogenesis into mature, amoeboid spermatozoa (sperm) (cf. GermFIG 9B&C) (Nelson and Ward, 1980 Ward et al., 1983). This process of activation is known as spermiogenesis.
Maturing spermatids and spermatozoa have highly condensed nuclei and tightly packed mitochondria (GermFIG 9B&C). In spermatids, MOs (now lacking the FB) locate near the cell periphery (GermFIG 9B). During spermatid activation, MOs fuse with the plasma membrane, releasing their contents (primarily glycoproteins) onto the cell surface. A fusion pore is generated on the cell surface by the MO collar (GermFIG 9C). Mutants affected in MO fusion produce sperm with defective motility, suggesting that MO content enhances sperm mobility (Ward et al., 1981 Roberts et al., 1986 Achanzar and Ward, 1997). Unlike sperm in many other animal phyla, C. elegans sperm are not flagellated. Rather, spermatid activation involves the formation of a foot or pseudopodium that allows the spermatozoon to attach to the walls of the spermathecal lumen and to crawl by projecting from and dragging the cell body. This motility is driven by dynamic polymerization of MSP, which, in addition to containing sequences that mediate extracellular signaling (described above Miller et al., 2001), has an intracellular cytoskeletal function (Italiano et al., 1996 Roberts and Stewart, 2000).
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Caenorhabditis elegans (Maupas, 1900)
C. elegans is unsegmented, vermiform, and bilaterally symmetrical. It has a cuticle (a tough outer covering, as an exoskeleton), four main epidermal cords, and a fluid-filled pseudocoelom (body cavity). It also has some of the same organ systems as larger animals. About one in a thousand individuals is male and the rest are hermaphrodites. The basic anatomy of C. elegans includes a mouth, pharynx, intestine, gonad, and collagenous cuticle. Like all nematodes, they have neither a circulatory nor a respiratory system. The four bands of muscles that run the length of the body are connected to a neural system that allows the muscles to move the animal's body only as dorsal bending or ventral bending, but not left or right, except for the head, where the four muscle quadrants are wired independently from one another. When a wave of dorsal/ventral muscle contractions proceeds from the back to the front of the animal, the animal is propelled backwards. When a wave of contractions is initiated at the front and proceeds posteriorly along the body, the animal is propelled forwards. Because of this dorsal/ventral bias in body bends, any normal living, moving individual tends to lie on either its left side or its right side when observed crossing a horizontal surface. A set of ridges on the lateral sides of the body cuticle, the alae, is believed to give the animal added traction during these bending motions. In relation to lipid metabolism, C. elegans does not have any specialized adipose tissues, a pancreas, a liver, or even blood to deliver nutrients compared to mammals. Neutral lipids are instead stored in the intestine, epidermis, and embryos. The epidermis corresponds to the mammalian adipocytes by being the main triglyceride depot. The pharynx is a muscular food pump in the head of C. elegans, which is triangular in cross-section. This grinds food and transports it directly to the intestine. A set of "valve cells" connects the pharynx to the intestine, but how this valve operates is not understood. After digestion, the contents of the intestine are released via the rectum, as is the case with all other nematodes. No direct connection exists between the pharynx and the excretory canal, which functions in the release of liquid urine. Males have a single-lobed gonad, a vas deferens, and a tail specialized for mating, which incorporates spicules. Hermaphrodites have two ovaries, oviducts, and spermatheca, and a single uterus. Anatomical diagram of a male C. elegans
C. elegans neurons contain dendrites which extend from the cell to receive neurotransmitters, and a process that extends to the nerve ring (the "brain") for a synaptic connection between neurons.Nonet, M. (2004) About the nematode Caenorhabdtis elegans The biggest difference is that C. elegans has motor excitatory and inhibitory neurons, known as cholinergic and gabaergic neurons, which simply act as further regulation for the tiny creature. They have no influence on the nervous system besides regulating neuron impulses.
