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.

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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 P 1 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 germlineinduced 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.

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