A forkhead protein controls sexual identity of theC. elegansmale somatic gonad

Abstract

In sex determination, globally acting genes control a spectrum of tissue-specific regulators to coordinate the overall development of an animal into one sex or the other. In mammals, primary sex determination initially occurs in the gonad, with the sex of other tissues specified as a secondary event. In insects and nematodes, globally acting regulatory pathways have been elucidated, but the more tissue- and organ-specific downstream effectors of these pathways remain largely unknown. We focus on the control of sexual dimorphism in the C. elegans gonad. We find that the forkhead transcription factor FKH-6 promotes male gonadal cell fates in XO animals. Loss-of-function fkh-6 mutant males have feminized gonads and often develop a vulva. In these mutant males, sex-specific cell divisions and migrations in the early gonad occur in the hermaphrodite mode, and hermaphrodite-specific gonadal markers are expressed. However, sexual transformation is not complete and the male gonad is malformed. By contrast, fkh-6 mutant hermaphrodites exhibit no sign of sex reversal. Most fkh-6 hermaphrodites form a two-armed symmetrical gonad resembling that of the wild type, but differentiation of the spermatheca and uterus is variably abnormal. The function of fkh-6 appears to be restricted to the gonad: fkh-6 mutants have no detectable defects in extra-gonadal tissues, and expression of a rescuing fkh-6 reporter is gonad-specific. Genetic and molecular analyses place fkh-6 downstream of tra-1, the terminal regulator of the global sex determination pathway, with respect to the first gonadal cell division. We conclude that fkh-6 regulates gonadogenesis in both sexes, but is male specific in establishing sexual dimorphism in the early gonad.

Functional Genomic Identification of Genes Required for Male Gonadal Differentiation in Caenorhabditis elegans

“…Genes required for male gonadal cell fates: Among the 67 genes in the feminized male gonad class was, as expected, fkh-6, which causes extensive gonadal feminization involving multiple cell types (Chang et al 2004). We also identified the Abd-B Hox gene homolog egl-5, which has been suggested to be partially feminized on the basis of gonadal morphology (Chisholm 1991).…”

“…We screened adults from two strains carrying gonad-specific GFP transgenes. The first, fkh-6TGFP (Chang et al 2004), is expressed in the gonadal primordium of both sexes during L1, but is hermaphrodite-specific during L4 and adult stages, when it is expressed in spermatheca and sheath cells. The second, K09C8.2TGFP (Thoemke et al 2005) is expressed in seminal vesicle and vas deferens cells of the male somatic gonad in late larvae and adults ( Figure 1A).…”

“…For both experimental strains (K09C8.2TGFP and fkh-6TGFP) eggs were recovered from gravid hermaphrodites by hypochlorite treatment, dropped onto prepared plates, and incubated at 22°for 3 days. Controls for each round of RNAi included fkh-6 (Chang et al 2004) and empty L4440 vector. Young adult animals were scored under fluorescence microscopy (Nikon SMZ1500) for missing, abnormal, or sexinappropriate marker expression.…”

“…Mutations compromising this pathway cause a ''symmetrical sisters'' phenotype in which both daughters adopt the same fate (Miskowski et al 2001;Siegfried and Kimble 2002;Phillips and Kimble 2009). Sex specificity is imposed on the SGPs by the global sex determining gene tra-1 (Hodgkin 1987) and the gonad-specific sex determining gene fkh-6 (Chang et al 2004). These genes play opposing roles in SGP sex determination, with tra-1 feminizing and fkh-6 masculinizing the somatic gonad, and they also act redundantly to promote mitotic proliferation of the SGP lineage (Chang et al 2004).…”

“…Sex specificity is imposed on the SGPs by the global sex determining gene tra-1 (Hodgkin 1987) and the gonad-specific sex determining gene fkh-6 (Chang et al 2004). These genes play opposing roles in SGP sex determination, with tra-1 feminizing and fkh-6 masculinizing the somatic gonad, and they also act redundantly to promote mitotic proliferation of the SGP lineage (Chang et al 2004). SGP sex determination is linked to cell cycle progression by cyclin D, which is required to overcome repression of fkh-6 expression in the SGPs by E2F (Tilmann and Kimble 2005).…”