Multiple levels of redundant processes inhibit Caenorhabditis elegans vulval cell fates

Abstract

Many mutations cause obvious abnormalities only when combined with other mutations. Such synthetic interactions can be the result of redundant gene functions. In Caenorhabditis elegans, the synthetic multivulva (synMuv) genes have been grouped into multiple classes that redundantly inhibit vulval cell fates. Animals with one or more mutations of the same class undergo wild-type vulval development, whereas animals with mutations of any two classes have a multivulva phenotype. By varying temperature and genetic background, we determined that mutations in most synMuv genes within a single synMuv class enhance each other. However, in a few cases no enhancement was observed. For example, mutations that affect an Mi2 homolog and a histone methyltransferase are of the same class and do not show enhancement. We suggest that such sets of genes function together in vivo and in at least some cases encode proteins that interact physically. The approach of genetic enhancement can be applied more broadly to identify potential protein complexes as well as redundant processes or pathways. Many synMuv genes are evolutionarily conserved, and the genetic relationships we have identified might define the functions not only of synMuv genes in C. elegans but also of their homologs in other organisms.

Figure 1.—

Regulation of lin-3 transcription by the synMuv genes. Real-time RT–PCR experiments were performed on the strains shown. Mean ΔΔC_T values were used to calculate relative changes in lin-3 expression normalized to levels of rpl-26. Mean values and standard deviations of relative lin-3/rpl-26 ratios for the trials are shown. (A) Mutation of each class A synMuv gene in a class B synMuv mutant background has increased lin-3 transcription. (B) Mutation of each class B synMuv gene in a class A synMuv mutant background has increased lin-3 transcription. lin-15A(n767) and lin-61(n3809) were used for this experiment. (C) Two class B–B–A synMuv triple mutants have increased levels of lin-3 transcription compared to their respective class B–A double mutants. lin-15A(n433) and lin-61(n3809) were used for this experiment.

Figure 2.—

Model: The synMuv classes function in two redundant pathways or processes, each composed of separate molecular pathways or processes, to mediate the inhibition of vulval cell fates. The class A and B synMuv genes act in two distinct pathways to inhibit the expression of vulval cell fates. Additionally, each class is composed of many separate pathways as defined by genetic redundancy tests. A few synMuv genes act nonredundantly with each other: lin-15A and lin-56 act together within the class A genes, while within the class B genes dpl-1 DP, efl-1 E2F, and lin-35 Rb; mys-1 HAT and trr-1 TRRAP; and let-418 Mi2 and met-1 Set2 act together to inhibit vulval fates.