Worm Breeder's Gazette 16(1): 37 (October 1, 1999)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
|1||Howard Hughes Medical Institute and Division of Biology 156-29 CALTECH, Pasadena CA 91125 USA.|
|2||Institut Jacques Monod, Tour 43, 2 place Jussieu, 75251 Paris Cedex 05, France.|
|3||Fred Hutchinson Cancer Research Center, Division of Basic Sciences,A3-013, 1100 Fairview Avenue North, Seattle, WA 98109-1024, USA.|
|4||Max-Planck Institut für Entwicklungsbiologie, Abteilung Elektronenmikroskopie, Spemannstrasse 35, 72076 Tübingen, Germany.|
|5||Institut für Zoologie, Freie Universität Berlin, Königin-Luise Str. 1-3, 14195 Berlin, Germany.|
|6||Max-Planck Institut für Entwicklungsbiologie, Abteilung Evolutionsbiologie, Spemannstrasse 35, 72076 Tübingen, Germany.|
The postembryonic development of nematodes is accompanied by periodic molting. The number of four molts is thought to be constant within the entire monophylum. In a majority of species, the first stage juvenile hatches from the ovum and all four larval molts occur in an external environment (outside the egg shell). This type of external molting is thought to be ancestral in nematodes. However, deviations from this pattern are seen in many different groups of Secernentea as well as Mermithidae and Tetradonematidae. In such cases, embryonization of molting occurs.
Studying the life cycle of Pristionchus pacificus revealed a surprising change of larval development. P. pacificus has three juvenile stages, instead of four, like in other nematodes. Embryogenesis is lengthened and many developmental events which take place during the first juvenile stage in C. elegans occur during late embryogenesis in P. pacificus. Video imaging and transmission electron microscopy revealed no embryonic molt. The timing of later developmental events relative to the molts differs between P. pacificus and C. elegans. In addition, the postembryonic blast cell divisons display a specific change in timing between the two species resulting in heterochrony between different cell lineages, like vulval and gonadal lineages. Developmental events appear to come into register during the last larval stage. Thus, differences in developmental timing between P. pacificus and C. elegans represent a deep heterochronic change. We designate the three juvenile stages of P. pacificus as J1 to J3. Comparison to other species of the family Diplogastridae indicates this pattern to represent an apomorphic character for the monophylum Diplogastridae.