Cell fusion is a common process in C. elegans . The C. elegans epidermis consists of several multinucleate syncytia that are generated by the fusion of cells throughout development. The largest such syncytium is
hyp7, which spans most of the length of the worm and which contains more than 130 nuclei. To study the formation of this syncytium, we isolated mutations that prevent fusion of one set of cells that fuse with
hyp7, the 12 Pn.p cells that line the ventral surface of the worm late in L1. We identified several mutations that affect the pattern of Pn.p cell fusion and have been characterizing two mutations that affect Pn.p cell fusion by altering Hox protein activity. The fusion decision of the 12 Pn.p cells is controlled by two Hox genes,
lin-39 and
mab-5 .
lin-39 is expressed in the mid-body [P(3-8).p] and in hermaphrodites prevents fusion of these cells.
mab-5 is expressed more posteriorly [in P(7-11).p] in both sexes, but is not active in hermaphrodite Pn.p cells. In
ref-1(
mu220) (REgulator of Fusion) hermaphrodites, P9.p and P10.p fail to fuse with
hyp7. This is due, at least in part, to inappropriate activation of MAB-5 in
ref-1 mutant hermaphrodites.
ref-1 encodes a gene with two basic helix-loop-helix DNA binding domains of the hairy/E(spl) subfamily. In males,
lin-39 and
mab-5 each individually prevent Pn.p cell fusion in P(3-6).p and P(9-11).p, respectively. However, in P7.p and P8.p, where both Hox genes are expressed in the same cell, the two Hox proteins neutralize one another's activities, so that P7.p and P8.p fuse with
hyp7. In
ref-2(
mu218) males, P7.p and P8.p fail to fuse with
hyp7, perhaps because LIN-39 and MAB-5 fail to cancel each other's activities.
ref-2(
mu218) is a dominant mutation in a non-coding region near a zinc finger transcription factor. This mutation affects the pattern of Pn.p cell fusion by affecting this transcription factor, perhaps by altering its expression. To determine the role of
ref-2 during development, we carried out RNAi of
ref-2 and also ectopically expressed
ref-2 using a heat shock promoter. These experiments demonstrate that the
ref-2 gene product is required for the generation of the Pn.p cells and also to keep the Pn.p cells unfused. We are currently trying to determine the effect of the
mu218 mutation on
ref-2 expression and are also trying to define the relationship between
ref-2 and the Hox genes.