Cell fusion is a major factor in the development of the C. elegans body plan. As the molecules and physical mechanisms responsible for cell membrane fusion are unknown in any developmental system, we have focused on C. elegans as a uniquely suited model for genetic and structural studies of the process of cell fusion. Some of the advantages of studying cell fusion in the worm are: A) the predictable postion and timing of cell fusion events based on the lineage of precursor cells [1]; B) the amenability of embryos to live-imaging and EM studies of the intermediate structural stages in a cell fusion event [2]; C) the surprising viability of worms with genetic defects in cell fusion [3,4]. In two distinct screens - one for embryonic hypodermal fusion defects and another for vulval morphogenesis/fusion defects - we have isolated recessive mutant alleles of a gene that is required for cell fusion among the precursors of epithelial syncytia [3,4]. Live-imaging experiments confirmed that correctly patterned syncytial precursors are blocked at the earliest stages of cell fusion in these mutants, before formation of an initial cytoplasmic bridge between partner cells. Mutations
oj55 and
hy21 both lie on Chromosome II and fail to complement in the trans-heterozygote. Allele
oj55 was further mapped to postion +0.79, and was rescued by injection of cosmid C26D10. This cosmid also rescues
hy21 , suggesting that both mutations lie within the same gene, which we have named
eff-1 (Epithelial Fusion Failure). Preliminary results from dsRNAi and a single rescue injection experiment appear to confine the functional
eff-1 gene to a 12.5 kb region of C26D10. Confirmation of the identity of the gene and characterization of the molecular lesions underlying
eff-1(
oj55) and
eff-1(
hy21) will be presented at the 13 th International C. elegans meeting. The two
eff-1 alleles vary in the numbers of unfused cells per animal and in the overall effects on body morphology:
hy21 has a stronger loss of function than
oj55. Yet it appears that all morphogenetic defects in both mutants can be attributed to failed cell fusions [3,4]. Other aspects of hypodermal and vulval patterning and differentiation appear normal. We therefore believe that the Eff-1 gene product functions specifically within the mechanism controlling the fusion of plasma membranes on neighboring cells. To test this hypothesis we will compare the developmental expression pattern of
eff-1 with the known temporospatial pattern of embryonic and larval cell fusions [1]. Furthermore, we will examine the dynamic sub-cellular localization of Eff-1 protein to assess whether it appears at the known sites for initiation and widening of the opening between actively fusing cells [2]. Podbilewicz and White (1994). Developmental Biology 161:406-424. Mohler et al. (1998). Current Biology 8: 1087-1090. Mohler and White (1999) 12 th International C. elegans meeting: 595. Shemer et al. (2001) 13 th International C. elegans meeting.