Worm Breeder's Gazette 13(3): 83 (June 1, 1994)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.


Carolyn Norris, Edward Hedgecock

Johns Hopkins University, Dept. of Biology.

One goal of our laboratory is to understand the genetic control of axonal growth and guidance in C. elegans. Here we describe two genes, unc( rh38 )and exc( rh162 ),that we think; function in cell-cell or cell-matrix; adhesion, both in neuronal and non-neuronal tissues.

Unc( rh38 )was isolated in a screen for mutants with abnormal PVPs, neurons that pioneer the ventral nerve cord (R. Durbin, 1985). Subsequent characterization revealed that the rh38 mutation disrupts the outgrowth of many axons, including pioneers. The outgrowth of another cellular process, the excretory canal, is frequently stunted in rh38 mutants. The rh38 mutation also causes two defects that have not been previously reported in C. elegans. First, the distal tip cell sometimes detaches from its germ cells. Second, the first polar body, which normally stays attached to the anterior end of the egg where it is expelled, often rotates to abnormal positions in rh38 worms. Taken together, we think unc( rh38 )disrupts cell-cell or cell-matrix attachment.

rh38 is probably a gain-of-function allele. rh38 / rh38 worms are quite uncoordinated, nearly 100% have amphid and phasmid axon abnormalities, and the excretory canal is short in ~60% of these homozygotes. rh38 /+is rarely (<1%) Unc, but the canal is short, and the amphid/phasmid axons are abnormal in 86% and 96%, respectively, of these heterozygotes. However, rh38 /Df[ mnDp111 , nDf23 , nDf25 ]is not Unc and rarely has any detectable cellular defects. Another gain-of-function allele isolated by non-complementation for the Unc phenotype, rh240 ,has properties similar to rh38 .To isolate a null mutant, we mutagened rh38 / rh38 worms and looked for non-Unc's in the first generation. So far we have obtained 4 new alleles that partially suppress the Unc and know that one, rh38r h272,is an intragenic revertant. rh38r h272is an impenetrant Unc, with frequent amphid and phasmid defects, and short canals in about 40% of the worms. Given that rh38r h272only partially suppresses the Unc phenotype of rh38 ,we think it is unlikely to be a null mutation.

The rh38 locus has not been separated from tax-2 .unc( rh38 )maps to chromosome I between unc-29 and aph-1 , where tax-2 is. The reason we suspect that rh38 and rh240 may be allelic to tax-2 is that they have the tax-2 ( p691 )phenotype of an amphid axon leaving the amphid commissure and traveling posteriorly in the ventral cord, and phasmid axons that are occasionally too long (Coburn and Bargman, WM '93). However, rh38 / tax-2 ( p691 )looks like rh38 /+,not rh38 /Df.. And only two of 103 worms of genotype rh38r h272/ tax-2 had the tax-2 amphid phenotype. So if rh38 is allelic to tax-2 , p691 may not be the null.

Exc( rh162 )is interesting for two reasons. First, exc( rh162 )has several phenotypes in common with unc( rh38 )including a short excretory canal, and the rare[ly reported] phenotypes of the detached distal tip cell and the floating polar body. Second, even though exc( rh162 )is not unc at all, preliminary experiments indicate exc( rh162 )is a synthetic lethal with unc-6 .That is exc( rh162 ); unc-6 ( ev400 )/+ double mutants arrest early in development. We therefore think that the function of exc( rh162 )is intimately related to that of unc-6 .This interaction with unc-6 is of great interest because UNC-6 is a laminin-like molecule that guides growth cones pioneering dorsal-ventral pathways (Hedgecock et al., 1990 and Ishii and Wadsworth et al., 1992).

exc( rh162 )is on chromosome V to the right of dpy-11 . nDf32 , sDf30 ,and sDf20 delete rh162 ,but nDf18 does not. That puts exc( rh162 )between let-337 and kra-l . rh162 /Dfmutants are not noticeably worse that rh162 homozygotes.

In summary, because unc( rh38 )and exc( rh162 )are the only two genes that are thus far known to cause detachment of the distal tip cell and polar body, we think these genes function similarly in cell-cell or cell-matrix attachment. Moreover, given the interaction of exc( rh162 )with UNC-6 ,these genes may be acting with UNC-6 ,a known axonal guidance molecule.