Vulval fate induction is controlled by several different signaling pathways, including an RTK/Ras/ERK pathway. Some genes that influence vulval development have wild-type or nearly wild-type null phenotypes, and their roles are only revealed in appropriate sensitized genetic backgrounds. For example, mutations in
ksr-1 and
sur-8 do not cause strong phenotypes singly, but do cause highly penetrant Vulvaless and Lethal phenotypes in combination with each other, or in combination with weak alleles of other Ras pathway genes such as
lin-45 raf. To identify additional genes acting with
ksr-1 or
sur-8 (or in other parallel pathways), we have used both forward and reverse genetic approaches to identify additional synthetic Vul/Let mutations. In the forward genetic approach, we screened for enhancers of two weak, "phenotypeless"
lin-45 raf alleles. Because most Ras pathway mutations will cause complete lethality in these
lin-45 backgrounds, this screen has allowed us to avoid most of the usual suspects and instead isolate mutations in more peripheral regulatory genes like
ksr-1,
sur-6,
eor-1 and
eor-2 (see abstracts by Howard et al. and by Kao and Sundaram). As a reverse genetic approach, we also directly tested candidate genes for synthetic effects using existing mutations or RNAi, and we observed synthetic effects for
mig-2(
rh17gf) and for R01H10.8 (
cnk-1) RNAi. By testing for synthetic effects of these different mutation/RNAi pairs, we have tentatively placed the various genes into three functional groups that may define different parallel pathways that positively regulate Ras signaling. The SynVul/Let effects of
mig-2(
rh17gf) are particularly intriguing since
mig-2 encodes a Rac-like GTPase, and Rac has been proposed to be an important target of Ras signaling in mammalian cells. The molecular lesion in
rh17 suggests this is an activating mutation; however, several of the
rh17 mutant phenotypes are also seen in
ced-10/rac-1(lf);
mig-2(lf) double mutants, suggesting that
ced-10 and
mig-2 have overlapping functions and that
rh17 somehow interferes with the functions of both genes.