Cell lineages, developmental timing, and spatial pattern formation in embryos of free-living soil nematodes.

From soils of various origins we have isolated a number of nematode strains and cultured them on agar plates. We have analyzed their anatomy, reproduction, and particularly their pattern of embryogenesis. With respect to early cleavage we can define six different classes. The basic scheme of embryogenesis is similar in all strains but considerable differences were observed in detail. Embryogenesis is more than five times longer in the slowest strain than in the fastest. The following general correlation was found: The slower embryogenesis proceeds in a strain, the relatively earlier the cleavage of germline cells occurs. In the fastest strain the primordial germ cell P4 is present at the 24-cell stage, while in the slowest strain it is already generated in the 5-cell stage. We hypothesize that germline cleavages have to occur within a certain time limit to preserve germline quality. The typical reversal of cleavage polarity in the division of the germline cell P2 is absent in the slowest, on other grounds apparently more primitive strain. This results in an unusual spatial arrangement of cells transiently. However, prior to gastrulation as a consequence of compensatory cell migrations (which may indicate the necessity for cell interactions), the pattern becomes very similar to that in the other strains. We propose that a standard cellular configuration is required at the beginning of gastrulation to ensure normal further development. Early cell interactions might be necessary to achieve this standard pattern. In about half of the analyzed strains cellular structures can be marked with an antibody raised against germline-specific granules of Caenorhabditis elegans. Our results do not support the notion that the staining pattern for P granules is a useful indicator for phylogenetic relationship.