A large close relative of C. elegans is slow-developing but not long-lived.

BACKGROUND: Variation in body size is thought to be a major driver of a wide variety of ecological and evolutionary patterns, including changes in development, reproduction, and longevity. Additionally, drastic changes in natural context often have profound effects on multiple fitness-related traits. Caenorhabditis inopinata is a recently-discovered fig-associated nematode that is unusually large relative to other members of the genus, including the closely related model system C. elegans. Here we test whether the dramatic increase in body size and shift in ecological context has led to correlated changes in key life history and developmental parameters within this species. RESULTS: Using four developmental milestones, C. inopinata was found to have a slower rate of development than C. elegans across a range of temperatures. Despite this, C. inopinata did not reveal any differences in adult lifespan from C. elegans after accounting for differences in developmental timing and reproductive mode. C. inopinata fecundity was generally lower than that of C. elegans, but fitness improved under continuous-mating, consistent with sperm-limitation under gonochoristic (male/female) reproduction. C. inopinata also revealed greater fecundity and viability at higher temperatures. CONCLUSION: Consistent with observations in other ectotherms, slower growth in C. inopinata indicates a potential trade-off between body size and developmental timing, whereas its unchanged lifespan suggests that longevity is largely uncoupled from its increase in body size. Additionally, temperature-dependent patterns of fitness in C. inopinata are consistent with its geographic origins in subtropical Okinawa. Overall, these results underscore the extent to which changes in ecological context and body size can shape life history traits.

Achatina fulica (Mollusca: Achatinidae) Naturally Infected with Caenorhabditis briggsae (Dougherty and Nigon, 1949) (Nematoda: Rhabditidae).

Specimens of the African snail Achatina fulica, collected in Bucaramanga, Colombia, were examined for parasites. Numerous specimens of Caenorhabditis briggsae were collected from the digestive tract of the snails and identified by the structure of male spiculum, caudal bursa, gubernaculum and precloacal lip in males, triangular tooth in metarhabdion, and protandrous hermaphrodites with a female:male ratio of 15:1 and with morphometry. DNA sequences of the ITS2 region of the ribosomal gene array from worms in this study matched with 99% similarity to published sequences of C. briggsae. A redescription of the species is provided. This is the first record of the species in South America.

Description of Caenorhabditis sinica sp. n. (Nematoda: Rhabditidae), a nematode species used in comparative biology for C. elegans.

We re-isolated in China a relative of the nematode model Caenorhabditis elegans that was previously referred to informally as C. sp. 5. In spite of its importance for comparative biology, C. sp. 5 has remained morphologically uncharacterized. Therefore, we now provide detailed description of morphology and anatomy, assigning the name of Caenorhabditis sinica sp. n. to this nematode that is found frequently in China. C. sinica sp. n. belongs to the Elegans group in the genus Caenorhabditis, being phylogenetically close to C. briggsae although differing in reproductive mode. The gonochoristic C. sinica sp. n. displays two significantly larger distal parts of uteri filled with sperms in the female/hermaphroditic gonad than does the androdioecious C. briggsae. The new species can be differentiated morphologically from all known Caenorhabditis species within the Elegans group by presenting a uniquely shaped, three-pointed hook structure on the male precloacal lip. The lateral field of C. sinica sp. n. is marked by three ridges that are flanked by two additional incisures, sometimes appearing as five ridges in total. This study ends the prolonged period of the 'undescribed' anonymity for C. sinica sp. n. since its discovery and use in comparative biological research. Significant and crossing-direction dependent hybrid incompatibilities in F1 and F2 crossing progeny make C. sinica sp. n. an excellent model for studies of population and speciation genetics. The abundance of nematode species lacking detailed taxonomic characterization deserves renewed attention to address the species description gap for this important yet morphologically 'difficult' group of animals.

Female, but not male, nematodes evolve under experimental sexual coevolution.

Coevolution between the sexes is often considered to be male-driven: the male genome is constantly scanned by selection for traits that increase relative male fertilization success. Whenever these traits are harmful to females, the female genome is scanned for resistance traits. The resulting antagonistic coevolution between the sexes is analogous to Red Queen dynamics, where adaptation and counteradaptation keep each other in check. However, the underlying assumption that male trait evolution precedes female trait counteradaptation has received few empirical tests. Using the gonochoristic nematode Caenorhabditis remanei, we now show that 20 generations of relaxed versus increased sexual selection pressure lead to female, but not to male, trait evolution, questioning the generality of a male-driven process.

