Differences in embryonic pattern formation between Caenorhabditis elegans and its close parthenogenetic relative Diploscapter coronatus.

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.