Gut granules Numerous gut granules are present in the intestine of C. elegans, the functions of which are still not fully known, as are many other aspects of this nematode, despite the many years that it has been studied. These gut granules are found in all of the Rhabditida orders. They are very similar to lysosomes in that they feature an acidic interior and the capacity for endocytosis, but they are considerably larger, reinforcing the view of their being storage organelles. A remarkable feature of the granules is that when they are observed under ultraviolet light, they react by emitting an intense blue fluorescence. Another phenomenon seen is termed 'death fluorescence'. As the worms die, a dramatic burst of blue fluorescence is emitted. This death fluorescence typically takes place in an anterior to posterior wave that moves along the intestine, and is seen in both young and old worms, whether subjected to lethal injury or peacefully dying of old age. Many theories have been posited on the functions of the gut granules, with earlier ones being eliminated by later findings. They are thought to store zinc as one of their functions. Recent chemical analysis has identified the blue fluorescent material they contain as a glycosylated form of anthranilic acid (AA). The need for the large amounts of AA the many gut granules contain is questioned. One possibility is that the AA is antibacterial and used in defense against invading pathogens. Another possibility is that the granules provide photoprotection the bursts of AA fluorescence entail the conversion of damaging UV light to relatively harmless visible light. This is seen a possible link to the melanin–containing melanosomes. A lateral (left) side anatomical diagram of an adult-stage C. elegans hermaphrodite
Embryonic development The fertilized zygote undergoes rotational holoblastic cleavage. Sperm entry into the oocyte commences formation of an anterior-posterior axis. The sperm microtubule organizing center directs the movement of the sperm pronucleus to the future posterior pole of the embryo, while also inciting the movement of PAR proteins, a group of cytoplasmic determination factors, to their proper respective locations. As a result of the difference in PAR protein distribution, the first cell division is highly asymmetric. C. elegans embryogenesis is among the best understood examples of asymmetric cell division. All cells of the germline arise from a single primordial germ cell, called the P4 cell, established early in embryogenesis.Kimble J, Crittenden SL. Germline proliferation and its control. 2005 Aug 15. In: WormBook: The Online Review of C. elegans Biology [Internet]. Pasadena (CA): WormBook 2005-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK19769/ This primordial cell divides to generate two germline precursors that do not divide further until after hatching.
Axis formation The resulting daughter cells of the first cell division are called the AB cell (containing PAR-6 and PAR-3) and the P1 cell (containing PAR-1 and PAR-2). A second cell division produces the ABp and ABa cells from the AB cell, and the EMS and P2 cells from the P1 cell. This division establishes the dorsal-ventral axis, with the ABp cell forming the dorsal side and the EMS cell marking the ventral side. Through Wnt signaling, the P2 cell instructs the EMS cell to divide along the anterior-posterior axis. Through Notch signaling, the P2 cell differentially specifies the ABp and ABa cells, which further defines the dorsal-ventral axis. The left-right axis also becomes apparent early in embryogenesis, although it is unclear exactly when specifically the axis is determined. However, most theories of the L-R axis development involve some kind of differences in cells derived from the AB cell.
Gastrulation Gastrulation occurs after the embryo reaches the 24-cell stage. C. elegans are a species of protostomes, so the blastopore eventually forms the mouth. Involution into the blastopore begins with movement of the endoderm cells and subsequent formation of the gut, followed by the P4 germline precursor, and finally the mesoderm cells, including the cells that eventually form the pharynx. Gastrulation ends when epiboly of the hypoblasts closes the blastopore.
