The embryonic lethal phenotypes ('no back end') resulting from psoralen-induced non-maternal mutations at two loci suggest roles for these genes in posterior embryonic morphogenesis. The 'nob-l' locus maps between
spe-6 and
unc-25 on the right arm of LGIII. The
ct223 allele is a neonatal lethal with a spherical gut and a round posterior, but a normal head; its earliest visible defect is a misarrangement of the E cells in the 100-cell embryo.
ct230 is a weaker homozygous viable allele, (approximately 65% fertility) which results in a lumpy deformed tail very similar to
vab-7.
ct230/eDf2 is nonviable, suggesting that
ct223 is a hypomorph, while
ct223/eDf2 has the same phenotypic range as
ct223/ct223. We have also isolated a larger deficiency of the locus with a break point between
spe-6 and 'nob-l' extending at least to bli-S. We are mapping Tcl polymorphisms in this region as an approach to cloning the gene.
ct224, which results in a similar but slightly more severe posterior embryonic defect, was isolated in a LGIII screen for more alleles of 'nob-l'. However, it complements 'nob' alleles, maps between
dpy17 and
unc-79 and fails to complement pal-l, the homeobox gene identified in Cynthia Kenyon's lab, which is involved in postembryonic cell interactions of V6 and T in the male tail. Because
ct224 is psoralen-induced and might be a deletion of more than one gene, we are trying to rescue the mutant phenotype by injecting the pal-l plasmids, given us by Cynthia ( thanks!). So far we have no established lines, but we have seen transient rescue in several injection series, including those using the smallest plasmid pWPK6 (5.5 kb). At present we believe that
ct224 is a strong, probably a null, allele of
pal1, indicating an embryonic role for this gene. We are beginning mosaic analysis to determine which embryonic lineages require the gene function. The double mutant
ct223ct224 has a somewhat additive phenotype (hatched embryos look like pears). Construction of double mutants with the weaker phenotypes is planned, and may be more informative as to gene interactions. It is of interest that the homeobox of pal-l shows sequence similarity to that of the caudal gene in Drosophila and its apparent mouse homolog, which also are implicated in posterior/endodermal development.