A phylogeny and molecular barcodes for Caenorhabditis, with numerous new species from rotting fruits.

BACKGROUND: The nematode Caenorhabditis elegans is a major laboratory model in biology. Only ten Caenorhabditis species were available in culture at the onset of this study. Many of them, like C. elegans, were mostly isolated from artificial compost heaps, and their more natural habitat was unknown. RESULTS: Caenorhabditis nematodes were found to be proliferating in rotten fruits, flowers and stems. By collecting a large worldwide set of such samples, 16 new Caenorhabditis species were discovered. We performed mating tests to establish biological species status and found some instances of semi-fertile or sterile hybrid progeny. We established barcodes for all species using ITS2 rDNA sequences. By obtaining sequence data for two rRNA and nine protein-coding genes, we determined the likely phylogenetic relationships among the 26 species in culture. The new species are part of two well-resolved sister clades that we call the Elegans super-group and the Drosophilae super-group. We further scored phenotypic characters such as reproductive mode, mating behavior and male tail morphology, and discuss their congruence with the phylogeny. A small space between rays 2 and 3 evolved once in the stem species of the Elegans super-group; a narrow fan and spiral copulation evolved once in the stem species of C. angaria, C. sp. 8 and C. sp. 12. Several other character changes occurred convergently. For example, hermaphroditism evolved three times independently in C. elegans, C. briggsae and C. sp. 11. Several species can co-occur in the same location or even the same fruit. At the global level, some species have a cosmopolitan distribution: C. briggsae is particularly widespread, while C. elegans and C. remanei are found mostly or exclusively in temperate regions, and C. brenneri and C. sp. 11 exclusively in tropical zones. Other species have limited distributions, for example C. sp. 5 appears to be restricted to China, C. sp. 7 to West Africa and C. sp. 8 to the Eastern United States. CONCLUSIONS: Caenorhabditis are "fruit worms", not soil nematodes. The 16 new species provide a resource and their phylogeny offers a framework for further studies into the evolution of genomic and phenotypic characters.

Mutations in two independent pathways are sufficient to create hermaphroditic nematodes.

Although the nematode Caenorhabditis elegans produces self-fertile hermaphrodites, it descended from a male/female species, so hermaphroditism provides a model for the origin of novel traits. In the related species C. remanei, which has only male and female sexes, lowering the activity of tra-2 by RNA interference created XX animals that made spermatids as well as oocytes, but their spermatids could not activate without the addition of male seminal fluid. However, by lowering the expression of both tra-2 and swm-1, a gene that regulates sperm activation in C. elegans, we produced XX animals with active sperm that were self-fertile. Thus, the evolution of hermaphroditism in Caenorhabditis probably required two steps: a mutation in the sex-determination pathway that caused XX spermatogenesis and a mutation that allowed these spermatids to self-activate.

Hakuna Nematoda: genetic and phenotypic diversity in African isolates of Caenorhabditis elegans and C. briggsae.

Caenorhabditis elegans and C. briggsae have many parallels in terms of morphology, life history and breeding system. Both species also share similar low levels of molecular diversity, although the global sampling of natural populations has been limited and geographically biased. In this study, we describe the first cultured isolates of C. elegans and C. briggsae from sub-Saharan Africa. We characterize these samples for patterns of nucleotide polymorphism and vulva precursor cell lineage, and conduct a series of hybrid crosses in C. briggsae to test for genetic incompatibilities. The distribution of genetic diversity confirms a lack of geographic structure to C. elegans sequences but shows genetic differentiation of C. briggsae into three distinct clades that may correspond to three latitudinal ranges. Despite low levels of molecular diversity, we find considerable variation in cell division frequency in African C. elegans for the P3.p vulva precursor cell, and in African C. briggsae for P4.p, a variation that was not previously observed in this species. Hybrid crosses did not reveal major incompatibilities between C. briggsae strains from Africa and elsewhere, and there was some evidence of inbreeding depression. These new African isolates suggest that important ecological factors may be shaping the patterns of diversity in C. briggsae, and that despite many similarities between C. elegans and C. briggsae, there may be more subtle differences in their natural histories than previously appreciated.