Post-embryonic development Under environmental conditions favourable for reproduction, hatched larvae develop through four larval stages - L1, L2, L3, and L4 - in just 3 days at 20 °C. When conditions are stressed, as in food insufficiency, excessive population density or high temperature, C. elegans can enter an alternative third larval stage, L2d, called the dauer stage (Dauer is German for permanent). A specific dauer pheromone regulates entry into the dauer state. This pheromone is composed of similar derivatives of the 3,6-dideoxy sugar, ascarylose. Ascarosides, named after the ascarylose base, are involved in many sex-specific and social behaviors. In this way, they constitute a chemical language that C. elegans uses to modulate various phenotypes. Dauer larvae are stress-resistant they are thin and their mouths are sealed with a characteristic dauer cuticle and cannot take in food. They can remain in this stage for a few months.http://www.wormatlas.org/hermaphrodite/introduction/mainframe.htm The stage ends when conditions improve favour further growth of the larva, now moulting into the L4 stage, even though the gonad development is arrested at the L2 stage. Each stage transition is punctuated by a molt of the worm's transparent cuticle. Transitions through these stages are controlled by genes of the heterochronic pathway, an evolutionarily conserved set of regulatory factors. Many heterochronic genes code for microRNAs, which repress the expression of heterochronic transcription factors and other heterochronic miRNAs. miRNAs were originally discovered in C. elegans. Important developmental events controlled by heterochronic genes include the division and eventual syncitial fusion of the hypodermic seam cells, and their subsequent secretion of the alae in young adults. It is believed that the heterochronic pathway represents an evolutionarily conserved predecessor to circadian clocks. Some nematodes have a fixed, genetically determined number of cells, a phenomenon known as eutely. The adult C. elegans hermaphrodite has 959 somatic cells and the male has 1033 cells, although it has been suggested that the number of their intestinal cells can increase by one to three in response to gut microbes experienced by mothers. Much of the literature describes the cell number in males as 1031, but the discovery of a pair of left and right MCM neurons increased the number by two in 2015. The number of cells does not change after cell division ceases at the end of the larval period, and subsequent growth is due solely to an increase in the size of individual cells.
The different Caenorhabditis species occupy various nutrient- and bacteria-rich environments. They feed on the bacteria that develop in decaying organic matter (microbivory). Soil lacks enough organic matter to support self-sustaining populations. C. elegans can survive on a diet of a variety of bacteria, but its wild ecology is largely unknown. Most laboratory strains were taken from artificial environments such as gardens and compost piles. More recently, C. elegans has been found to thrive in other kinds of organic matter, particularly rotting fruit. C. elegans can also use different species of yeast, including Cryptococcus laurentii and C. kuetzingii, as sole sources of food. Although a bacterivore, C. elegans can be killed by a number of pathogenic bacteria, including human pathogens such as Staphylococcus aureus, Pseudomonas aeruginosa, Salmonella enterica or Enterococcus faecalis. Invertebrates such as millipedes, insects, isopods, and gastropods can transport dauer larvae to various suitable locations. The larvae have also been seen to feed on their hosts when they die. Nematodes can survive desiccation, and in C. elegans, the mechanism for this capability has been demonstrated to be late embryogenesis abundant proteins. C. elegans, as other nematodes, can be eaten by predator nematodes and other omnivores, including some insects.Elaine R. Ingham Soil biology primer USDA The Orsay virus is a virus that affects C. elegans, as well as the Caenorhabditis elegans Cer1 virus and the Caenorhabditis elegans Cer13 virus.
Wild isolates of Caenorhabditis elegans are regularly found with infections by Microsporidia fungi. One such species, Nematocida parisii, replicates in the intestines of C. elegans. Arthrobotrys oligospora is the model organism for interactions between fungi and nematodes. It is the most common and widespread nematode capturing fungus.
C. elegans was the first multicellular organism to have its whole genome sequenced. The sequence was published in 1998, although some small gaps were present the last gap was finished by October 2002.
Size and gene content The C. elegans genome is about 100 million base pairs long and consists of six pairs of chromosomes in hermaphrodites or five pairs of autosomes with XO chromosome in male C.elegans and a mitochondrial genome. Its gene density is about one gene per five kilo-base pairs. Introns make up 26% and intergenic regions 47% of the genome. Many genes are arranged in clusters and how many of these are operons is unclear. C. elegans and other nematodes are among the few eukaryotes currently known to have operons these include trypanosomes, flatworms (notably the trematode Schistosoma mansoni), and a primitive chordate tunicate Oikopleura dioica. Many more organisms are likely to be shown to have these operons. The genome contains an estimated 20,470 protein-coding genes. About 35% of C. elegans genes have human homologs. Remarkably, human genes have been shown repeatedly to replace their C. elegans homologs when introduced into C. elegans. Conversely, many C. elegans genes can function similarly to mammalian genes. The number of known RNA genes in the genome has increased greatly due to the 2006 discovery of a new class called 21U-RNA genes, and the genome is now believed to contain more than 16,000 RNA genes, up from as few as 1,300 in 2005. Scientific curators continue to appraise the set of known genes new gene models continue to be added and incorrect ones modified or removed. The reference C. elegans genome sequence continues to change as new evidence reveals errors in the original sequencing. Most changes are minor, adding or removing only a few base pairs of DNA. For example, the WS202 release of WormBase (April 2009) added two base pairs to the genome sequence. Sometimes, more extensive changes are made as noted in the WS197 release of December 2008, which added a region of over 4,300 bp to the sequence.
