Roles of
let-60 ras and
sur-3 in vulva development and sex myoblast migration Meera Sundaram and Min Han Dept. of MCD Biology, Univ. of Colorado, Boulder CO 80309-0347 During the L2 stage in the hermaphrodite, the two sex myoblasts (SMs) migrate from the posterior to positions precisely flanking the center of the somatic gonad Several apparent
let-60 loss-of-function mutations cause defects in SM migration. We have previously described two such alleles,
let-60(nl O46
ku48) and
let-60(nl O46
ku75), which are intragenic revertants of
let-60(nlO46gu [WBG 13(1)]. We sequenced these alleles and found that they contain identical or very similar lesions to those in the previously described loss-of-function alleles let 60
(n2022J and
let-60(
n2021) [Beitel, Clark and Horvitz 1990]. In vulva development, nl O46gf causes a Muv phenotype,
n2022 or
n2021 causes a partial Vul phenotype, while
n1O46
ku48 or nl O46
ku75 is wild type (i.e. the gain of function and loss of function mutations mutually suppress each other). However, with respect to SM migration, both
n2022 and
n2021 homozygotes (like nl O46 homozygotes), are wild-type. Therefore, the SM migration phenotype in our intragenic revertants must require the presence of both nl O46 and either
ku48 or
ku75 in cis. A low penetrance SM migration defect also is seen when nl O46 is placed in trans to either
n2022 or
n2021. These complex genetic interactions between nl O46 and other
let-60 mutations raise questions as to the nature of the nl O46 allele with respect to SM migration. kt-60(nl O46J also displays interesting genetic interactions with mutations in other genes affecting SM migration. Although
let-60(nl O46) does not suppress the SM migration defects of either egl-l S
(n484J or
sem-5(nl 779J, it does partially suppress
egl-17(
el313) and
egl-17(
nl377). kt- 60(nlO46) also enhances the weak SM migration defect caused by mutations in
lin-45 raf, sur-l MAPK, or
sur-3. From these results, we think that the ras pathway normally plays some role in regulating SM migration, although the nature of that role is not yet clear. We isolated two apparent gain-of-function mutations in
sur-3 as semi dominant suppressors of the
let-60(nlO46gf) Muv phenotype.
sur-3(
ku68J also enhances the Vul and/or Let phenotypes caused by weak alleles of
let-60 and
lin45, but does not cause any vulval defects in a wild-type background.
sur-3(
ku68J causes a weak posterior displacement (or possibly randomization) of SM positions. Double mutants between sur- 3
(ku68J and either egl-lS
(n484J or
egl-17(
nl377J have strongly randomized SM positions, similar to what is seen when the gonad is ablated h
egl-15 or
egl-17 mutants [Stern and Horvitz, 1991]. Gonad ablation experiments in
sur-3, let- 60 and various double mutant animals may help to clarify the roles of these genes h regulating SM migration. We are currently screening for
sur-3 loss-of-function mutations. We have also cloned the
sur-3 gene by transformation rescue in order to begin a molecular characterization of
sur-3. References: Beitel, G. J., Clark, S. G., and Horvitz, H. R. (
l990). Nature 348, 503-509. Stern, M. J. and Horvitz H. R. (
l990). Development 113, 797-803.