Genetic control of vulval development in Caenorhabditis briggsae.

The nematode Caenorhabditis briggsae is an excellent model organism for the comparative analysis of gene function and developmental mechanisms. To study the evolutionary conservation and divergence of genetic pathways mediating vulva formation, we screened for mutations in C. briggsae that cause the egg-laying defective (Egl) phenotype. Here, we report the characterization of 13 genes, including three that are orthologs of Caenorhabditis elegans unc-84 (SUN domain), lin-39 (Dfd/Scr-related homeobox), and lin-11 (LIM homeobox). Based on the morphology and cell fate changes, the mutants were placed into four different categories. Class 1 animals have normal-looking vulva and vulva-uterine connections, indicating defects in other components of the egg-laying system. Class 2 animals frequently lack some or all of the vulval precursor cells (VPCs) due to defects in the migration of P-cell nuclei into the ventral hypodermal region. Class 3 animals show inappropriate fusion of VPCs to the hypodermal syncytium, leading to a reduced number of vulval progeny. Finally, class 4 animals exhibit abnormal vulval invagination and morphology. Interestingly, we did not find mutations that affect VPC induction and fates. Our work is the first study involving the characterization of genes in C. briggsae vulva formation, and it offers a basis for future investigations of these genes in C. elegans.