Diversity in mating behavior of hermaphroditic and male-female Caenorhabditis nematodes.

In this study, we addressed why Caenorhabditis elegans males are inefficient at fertilizing their hermaphrodites. During copulation, hermaphrodites generally move away from males before they become impregnated. C. elegans hermaphrodites reproduce by internal self-fertilization, so that copulation with males is not required for species propagation. The hermaphroditic mode of reproduction could potentially relax selection for genes that optimize male mating behavior. We examined males from hermaphroditic and gonochoristic (male-female copulation) Caenorhabditis species to determine if they use different sensory and motor mechanisms to control their mating behavior. Instead, we found through laser ablation analysis and behavioral observations that hermaphroditic C. briggsae and gonochoristic C. remanei and Caenorhabditis species 4, PB2801 males produce a factor that immobilizes females during copulation. This factor also stimulates the vulval slit to widen, so that the male copulatory spicules can easily insert. C. elegans and C. briggsae hermaphrodites are not affected by this factor. We suggest that sensory and motor execution of mating behavior have not significantly changed among males of different Caenorhabditis species; however, during the evolution of internal self-fertilization, hermaphrodites have lost the ability to respond to the male soporific-inducing factor.

Sex determination: ways to evolve a hermaphrodite.

Most species of the nematode genus Caenorhabditis reproduce through males and females; C. elegans and C. briggsae, however, produce self-fertile hermaphrodites instead of females. These transitions to hermaphroditism evolved convergently through distinct modifications of germline sex determination mechanisms.

Evolution of dnmt-2 and mbd-2-like genes in the free-living nematodes Pristionchus pacificus, Caenorhabditis elegans and Caenorhabditis briggsae.

Whole genome sequencing of several metazoan model organisms provides a platform for studying genome evolution. How representative are the genomes of these model organisms for their respective phyla? Within nematodes, for example, the free-living soil nematode Caenorhabditis elegans is a highly derived species with unusual genomic characters, such as a reduced Hox cluster (Curr. Biol., 13, 37-40) and the absence of a Hedgehog signaling system. Here, we describe the recent loss of a DNA methyltransferase-2 gene (dnmt-2) in C.elegans. A dnmt-2-like gene is present in the satellite model organism Pristionchus pacificus, another free-living nematode that diverged from C.elegans 200-300 million years ago. In contrast, C.elegans, Caenorhabditis briggsae and P.pacificus all contain an mbd-2-like gene, which encodes another essential component of the methylation system of higher animals and fungi. Cel-mbd-2 is expressed throughout development and RNA interference (RNAi) experiments result in variable phenotypes. In contrast, Cbr-mbd-2 RNAi results in paralyzed larval or adult worms suggesting recent changes of gene function within the genus Caenorhabditis. We speculate that both genes were part of an ancestral DNA methylation system in nematodes and that gene loss and sequence divergence have abolished DNA methylation in C.elegans.

Genomes: a helpful cousin for our favourite worm.

The recently published genome of the nematode Caenorhabditis briggsae provides a drastic improvement in structural annotation of the C. elegans genome, as well as a promising source of evolutionary comparisons.

Caenorhabditis elegans lin-25: a study of its role in multiple cell fate specification events involving Ras and the identification and characterization of evolutionarily conserved domains.

Caenorhabditis elegans lin-25 functions downstream of let-60 ras in the genetic pathway for the induction of the 1 degrees cell fate during vulval development and encodes a novel 130-kD protein. The biochemical activity of LIN-25 is presently unknown, but the protein appears to function together with SUR-2, whose human homologue binds to Mediator, a protein complex required for transcriptional regulation. We describe here experiments that indicate that, besides its role in vulval development, lin-25 also participates in the fate specification of a number of other cells in the worm that are known to require Ras-mediated signaling. We also describe the cloning of a lin-25 orthologue from C. briggsae. Sequence comparisons suggest that the gene is evolving relatively rapidly. By characterizing the molecular lesions associated with 10 lin-25 mutant alleles and by assaying in vivo the activity of mutants lin-25 generated in vitro, we have identified three domains within LIN-25 that are required for activity or stability. We have also identified a sequence that is required for efficient nuclear translocation. We discuss how lin-25 might act in cell fate specification in C. elegans within the context of models for lin-25 function in cell identity and cell signaling.

