BACKGROUND:MAP (mitogen-activated protein) kinase activation is a prerequisite for oocyte maturation, ovulation and fertilisation in many animals. In the hermaphroditic nematode Caenorhabditis elegans, an MSP (major sperm protein) dependent pathway is utilised for MAP kinase activation and successive oocyte maturation with extracellular MSP released from sperm acting as activator. How oocyte-to-embryo transition is triggered in parthenogenetic nematode species that lack sperm, is not known.RESULTS:We investigated two key elements of oocyte-to-embryo transition, MSP expression and MAP kinase signaling, in two parthenogenetic nematodes and their close hermaphroditic relatives. While activated MAP kinase is present in all analysed nematodes irrespective of the reproductive mode, MSP expression differs. In contrast to hermaphroditic or bisexual species, we do not find MSP expression at the protein level in parthenogenetic nematodes. However, genomic sequence analysis indicates that functional MSP genes are present in several parthenogenetic species.CONCLUSIONS:We present three alternative interpretations to explain our findings. (1) MSP has lost its function as a trigger of MAP kinase activation and is not expressed in parthenogenetic nematodes. Activation of the MAP kinase pathway is achieved by another, unknown mechanism. Functional MSP genes are required for occasionally emerging males found in some parthenogenetic species. (2) Because of long-term disadvantages, parthenogenesis is of recent origin. MSP genes remained intact during this short interval although they are useless. As in the first scenario, an unknown mechanism is responsible for MAP kinase activation. (3) The molecular machinery regulating oocyte-to-embryo transition in parthenogenetic nematodes is conserved with respect to C. elegans, thus requiring intact MSP genes. However, MSP expression has been shifted to non-sperm cells and is reduced below the detection limits, but is still sufficient to trigger MAP kinase activation and embryogenesis.

In order to evaluate the evolutionary preservation of developmental programs during nematode embryogenesis, we searched for close relatives of the model system Caenorhabditis elegans with deviant patterns. The parthenogenetically reproducing species Diploscapter coronatus shows prominent differences to C. elegans. While in the 2-cell stage of C. elegans a rotation of the nuclear/centrosome complex is found only in the posterior P1 cell, in D. coronatus cell isolation indicates that rotation takes place in a cell-autonomous manner in both blastomeres, resulting in a linear 4-cell array. In C. elegans, the ABp cell becomes different from its ABa sister via a germline-induced induction. In D. coronatus, AB daughters do not touch the germline but nevertheless execute different fates, suggesting a cell-autonomous mechanism or signaling over distance. Laser ablation experiments revealed that active migration of the EMS cell is required to transform the linearly ordered blastomeres into a 3-dimensional embryo, and the difference can be most easily explained with a heterochronic shift with respect to cell mobility. In D. coronatus, reversal of cleavage polarity in the germline, typical for C. elegans, is absent. This results in four different transient variants of posterior blastomeres which eventually merge into a single pattern prior to the onset of gastrulation. This merging includes primordial germ cell migrations of variable extent toward the gut precursor cell and suggests a specific cell-cell recognition mechanism. Cell distribution in advanced embryos is essentially indistinguishable between both species.

BACKGROUND:The zinc finger (ZF) protein CTCF (CCCTC-binding factor) is highly conserved in Drosophila and vertebrates where it has been shown to mediate chromatin insulation at a genomewide level. A mode of genetic regulation that involves insulators and insulator binding proteins to establish independent transcriptional units is currently not known in nematodes including Caenorhabditis elegans. We therefore searched in nematodes for orthologs of proteins that are involved in chromatin insulation.RESULTS:While orthologs for other insulator proteins were absent in all 35 analysed nematode species, we find orthologs of CTCF in a subset of nematodes. As an example for these we cloned the Trichinella spiralis CTCF-like gene and revealed a genomic structure very similar to the Drosophila counterpart. To investigate the pattern of CTCF occurrence in nematodes, we performed phylogenetic analysis with the ZF protein sets of completely sequenced nematodes. We show that three ZF proteins from three basal nematodes cluster together with known CTCF proteins whereas no zinc finger protein of C. elegans and other derived nematodes does so.CONCLUSION:Our findings show that CTCF and possibly chromatin insulation are present in basal nematodes. We suggest that the insulator protein CTCF has been secondarily lost in derived nematodes like C. elegans. We propose a switch in the regulation of gene expression during nematode evolution, from the common vertebrate and insect type involving distantly acting regulatory elements and chromatin insulation to a so far poorly characterised mode present in more derived nematodes. Here, all or some of these components are missing. Instead operons, polycistronic transcriptional units common in derived nematodes, seemingly adopted their function.

In the well studied model nematode Caenorhabditis elegans entrance of the sperm induces an anterior-posterior polarity in the egg and determines the orientation of the primary embryonic axis. Subsequently, fusion of two haploid gamete nuclei results in a diploid zygote as a prerequisite for normal embryogenesis. Here we analyze the establishment of embryonic polarity and diploidy in the absence of sperm in three parthenogenetic nematode species from three different families, Diploscapter coronatus (Diploscapteridae), Acrobeloides nanus (Cephalobidae) and Plectus sp. (Plectidae). We find that they not only differ from C. elegans in these two aspects but also from each other, indicating variant solutions for the same developmental challenges and supporting the view that the parthenogenetic mode of reproduction has been acquired multiple times independently.

Early cell lineages and arrangement of blastomeres in C. elegans are similar to the pattern found in Ascaris and other studied nematodes leading to the assumption that embryonic development shows little variation within the phylum Nematoda. However, analysis of a larger variety of species from various branches of the phylogenetic tree demonstrate that prominent variations in crucial steps of early embryogenesis exist among representatives of this taxon. So far, most of these variations have only been studied on a descriptive level and thus essentially nothing is known about their molecular or genetic basis. Nevertheless, it is obvious that the limited morphological diversity of the freshly hatched juvenile and the uniformity of the basic body plan contrast with the many modifications in the way a worm is generated from the egg cell. This chapter focuses on the initial phase between egg activation and gastrulation and deals with the following aspects: reproduction and diploidy, polarity, cleavage and germ line, cell lineages; cell cycles and maternal contribution, cell-cell communication and cell specification, gastrulation.

Comparative analyses revealed considerable differences in embryonic pattern formation and cell-specification between Caenorhabditis elegans and Acrobeloides nanus, members of two neighboring nematode clades. While C.elegans develops very rapidly, A. nanus needs 4-5 times as long. To investigate whether differences during early embryogenesis could be related to developmental tempo, we studied three more slowly developing representatives of the genus Rhabditis, thus close relatives of C.elegans. Besides differences in body size and mode of reproduction, they differ from C.elegans in the order of cleavages, germline behavior and requirement for early zygotic transcription, showing evident similarities to A. nanus. The distinct variations in cell-cycle rhythms and arrest after inhibition of transcription appear to reflect a species-specific interplay in the timing between exhausting maternal supplies and making available newly transcribed gene products. Looking for the reversal of cleavage polarity in the germline present in C.elegans but not in A. nanus, two of the studied species express this distinct feature only in a later cell generation. We found that a C.elegans mutant in the mes-1 gene shows a similar deviation. Concerning specification of the gut cell lineage and the potential to compensate for lost cells, the three tested Rhabditis species behave less regulatively, like C.elegans; in contrast to A. nanus, the gut precursor EMS requires an inductive signal from the germline cell P2 and an experimentally eliminated EMS cell is not replaced by a neighboring blastomere. In conclusion, embryogenesis of the examined Rhabditis species includes features of both the fast-developing C. elegans and the slow-developing A. nanus.