2000 West Coast Worm Meeting abstract 14
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University of California, San Francisco
Cell fusion is a common process in C. elegans. The C. elegans hypodermis consists of several multinucleate syncytia that are generated by the fusion of cells throughout development. The largest syncytium is hyp7, which spans most of the length of the worm and which contains more than 130 nuclei. We are carrying out two screens to identify mutations affecting cell fusion. First, we screened for mutations that prevent fusion of the Pn.p cells with hyp7 on the ventral surface of the worm. Mutations identified in this screen affect the decision of these cells to fuse. Second, in order to identify mutations in genes that carry out the cell fusion process, we are screening for mutations that affect fusion of the seam cells that line the lateral surface of the worm.
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.ps. In ref-1(mu220) (REgulator of Fusion) hermaphrodites, P9.p and P10.p fail to fuse with hyp7. This is due, in part, to inappropriate activation of MAB-5 in ref-1 hermaphrodites. ref-1 encodes a gene with two basic-helix-loop-helix DNA binding domains of the hairy/E(spl) family.
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, they somehow 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 others activities. ref-2 has been mapped to a 40 kb region on the center of the X chromosome and transformation rescue experiments are in progress.
Descendants of the seam cells fuse with hyp7 at all larval stages and ultimately the seam cells fuse with each other during L4. We are currently using a membrane localized gfp fusion expressed in the seam cells to identify mutants in which seam cell fusions fail to occur.