Related genomes In 2003, the genome sequence of the related nematode C. briggsae was also determined, allowing researchers to study the comparative genomics of these two organisms. The genome sequences of more nematodes from the same genus e.g., C. remanei, C. japonica and C. brenneri (named after Brenner), have also been studied using the shotgun sequencing technique. These sequences have now been completed.
Other genetic studies C. elegans adult with GFP coding sequence inserted into a histone-encoding gene by Cas9-triggered homologous recombination
As of 2014, C. elegans is the most basal species in the 'Elegans' group (10 species) of the 'Elegans' supergroup (17 species) in phylogenetic studies. It forms a branch of its own distinct to any other species of the group. Tc1 transposon is a DNA transposon active in C. elegans.
The hermaphroditic worm is considered to be a specialized form of self-fertile female, as its soma is female. The hermaphroditic germline produces male gametes first, and lays eggs through its uterus after internal fertilization. Hermaphrodites produce all their sperm in the L4 stage (150 sperm cells per gonadal arm) and then produce only oocytes. The hermaphroditic gonad acts as an ovotestis with sperm cells being stored in the same area of the gonad as the oocytes until the first oocyte pushes the sperm into the spermatheca (a chamber wherein the oocytes become fertilized by the sperm). The male can inseminate the hermaphrodite, which will preferentially use male sperm (both types of sperm are stored in the spermatheca). Once he recognizes a hermaphrodite worm, the male nematode begins tracing the hermaphrodite with his tail until he reaches the vulval region. The male then probes the region with his spicules to locate the vulva, inserts them, and releases sperm. The sperm of C. elegans is amoeboid, lacking flagella and acrosomes. When self-inseminated, the wild-type worm lays about 300 eggs. When inseminated by a male, the number of progeny can exceed 1,000. Hermaphrodites do not typically mate with other hermaphrodites. At 20 °C, the laboratory strain of C. elegans (N2) has an average lifespan around 2–3 weeks and a generation time of 3 to 4 days. C. elegans has five pairs of autosomes and one pair of sex chromosomes. Sex in C. elegans is based on an X0 sex-determination system. Hermaphrodites of C. elegans have a matched pair of sex chromosomes (XX) the rare males have only one sex chromosome (X0).
Sex determination C. elegans are mostly hermaphroditic organisms, producing both sperms and oocytes. Males do occur in the population in a rate of approximately 1 in 200 hermaphrodites, but the two sexes are highly differentiated. Males differ from their hermaphroditic counterparts in that they are smaller and can be identified through the shape of their tail. C.elegans reproduce through a process called androdioecy. This means that they can reproduce in two ways: either through self-fertilization in hermaphrodites or through hermaphrodites breeding with males. Males are produced through non-disjunction of the X chromosomes during meiosis. The worms that reproduce through self-fertilization are at risk for high linkage disequilibrium, which leads to lower genetic diversity in populations and an increase in accumulation of deleterious alleles. In C. elegans, somatic sex determination is attributed to the tra-1 gene. The tra-1 is a gene within the TRA-1 transcription factor sex determination pathway that is regulated post-transcriptionally and works by promoting female development. In hermaphrodites (XX), there are high levels of tra-1 activity, which produces the female reproductive system and inhibits male development. At a certain time in their life cycle, one day before adulthood, hermaphrodites can be identified through the addition of a vulva near their tail. In males (XO), there are low levels of tra-1 activity, resulting in a male reproductive system. Recent research has shown that there are three other genes, fem-1, fem-2, and fem-3, that negatively regulate the TRA-1 pathway and act as the final determiner of sex in C. elegans.