The cuticle of the nematode Caenorhabditis elegans: a complex collagen structure.

The cuticle of the nematode Caenorhabditis elegans forms the barrier between the animal and its environment. In addition to being a protective layer, it is an exoskeleton which is important in maintaining and defining the normal shape of the nematode. The cuticle is an extracellular matrix consisting predominantly of small collagen-like proteins that are extensively crosslinked. Although it also contains other protein and non-protein compounds that undoubtedly play a significant part in its function, the specific role of collagen in cuticle structure and morphology is considered here. The C. elegans genome contains between 50 and 150 collagen genes, most of which are believed to encode cuticular collagens. Mutations that result in cuticular defects and grossly altered body form have been identified in more than 40 genes. Six of these genes are now known to encode cuticular collagens, a finding that confirms the importance of this group of structural proteins to the formation of the cuticle and the role of the cuticle as an exoskeleton in shaping the worm. It is likely that many more of the genes identified by mutations giving altered body form, will be collagen genes. Mutations in the cuticular collagen genes provide a powerful tool for investigating the mechanisms by which this group of proteins interact to form the nematode cuticle.

Localization of FMRFamide-like peptides in Caenorhabditis elegans.

The neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2) is a member of a large family of related peptides that have been found throughout the animal kingdom. By using an antiserum specific for the Arg-Phe-NH2 moiety, we have found that about 10% of the neurons in the nematode Caenorhabditis elegans are immunoreactive. Most of these neurons, which include sensory, motor, and interneurons, were identified on the basis of their number, position, and projection pattern and by analysis of characterized mutants. Neurons that were immunoreactive in hermaphrodite animals were generally also found in males, but each sex had, in addition, sex-specific immunoreactive cells. Staining of hermaphrodite animals from different larval stages suggests that the onset of FMRFamide-like expression is differentially regulated among the cells. We have found a possible neuromodulatory role for the related peptide FLRFamide (Phe-Leu-Arg-Phe-NH2). In an egg-laying assay, FLRFamide by itself was not active but could potentiate a serotonin effect. The FMRFamide-like immunoreactivity was also used as a marker to examine the differentiation of cells that normally undergo programmed cell death. Cells that are destined to die in the Pn.a lineages appear to differentiate and adopt the fate of lineally equivalent cells before cell death.

Evidence from reversal of handedness in C. elegans embryos for early cell interactions determining cell fates.

Many animals with overall bilateral symmetry also exhibit some left-right asymmetries with generally invariant handedness. Therefore, the left-right embryonic axis must have a consistent polarity, whose origins and subsequent effects on development are not understood. Caenorhabditis elegans exhibits such left-right asymmetries at all developmental stages. The embryonic cell lineage is asymmetric as well: although the animal is generally bilaterally symmetric, many of its contralaterally analogous cells arise from different lineages on the two sides of the embryo. I accomplished reversal of embryonic handedness by micromanipulation at the 6-cell stage, which resulted in mirror-image but otherwise normal development into healthy, fertile animals with all the usual left-right asymmetries reversed. This result demonstrates that in the 6-cell embryo the pair of anterior (AB) blastomeres on the right is equivalent to the pair on the left, and that the extensive differences in fates between lineally homologous derivatives of these cells on the two sides of the animal must be dictated by cell interactions, most of which are likely to occur early in embryogenesis.

The posterior nervous system of the nematode Caenorhabditis elegans: serial reconstruction of identified neurons and complete pattern of synaptic interactions.

