Maternal-effect sterile (mes) gene function is essential for germline viability. Four mes genes (
mes-2,
mes-3,
mes-4, and
mes-6) are involved in controlling gene expression in the germline, probably by altering chromatin structure. To identify genes that genetically interact with
mes-4, we have tested candidate genes for enhancement of the Mes-4 sterile phenotype. We feed
mes-4/+ worms dsRNA to candidate genes and screen for sterile
mes-4 F1 offspring (normally sterility is not observed until the F2 generation, so "enhancement" is observing sterility a generation early). As candidate enhancers, we selected 200 germline-enriched potential chromatin regulators, identified by V. Reinke1 based on microarray and sequence analysis. Thus far, of these 200 genes, 50 from chromosome I have been tested. RNAi depletion of 3 genes resulted in strong enhancement of the Mes-4 phenotype: more than 50% of F1 homozygous animals became sterile. One of the enhancer genes is
lin-35, the only Rb homolog in C. elegans.
lin-35 is a member of the synthetic multivulva B (SynMuv B) family. Mutations in SynMuv B genes result in a multivulva phenotype only when in combination with a SynMuv A class mutation. Depletion of
lin-35 strongly enhances Mes-4 and also Mes-3 sterility, but only weakly enhances Mes-2 and Mes-6 sterility. We generated a
lin-35;
mes-4/nT1 double mutant and observed significant sterility among their
mes-4 progeny (i.e. sterility a generation early, as seen by RNAi). Surprisingly, however, in subsequent generations
lin-35 suppressed the Mes-4 phenotype:
mes-4 homozygous animals were able to produce fertile progeny. Furthermore, the suppressing effect of
lin-35 increased with each generation. This study suggests a potential link between MES function and LIN-35/Rb function in the germline. Rb in other organisms forms a complex with histone deacetylase (HDAC), whose activity generally represses gene expression.
hda-1 in C. elegans is another member of the SynMuv B class. MES-4 and MES-2 each have a SET domain, and many SET-domain-containing proteins are now known to possess histone methyltransferase (HMT) activity. We speculate that LIN-35 and MES proteins function in a balanced way to maintain proper histone modification patterns in the germline. Analysis of histone modifications in mes;
lin-35 double mutants and tests of the ability of other SynMuv mutants to enhance/suppress Mes sterility will help us figure out how these proteins function in the germline. 1) Reinke et al., 2000, Mol. Cell. 6, 605 2) Thanks to Erik Anderson in Horvitz lab for sharing his SynMuv; mes results.