Evolution The sex determination system in C. elegans is a topic that has been of interest to scientists for years. Since C. elegans are used as a model organism, any information discovered about the way their sex determination system might have evolved could further the same evolutionary biology research in other organisms. After almost 30 years of research, scientists have began to put together the pieces in the evolution of such a system. What they have discovered is that there is a complex pathway involved that has several layers of regulation. The closely related organism Caenorhabditis briggsae has been studied extensively and its whole genome sequence has helped put together the missing pieces in the evolution of C. elegans sex determination. It has been discovered that two genes have assimilated, leading to the proteins XOL-1 and MIX-1 having an effect on sex determination in C. elegans as well. Mutations in the XOL-1 pathway leads to feminization in C. elegans . The mix-1 gene is known to hypoactivate the X chromosome and regulates the morphology of the male tail in C. elegans. Looking at the nematode as a whole, the male and hermaphrodite sex likely evolved from parallel evolution. Parallel evolution is defined as similar traits evolving from an ancestor in similar conditions simply put, the two species evolve in similar ways over time. An example of this would be marsupial and placental mammals. Scientists have also hypothesized that hermaphrodite asexual reproduction, or "selfing", could have evolved convergently by studying species similar to C. elegans Other studies on the sex determination evolution suggest that genes involving sperm evolve at the faster rate than female genes. However, sperm genes on the X chromosome have reduced evolution rates. Sperm genes have short coding sequences, high codon bias, and disproportionate representation among orphan genes. These characteristics of sperm genes may be the reason for their high rates of evolution and may also suggest how sperm genes evolved out of hermaphrodite worms. Overall, scientists have a general idea of the sex determination pathway in C. elegans, however, the evolution of how this pathway came to be is not yet well defined.
Asymmetric cell divisions during early embryogenesis of wild-type C. elegans
In 1963, Sydney Brenner proposed using C. elegans as a model organism for the investigation primarily of neural development in animals. It is one of the simplest organisms with a nervous system. The neurons do not fire action potentials, and do not express any voltage-gated sodium channels. In the hermaphrodite, this system comprises 302 neurons the pattern of which has been comprehensively mapped, in what is known as a connectome, and shown to be a small-world network. Research has explored the neural and molecular mechanisms that control several behaviors of C. elegans, including chemotaxis, thermotaxis, mechanotransduction, learning, memory, and mating behaviour. In 2019 the connectome of the male was published using a technique distinct from that used for the hermaphrodite. The same paper used the new technique to redo the hermaphrodite connectome, finding 1,500 new synapses. It has been used as a model organism to study molecular mechanisms in metabolic diseases. Brenner also chose it as it is easy to grow in bulk populations, and convenient for genetic analysis.Alt. URL It is a multicellular eukaryotic organism, yet simple enough to be studied in great detail. The transparency of C. elegans facilitates the study of cellular differentiation and other developmental processes in the intact organism. The spicules in the male clearly distinguish males from females. Strains are cheap to breed and can be frozen. When subsequently thawed, they remain viable, allowing long-term storage. Maintenance is easy when compared to other multicellular model organisms. A few hundred nematodes can be kept on a single agar plate and suitable growth medium. Brenner described the use of a mutant of E. coli – OP50. OP50 is a uracil-requiring organism and its deficiency in the plate prevents the overgrowth of bacteria which would obscure the worms. The use of OP50 does not demand any major laboratory safety measures, since it is non-pathogenic and easily grown in Luria-Bertani (LB) media overnight.
