Differences in maternal supply and early development of closely related nematode species.

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.