Comparative Genetics of Sex Determination: Masculinizing Mutations in Caenorhabditis briggsae

Abstract

The nematodes Caenorhabditis elegans and C. briggsae independently evolved self-fertile hermaphroditism from gonochoristic ancestors. C. briggsae has variably divergent orthologs of nearly all genes in the C. elegans sex determination pathway. Their functional characterization has generally relied on reverse genetic approaches, such as RNA interference and cross-species transgene rescue and more recently on deletion mutations. We have taken an unbiased forward mutagenesis approach to isolating zygotic mutations that masculinize all tissues of C. briggsae hermaphrodites. The screens identified loss-of-function mutations in the C. briggsae orthologs of tra-1, tra-2, and tra-3. The somatic and germline phenotypes of these mutations are largely identical to those of their C. elegans homologs, including the poorly understood germline feminization of tra-1(lf) males. This overall conservation of Cb-tra phenotypes is in contrast to the fem genes, with which they directly interact and which are significantly divergent in germline function. In addition, we show that in both C. briggsae and C. elegans large C-terminal truncations of TRA-1 that retain the DNA-binding domain affect sex determination more strongly than somatic gonad development. Beyond these immediate results, this collection of mutations provides an essential foundation for further comparative genetic analysis of the Caenorhabditis sex determination pathway.

Context-dependent function of a conserved translational regulatory module

“…Therefore, the Mog phenotype of Cbr-tra-2(nm1)/+;Cbr-puf-2/1.2(RNAi) animals requires a maternal nm1 allele, and the poorly differentiated germ line of their Cbr-dpy(nm4);Cbr-puf-2/1.2(RNAi) siblings is a dominant maternal effect of the Cbrtra-2(nm1) mutation. Another Cbr-tra-2 allele, nm9ts (Kelleher et al, 2008), produced the same result, suggesting that the interaction between Cbr-tra-2 and Cbr-puf-2/1.2 is general.…”

“…XX Cbr-tra-2(nm1) homozygotes develop imperfect male bodies and produce only sperm (Kelleher et al, 2008), whereas heterozygotes are normal hermaphrodites. All XX Cbr-tra-2(nm1);Cbr-puf-2/1.2(RNAi) animals developed male somas, but roughly half of these had tumorous germ lines lacking differentiated gametes and half produced sperm proximal to a tumor (Fig.…”

Independent recruitments of a translational regulator in the evolution of self-fertile nematodes

“…Mutations in Cbr-tra-2 and Cbr-tra-1 that masculinize the XX germ line and soma (34) suppress Cbr-gld-1(nm68) tumors, and the germ line remains masculinized (Table S1). When only the hermaphrodite germ line is feminized with Cbr-fog-3(RNAi) (35), Cbr-gld-1(RNAi) produces germ-line tumors (14) (Table S1).…”

“…Thus, Cbr-gld-1(lf)-mediated tumors form in oocyte-fated germ lines, regardless of somatic sex or karyotype, as in C. elegans (31). Cbr-tra-1 mutants also produce robust oocytes as they age (34,36). Cbr-gld-1; Cbr-tra-1 mutants produce neither tumors nor differentiated oocytes, but do often produce sperm normally (Table S1).…”

“…C. briggsae tra-2 plays a conserved female-promoting role in both the soma and germ line (34,47). Cbr-tra-2 mRNA can also be repressed via its 3′ UTR in the soma (48), which in C. elegans is mediated by SUP-26 (49), and potentially the GLD-1 paralog ASD-2 (50) (Fig.…”