Notable findings The developmental fate of every single somatic cell (959 in the adult hermaphrodite 1031 in the adult male) has been mapped. These patterns of cell lineage are largely invariant between individuals, whereas in mammals, cell development is more dependent on cellular cues from the embryo. As mentioned previously, the first cell divisions of early embryogenesis in C. elegans are among the best understood examples of asymmetric cell divisions, and the worm is a very popular model system for studying developmental biology. Programmed cell death (apoptosis) eliminates many additional cells (131 in the hermaphrodite, most of which would otherwise become neurons) this "apoptotic predictability" has contributed to the elucidation of some apoptotic genes. Cell death-promoting genes and a single cell-death inhibitor have been identified. Wild-type C. elegans hermaphrodite stained with the fluorescent dye Texas Red to highlight the nuclei of all cells
The hermaphrodite intestine produces yolk proteins, while the male intestine does not. This sex-specific cell fate is controlled by mab-3 and tra-1. MAB-3 is a DM domain containing transcription factor related to Drosophila Doublesex and mouse Dmrt1 (Raymond et al., 1998). Mutations in mab-3 cause yolk to be produced in males (Shen and Hodgkin, 1988). MAB-3 binds to a site in the promoter of the vit-2 vitellogenin gene in males, repressing vit-2 transcription (Yi et al., 2000). TRA-1A binds to a site in the mab-3 promoter in hermaphrodites, repressing mab-3 transcription (Yi and Zarkower, 1999). Thus, the hermaphrodite specific fate of the intestinal cells is controlled directly by the sex determination pathway.
Materials and Methods
Strains were grown at 20° on NGM plates containing E. coli OP50. We used the laboratory C. elegans strain N2 as our wild-type strain (Sulston and Brenner 1974). We also used the N2 mutant strain JK574, which contains the fog-2(q71) allele, for our experiments.
Synchronized worms were grown to either young adulthood or the sixth day of adulthood prior to RNA extraction. Synchronization and aging were carried out according to protocols described previously (Leighton et al. 2014). 1000–5000 worms from each replicate were rinsed into a microcentrifuge tube in S basal (5.85 g/liter NaCl, 1 g/liter 6 g/liter ), and then spun down at 14,000 rpm for 30 sec. The supernatant was removed and 1 ml of TRIzol was added. Worms were lysed by vortexing for 30 sec at room temperature and then 20 min at 4°. The TRIzol lysate was then spun down at 14,000 rpm for 10 min at 4° to allow removal of insoluble materials. Thereafter, the Ambion TRIzol protocol was followed to finish the RNA extraction (MAN0001271 rev. date: 13 Dec 2012). Three biological replicates were obtained for each genotype and each time point.
RNA integrity was assessed using an RNA 6000 Pico Kit for Bio-Analyzer (Agilent Technologies #5067–1513) and mRNA was isolated using a NEBNext Poly(A) mRNA Magnetic Isolation Module (New England Biolabs, NEB, #E7490). RNA-seq libraries were constructed using the NEBNext Ultra RNA Library Prep Kit for Illumina (NEB #E7530), following manufacturer’s instructions. Briefly, mRNA isolated from ∼1 μg of total RNA was fragmented to the average size of 200 nt by incubating at 94° for 15 min in first-strand buffer, cDNA was synthesized using random primers and ProtoScript II Reverse Transcriptase, followed by second-strand synthesis using Second Strand Synthesis Enzyme Mix (NEB). Resulting DNA fragments were end-repaired, dA-tailed and ligated to NEBNext hairpin adaptors (NEB #E7335). After ligation, adaptors were converted to the ‘Y’ shape by treating with USER enzyme, and DNA fragments were size-selected using Agencourt AMPure XP beads (Beckman Coulter #A63880) to generate fragment sizes between 250 and 350 bp. Adaptor-ligated DNA was PCR amplified, followed by AMPure XP bead clean-up. Libraries were quantified with Qubit dsDNA HS Kit (ThermoFisher Scientific #Q32854) and the size distribution was confirmed with High Sensitivity DNA Kit for Bioanalyzer (Agilent Technologies #5067–4626). Libraries were sequenced on Illumina HiSeq2500 in single-read mode with a read length of 50 nt, following manufacturer’s instructions. Base calls were performed with RTA 22.214.171.124 followed by conversion to FASTQ with bcl2fastq 1.8.4.
RNA-seq alignment was performed using Kallisto (Bray et al. 2016) with 200 bootstraps. Kallisto was run in single-end read mode, setting the average fragment length of 200bp, and a standard deviation of 60bp for all samples. Differential expression analysis was performed using Sleuth (Pimentel et al. 2016). The following general linear model (GLM) was fitted: where is the TPM count for the ith gene is the intercept for the ith gene is the regression coefficient for variable X for the ith gene A is a binary age variable indicating first-day adult (0) or sixth-day adult (1) G is the genotype variable indicating wild type (0) or fog-2(lf) (1) and refers to the regression coefficient accounting for the interaction between the age and genotype variables in the ith gene. Genes were called significant if the FDR-adjusted q-value for any regression coefficient was <0.1. Our script for differential analysis is available on GitHub.
