Worm Breeder's Gazette 15(2): 27 (February 1, 1998)
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.
MCD Biology, University of Colorado, Boulder, CO 80309
Deletion mutations of the caudal homolog pal-1 cause severe posterior defects in embryonic morphogenesis. We have continued analysis of embryonic pal-1 expression by lineage analysis of a pal-1::GFP transgenic line which shows an expression pattern very close to that of antibody staining (Craig Hunter, thanks again for the antibodies), to extend and correct our earlier lacZ reporter data. In the GFP line, all cells of the early C and D lineages express PAL-1::GFP from the 4C and 4D stage until early morphogenesis; this corresponds to the antibody staining seen in staged embryos. The MS descendants M and mu int R and their dying sisters (rather than Z1 and Z4 as we earlier thought) express the reporter. Ventral AB descendants plpappp, plppppp, and prppppp, and their daughters express strongly; other ABpxpppxx neighboring cells stain weakly with antibodies and occasionally express GFP. These cells gastrulate to interior positions and generate a mixed bag of mesectodermal cell types: neurons, muscles, and hypodermis of the rectal and tail regions. Their pal-1 expression, except in the rectal cells, is gone before hatching. Transient antibody staining is additionally seen in the posterior P and some V cells as they move to enclose the body; this has not been seen with the GFP reporter. Finally, the two int-5 intestinal cells express the GFP reporter from early morphogenesis through hatching. Overall, pal-1 is expressed transiently in the posterior in a number of cell lineages during the period of morphogenesis. 4D lineage analysis of pal-1(ct224) mutant embryos reveals only minor lineage defects, but gross defects in cell positions and movements of PAL-1-expressing cells. About 80% of ct224 homozygous embryos hatch with a Nob terminal phenotype; the remainder fail in hypodermal enclosure. We previously reported mispositioning of the dorsal hypodermal C cells as early as the 8C-cell stage, resulting in an ill-defined or skewed dorsal midline; and a failure of the contralateral nuclear migration and elongation in these cells during early morphogenesis. We have now analysed two ventrally oriented embryos, one enclosed and one unenclosed. In the C muscle lineage, some cells failed to make the terminal division and appeared to differentiate early, albeit as muscle. C muscle was spatially disorganized, with some Cpp cells not moving to the normal right-side positions. Severe ventral defects were observed in the process of late gastrulation (just pre-lima, ~350 cells). Normally, the mesectodermal contralateral homologs Abplpappp/prpappp and plppppp/prppppp initially border a 'ventral cleft' [Sulston et al.(1983), Dev. Biol. 100:64-119]. At about 3.5 hr post-fertilization these cells move from lateral surface positions to meet at the ventral midline; at the same time the ABpxppppp cells contact the most posterior of the C hypodermals Caapv and Cpapv. The cells appear to adhere on contact and the ventral cleft rapidly closes from posterior to anterior as these cells divide. Subsequently the lateral hypodermal row of P cells moves ventrally to effect hypodermal enclosure [Williams-Masson et al.(1997), Dev. Biol.124:2889-2901]. All of these cells except ABprpappp express pal-1 during this period. In the unenclosed ct224 embryo, these AB:ventral midline and C:posterior contacts never occurred and the ventral cleft remained open; hypodermal enclosure also failed. In the Nob ct224 embryo the ventral cleft closed late, after the bordering cells divided, and the posterior AB:C contact was transient. Hypodermal enclosure was delayed at least 2 hr, and no posterior elongation occurred. In summary, patterning defects suggesting failures in cell-cell recognition or some other positional cues have been observed in most of the cell lineages expressing pal-1. Together these defects appear sufficient to explain the observed embryonic phenotypes.