Serial-section electron microscopy has been used to reconstruct the cellular architecture of the posterior nervous system of the nematode Caenorhabditis elegans. Each of 40 neurons in the tail of the adult hermaphrodite can be reproducibly and unambiguously identified by a set of morphological features, including cell body position, fiber geometry and size, and staining properties. A complete list of synapses has been assembled for 2 isogenic animals, and these lists are compared with a third isogenic animal reconstructed by White et al. (1986). The set of neurons and their pattern of synaptic interactions is simple and reproducible. Most of the cells are involved in sensory transduction or in local signal processing to relay signals via a few interneurons to motoneurons and thence to body muscles. Because the tail neurons are well separated and fairly reproducible in position, the hermaphrodite tail lends itself to laser-ablation studies of sensory processing (cf. Chalfie et al., 1985). Most of the synapses in the tail are concentrated in the preanal ganglion. Among the approximately 150 synapses there, about 85% are dyadic chemical synapses. The dyadic synapses are involved in reproducible patterns that have several interesting features. Most neurons synapse onto a few preferred pairs of target cells, in patterns that suggest a combinatorial model of synapse specification that may be open to genetic analysis. Furthermore, most dyadic contacts A----B,C fit a pattern in which the 2 postsynaptic partners are involved elsewhere in unidirectional synapses B----C. Thus, the dyadic synapse may serve to diverge sensory signals into parallel pathways, which then reconverge. This divergence/reconvergence pattern eventually directs processed sensory signals to the ventral cord interneurons PVCL and PVCR. About 80-90% of the synapses fall into repeated classes of synapses. Many of the remaining synapses are widely scattered and irreproducible from one animal to the next. Some of these contacts may be developmental mistakes reflecting a degree of "noise" in synapse specification (Waddington, 1957).

Handed asymmetry, handedness reversal and mechanisms of cell fate determination in nematode embryos.

Embryos of the nematode Caenorhabditis elegans exhibit left-right asymmetry with an invariant handedness. The embryonic cell lineage is asymmetrical: although the animal is generally bilaterally symmetrical with only a few left-right asymmetries, many of its contralaterally analogous cells arise via different lineages on the two sides of the embryo. Larvae and adults also exhibit left-right asymmetries with a handedness that is normally invariant. The frequency of animals with opposite handedness was increased among the progeny of adults exposed to the mutagen ethyl methanesulphonate and among animals that developed from embryos treated in early cleavage with chitinase to destroy the egg shell. Reversal of embryonic handedness was accomplished directly by micromanipulation at the 6-cell stage, resulting in mirror-image but otherwise normal development into healthy, fertile animals with all the usual left-right asymmetries reversed. This demonstrates that (1) the handedness of cell positions in the 6-cell embryo dictates handedness throughout development; (2) at this stage the pair of anterior blastomeres on the right is equivalent to the pair on the left; and (3) the extensive differences in fates of lineally homologous cells on the two sides of the animal must be dictated by cellular interactions, most of which are likely to occur early in embryogenesis and appear to have been conserved in widely diverged nematode species.

Organogenesis in C. elegans: positioning of neurons and muscles in the egg-laying system.

One of the final stages in the development of egg-laying behavior in the nematode C. elegans is the organization of 8 motor neurons (2 HSN and 6 VC cells) and 8 muscles into a motor system to control the opening of the vulva. Using mutations that disrupt the development of specific components of the egg-laying system and laser microsurgery to ablate selected precursor cells, we have determined that the guidance of the egg-laying neurons and muscles, and in particular the VC neurons and vulval muscles, into the vulval region is dependent on interactions with surrounding epithelial and gonadal tissue and appears to be independent of neuron-neuron and neuron-muscle interactions. The development of the egg-laying system can be described as a series of cell interactions in which certain cells arise through induction and subsequently provide inductive cues themselves.

A new kind of informational suppression in the nematode Caenorhabditis elegans.

Independent reversions of mutations affecting three different Caenorhabditis elegans genes have each yielded representatives of the same set of extragenic suppressors. Mutations at any one of six loci act as allele-specific recessive suppressors of certain allels of unc-54 (a myosin heavy chain gene), lin-29 (a heterochronic gene), and tra-2 (a sex determination gene). The same mutations also suppress certain alleles of another sex determination gene, tra-1, and of a morphogenetic gene, dpy-5. In addition to their suppression phenotype, the suppressor mutations cause abnormal morphogenesis of the male bursa and the hermaphrodite vulva. We name these genes smg-1 through smg-6 (suppressor with morphogenetic effect on genitalia), in order to distinguish them from mab (male abnormal) genes that can mutate to produce abnormal genitalia but which do not act as suppressors (smg-1 and smg-2 are new names for two previously described genes, mab-1 and mab-11). The patterns of suppression, and the interactions between the different smg genes, are described and discussed. In general, suppression is recessive and incomplete, and at least some of the suppressed mutations are hypomorphic in nature. A suppressible allele of unc-54 contains a deletion in the 3' noncoding region of the gene; the protein coding region of the gene is apparently unaffected. This suggests that the smg suppressors affect a process other than translation, for example mRNA processing, transport, or stability.