An important aspect of vulva morphogenesis is to ensure that a proper connection forms between the lumens of the vulva and the uterus. In screens for mutants defective in vulva morphogenesis, we have isolated multiple mutations that cause the uterus and the vulva to fail to make a proper connection resulting in an Egl- phenotype. These mutants have been named cog or connection of gonad defective. We have isolated 8 alleles with a Cog phenotype that define the genes
cog-2,
cog-3 and
cog-4. In wild type animals the anchor cell (AC) fuses with a subset of pi cells to form the uterine seam cell (utse) (1). In animals carrying any one of the five
cog-2 alleles, the vulval lineages are normal, but the AC fails to fuse with the utse and, instead, remains at the apex of the vulva, blocking the entrance from the uterus. The
cog-2 defect is not a result of the complete failure to form any pi cells to which the AC might fuse. In
cog-2 animals at least some pi cell precursor cells divide in a manner characteristic of the generation of pi cells. However, it remains possible that although these cells take on the appearance of pi cells, they may not actually fully adopt a pi fate which may in turn block AC fusion.
cog-2 encodes a transcription factor that belongs to a large family of proteins called SOX, SRY related HMG box, proteins. COG-2 is greater than 70% identical within the 110 amino acid HMG box region to the mammalian proteins SOX-5 and SOX-6. The mouse SOX-5 protein is a sequence-specific, DNA-binding protein with the ability to bend DNA upon binding, and it is thought to play an architectural role in the transcription process (2). We have identified the lesions in the
cog-2 alleles, and two are suspected molecular nulls that truncate the predicted protein before the HMG box. We are currently examining the expression pattern of COG-2.
cog-4 was originally defined by two alleles. In animals carrying lesions in the
cog-4 gene, the thin process at the top of the vulva that is normally formed by the utse is replaced by a thick layer of tissue that appears to block the entrance to the vulva from the uterus. The vulval lineages in
cog-4 mutant animals are normal, and, unlike in
cog-2 animals, the AC successfully fuses to the utse. However, at a later stage, the vulval cells (and possibly some uterine cells) become disorganized and take up aberrant positions.
cog-4 mutant males mate inefficiently and appear to have defects in late stage male tail morphogenesis.
cog-4 encodes a novel, 317 amino acid protein, and our
cog-4 alleles are allelic to two previously identified mutations in
egl-26. All four alleles result in missense mutations in the second or fourth exons of the gene. A rescuing fusion of COG-4 to GFP is expressed transiently in the AC cytoplasm and then in some cells of the uterus where it becomes localized to what appears to be the lining of the uterine lumen. Weaker expression is also seen lining parts of the lumen of the vulva. Expression in a few other cells in the hermaphrodite tail region and near the pharynx is also observed and is being further characterized. We are currently examining expression patterns in the male. 1. A. P. Newman, J. G. White and P. W. Sternberg. (1996) Development 122: 3617-3626. 2. F. Connor et. al. (1994) Nucleic Acids Research 22: 3339-3346.