The SEMAPHORINS are known as chemorepulsive molecules involved in axon guidance (Raper,Curr.Opian.Neurobiol.10,2000). We took a genetic approach to understand the role of SEMAPHORIN in cell migration and signalling in C.elegans . A Ce-SEMAII-A mutant (
mab-20 ) was isolated and showed limited axon guidance abnormalities (Roy et al., Dev.127(2000)). Surprisingly, a null allele (
ev574 ) exhibits penetrant epidermal cell migration defects affecting morphogenesis and male tail development. We performed an enhancer screen on a hypomorph of
mab-20 allele (
bx61 ) and isolated different mutant genes enhancing its morphogenesis defects. One of these mutations (
ev732 ) has been genetically mapped to linkage group IV, where a known CE-PLEXIN-A homologue ( Y55F3AL.1) has been previously identified by the GENOME SEQUENCING CONSORTIUM (GSC). By performing PCR-screening on a frozen chemically mutagenized worm library, we isolated a deletion allele in the Y55F3AL.1 gene (
ev724 ) and found it is allelic to
ev732 for the Mab ( m ale ab normal) phenotype. The C.elegans male tail consists of a fan shaped acellular structure in which are encasted two bilaterally symmetric sets of nine sensory rays, each ray consisting of a structural cell sheathing two sensory neurons. For these two alleles, ray 1 is anteriorly positioned to the other rays when compared to wildtype (WT) males. A developmental phenotypic characterization revealed that ray 1 precursor cells are generated normally in Ce-PLEXIN-A alleles but later on fail to detach from the SET cell syncitia. Interestingly, the same phenotype has been observed in knockout alleles of Ce-SEMAI-A (
ev709 ) and Ce-SEMAI-B (
ev715 ) (Val Ginzburg, personal communication). This striking similarity between Ce-SEMAI-A/B and Ce-PLEXIN-A mutant phenotype, in contrast to Ce-SEMAII-A Mab phenotype which consists of fusions between the different rays, suggests that Ce-SEMAI-A/B and Ce-PLEXIN-A are part of a common genetic pathway controlling ray 1 position. This hypothesis is being tested by constructing double Ce-SEMAI-A/B ; Ce-PLEXIN-A mutants. Little is known about the role of NEUROPILIN/SEMAPHORIN complexes and their interactions with the VEGF (vascular endothelial growth factor) system in vertebrate cardiovascular development (Soker et al., Cell92(1998) ;Miao et al.,JCB146(1999)). As we did for SEMAPHORINS , we undertook a genetic approach to understand VEGF signaling and function in C.elegans . No NEUROPILIN like molecule have yet been identified in the C.elegans sequenced genome. However, a VEGF like ligand ( Y39A3CL.6 ) and two VEGF receptor (VEGFr1/Flt-1 ; VEGFr2/Flk-1) candidates ( F59F3.1 ; F59F3.5 ) have been identified by the GSC. Using a reverse genetic approach we isolated deletion alleles in both receptor genes ( F59F3.1 -
ev760 ; F59F3.5 -
ev761 ). So far, the phenotypic analysis revealed an anteriorly positioned ray 1 in male tail of F59F3.1 (
ev760) mutant males. The future work will include determining the expression patterns of both receptors and a phenotypic characterization of the second homologous receptor mutant F59F3.5 (
ev761) . We recently obtained a knockout allele of Y39A3CL.6 which should help characterizing this pathway. We have shown that Ce-SEMAI-A/B and Ce-PLEXIN-A affect ray 1 positioning during C.elegans male tail development in contrast to Ce-SEMAII-A which likely has a broader role in keeping each ray distinct. This is consistent with the fact that Ce-SEMAI-A/B are transmembrane anchored semaphorins and that Ce-SEMAII-A is a secreted semaphorin, thus likely having a broader action range. The phenotypic similarity observed so far between the SEMAPHORIN ligand/receptor system and the VEGFr1/Flt-1 receptor homologue in C.elegans suggests a role for both systems in ray cell migration and patterning. We think that the C.elegans male tail offers a simple system to study cell migration and we plan to use it in order to genetically understand the interaction between these two systems.