Worm Breeder's Gazette 12(5): 39 (February 1, 1993)
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.
The homeotlc selector gene mab-5 is a member of a cluster of homeotic selector genes which is conserved from C. elegans to humans. These genes are expressed in a region specific manner and are required to correctly determine cell fate in the regions in which they are expressed (Wang and Kenyon, unpub.; Clark and Horvitz, personal communication). mab-5 is expressed in the posterior of the worm and is required in those cells to specify posterior fates. For example, mab-5 is required in males for the V5 and V6 lineages to produce rays. In males carrying a loss-of-function mab-5 mutation, the V5 and V6 lineages produce alae instead of rays, just like their anterior homologs. Conversely, ectopic mab-5 can cause V cells anterior to V5 to produce rays instead of alae (Costa et al., 1988; Kenyon, 1986; Salser. unpub.).
In an effort to understand how the wild type pattem of expression is obtained, we decided to screen mab-5 ( e2088 ); muls3 animals, which carry a mab-5 -lacZreporter construct, for mutations which affect the mab-5 pattern of expression. One of the most striking mutations found, mu38 ,causes ectopic expression of this construct. In a wild type background the reporter construct is expressed in a number of cells in the posterior of the worm, including the V6 and QL lineages. When newly hatched mu38 ; muls3 worms are stained, expression appears normal, but when late L1 sare stained, expression extends in the Vn.p cells anteriorly all the way to V1 .pand is also seen in the descendants of QR. There is a concomitant phenotype: the descendants of QR can stay in the posterior (as has been seen in animals carrying a mab-5 gain-of-function mutation (Salser and Kenyon, 1992)), and in males the V1 -V5lineages make rays instead of alae. These effects are due to ectopic mab-5 expression, because in the mu38 ; mab-5 ( e2088 )double mutant they do not occur. The changes can be thought of as homeotic transformations of anterior fates to posterior fates induced by the ectopic mab-5 .
mu38 worms exhibit other phenotypes as well: they are sometimes Muv, are Unc, and in general are quite sickly. Not all of these phenotypes can be explained by ectopic mab-5 ,so we decided to see if other genes in the homeotic cluster are misexpressed. Indeed lacZ fusion constructs made with the lin-39 or egl-5 genes (which are respectively anterior and posterior to mab-5 in the cluster (Wang et al., submitted)) are ectopically expressed in a mu38 background.
We have used PCR mapping (Williams et al., 1992) to determine that the mu38 mutation maps to the right amm of chromsome I. It is closely linked to TC bn2 and hp4 :0/94 chromosomes examined showed recombination between mu38 and these markers.
The mu38 mutation causes ectopic expression of at least three genes in the C. elegans homeotic gene cluster. For at least one of these, mab-5 ,the initial pattern of expression is normal; the gene becomes derepressed later in development (the lin-39 and egl-5 expression pattems haven't been checked yet). It appears that the wild type function of the gene mutated by mu38 is to maintain the homeotic selector genes in a stably repressed state once an initial pattern of expression and repression has been obtained. If this is the case, then mu38 could be considered to be equivalent to the Drosophila Polycomhgroup of genes (which also are required to maintain homeotic gene repression). The initial pattern of mab-5 expression changes during development (Salser, unpub.); we are interested in understanding both how the maintenance system revealed by mu38 works to preserve the initial pattern and whether it is modified to allow for changes in expression.