Regression coefficients and TPM counts were processed using Python 3.5 in a Jupyter Notebook (Pérez and Granger 2007). Data analysis was performed using the Pandas, NumPy and SciPy libraries (McKinney 2011 Van Der Walt et al. 2011 Oliphant 2007). Graphics were created using the Matplotlib and Seaborn libraries (Waskom et al. 2016 Hunter 2007). Interactive graphics were generated using Bokeh (Bokeh Development Team 2014).
Tissue, phenotype, and gene ontology enrichment analyses (TEA, PEA, and GEA, respectively) were performed using the WormBase Enrichment Suite for Python (Angeles-Albores et al. 2016, 2017a). Briefly, the WormBase Enrichment Suite accepts a list of genes and identifies the terms with which these genes are annotated. Terms are annotated by frequency of occurrence, and the probability that a term appears at this frequency under random sampling is calculated using a hypergeometric probability distribution. The hypergeometric probability distribution is extremely sensitive to deviations from the null distribution, which allows it to identify even small deviations from the null.
Strains are available from the Caenorhabditis Genetics Center. All of the data and scripts pertinent to this project, except the raw reads, can be found on our Github repository https://github.com/WormLabCaltech/Angeles_Leighton_2016. Supplementary Material, File S1 contains the list of genes that were altered in aging regardless of genotype. File S2 contains the list of genes and their associations with the fog-2(lf) phenotype. File S3 contains genes associated with the female-like state. Raw reads were deposited to the Sequence Read Archive under the accession code SUB2457229.
Sex, Worms, and Videotape
The brain reigns as the most important sex organ--even for microscopic worms. By "masculinizing" the tiny brains of genetically female nematodes, researchers have given these ladies sexual behavior typical of male worms and begun to unravel the neuronal circuits behind worm attraction.
Sex isn't to Caenorhabditis elegans--a 1-millimeter worm that feeds on soil bacteria--what it is to humans. The vast majority of individuals are genetically female, but they are really hermaphrodites, producing enough sperm to self-fertilize as many as 300 eggs. When food is abundant, females can release pheromones to attract the rare males, whose more robust sperm can fertilize as many as 1200 eggs. Whereas males actively seek out the pheromones, hermaphrodites don't.
To understand what makes the two sexes behave differently, Jamie White of the University of Utah, Salt Lake City, and his colleagues looked to the worms' brains, which number fewer than 400 neurons and are very different in males and hermaphrodites. In one experiment, White's group enlisted the help of a gene called fem-3, which, when it is overexpressed throughout the developing hermaphrodites' bodies, makes them males. The researchers overexpressed it in just the nervous systems, producing worms that had the bodies of hermaphrodites--they lacked the tail males use to copulate--but with intact male nervous systems.
The researchers filmed these mixed-up worms on a petri dish dabbed with the female pheromone. The modified worms moved toward the pheromone in fact, their behavior was indistinguishable from a male's. Apparently, a masculinized brain is all it takes for the worms to develop male-typical behavior, says White. That finding matches studies on mice and fruit flies that show that, depending on which genetic cues are activated in the nervous system, "core anatomy is capable of generating either male or female behavior," says neuroscientist Cori Bargmann of Rockefeller University in New York City.
The researchers also discovered that three small groups of cells in males' nervous systems--called AWA, AWC, and CEM--play a key role in the sexual attraction of males to females. When both the AWA and AWC groups, or just the CEM group, were zapped with a tiny laser beam, males lost interest in pheromones, the videotape revealed. When the same was done to the cell groups of immature males, however, the worms still displayed sexual attraction when they became adults. Apparently, the remaining neural groups take over the job of the lost cells. That makes sense, White says: "Sexual behavior needs to be very robust." This way, if something goes wrong in one type of neuron during development, "the male can still reproduce."