Pristionchus pacificus, a nematode with only three juvenile stages, displays major heterochronic changes relative to Caenorhabditis elegans

The nematode Pristionchus pacificus (Diplogastridae) has been described as a satellite organism for a functional comparative approach to the model org anism Caenorhabditis elegans because genetic, molecular, and cell-biologica l tools can be used in a similar way in both species. Here we show that P.p acificus has three juvenile stages, instead of the usual four found in othe r nematodes. Embryogenesis is lengthened and many developmental events that take place during the first juvenile stage in C. elegans occur during late embryogenesis in P pacificus. Video imaging and transmission electron micr oscopy revealed no embryonic moult. The timing of later developmental event s relative to the moults differs between P. pacificus and C. elegans. In ad dition, the post-embryonic blast-cell divisions display a specific change i n timing between the two species, resulting in heterochrony between differe nt cell lineages, such as vulval and gonadal lineages. Developmental events appear to come into register during the last larval stage. Thus, differenc es in developmental timing between P. pacificus and C. elegans represent a deep heterochronic change. We designate the three juvenile stages of P. pac ificus as J1 to J3. Comparison with other species of the family Diplogastri dae indicates that this pattern represents an apomorphic character for the monophylum Diplogastridae.