It is vital that the sex determination of sexually dimorphic tissues is coordinated with that of the gonad. In invertebrates, this coordination is achieved by master regulators of sex determination (such as tra-1 in Caenorhabditis elegans), which are expressed in all sexually dimorphic tissues. The tissue-specific downstream effectors of these master regulators remain largely unknown, but Chang and co-workers(p. 1425) have now identified the first gonad-specific gene involved in invertebrate sex determination. They report that C. elegans males lacking the forkhead transcription factor FKH-6 have feminised gonads that express hermaphrodite markers. By contrast, fkh-6-null hermaphrodites have no sex reversal,although the disruption of somatic tissues of the gonad results in disorganised spermathecae. The authors go on to use genetic and molecular analyses to place fkh-6 downstream of tra-1 in establishing gonadal sexual dimorphism, concluding that FKH-6 has a male-specific role in regulating sexual dimorphism in the early...

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