Worm Breeder's Gazette 14(4): 36 (October 1, 1996)
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.
Zoological Institute, University of Cologne, 50923 Koeln, Germany
To better understand the evolution of cell-specification mechanisms we study development in other nematode species. One of them is Cephalobus spec., an apparently more primitive representative of rhabditid nematodes (Skiba and Schierenberg, 1992, Dev. Biol. 151:597). Cephalobus lacks autofluorescence and birefringence of the gut granules as found in C.elegans. In order to analyse development of the intestine we therefore take advantage of 1.) its tissue-specific endocytotic activity leading to accumulation of fluorescently labelled transferrin (Bossinger et al., 1996, Roux's Arch. Dev. Biol. 205: 494) and 2.) the mAb 1CB4 which specifically recognizes gut cells in C.elegans (Okamoto and Thomson, 1985, J. Neurosci. 5: 643) and Cephalobus. In the last WBG we reported that gut differentiation in Cephalobus does not depend on an induction by P2 but may depend on AB. Here we present additional data leading to a modified interpretation. When AB was extruded at the 2-cell-stage, 87% (35/40) of the P1-derived partial embryos displayed gut differentiation while 13% (5/40) did not. When AB was extruded at the 3-cell stage 45% (15/33) of the partial embryos showed gut differentiation while 55% (18/33) did not (or very faintly). From this we conclude that in Cephalobus gut differentiation can take place without inductions from either P2 or AB (descendants). In another set of experiments we extruded P1 from 2-cell stages. Much to our surprise, 62% (20/32) of the emerging AB-derived embryos developed strong gut differentiation although lineage analysis shows that normally the gut in Cephalobus comes exclusively from descendants of the E- cell as in C. elegans. To exclude a potential confusion of AB/P1 (despite their size differences), we ascertained the typical different early lineage patterns in the non-extruded and the extruded cell. We found that in 65% (17/26) of the embryos AB was still able to produce gut-like cells when EMS and P2 were extruded in late 3-cell stages. In 35% (6/17) of these we even observed an overexpression of the gut markers in the AB lineage with 40-60 cells being recognized by the transferrin assay and nicely outlined by the antibody. These cells were considerably smaller than the normal 20-24 gut cells in untreated Cephalobus embryos, suggesting one or two additional rounds of cell division. Our results indicate that in Cephalobus both of the first two blastomeres carry the potential to develop gut and that an inhibitory interaction between AB and EMS (or their descendants) is necessarty to restrict the gut fate to the E-lineage. They also show that the relatively few divisions that normally take place in the gut lineage are not a prerequisite for proper differentiation. They can be interpreted as an early example of two cells competing for a primary fate as observed later in equivalence groups. Preliminary observations suggest that in manipulated embryos after the 8-AB cell stage some AB descendants acquire a slower cell-cycle rhythm and give rise to descendants with gut characteristics. This pattern is reminiscent of early embryogenesis in Enoplus brevis, a marine nematode in which a visible soma/germline differentiation is absent. In addition, other nematode species have been described in which gut is derived from the AB blastomere (V.V. Malakhov, 1994, Nematodes, Smithsonian Institution Press).