Elucidating the molecular mechanisms that created ancient complex traits like insect wings is difficult. Fortunately, some complex traits have arisen recently. For example, hermaphroditic reproduction evolved independently many times during recent nematode evolution. Although C. elegans hermaphrodites require
fog-2, which encodes an F box protein that regulates the translation of
tra-2 mRNAs, the related species C. briggsae lacks
fog-2. We identified a critical regulator of hermaphrodite development in C. briggsae, named
she-1. Analysis of double mutants indicates that
she-1 acts upstream of
tra-2 in C. briggsae, just as
fog-2 does in C. elegans. Molecular cloning shows that
she-1 encodes a novel F box protein that was created by a recent gene duplication. Whereas FOG-2 acts through GLD-1 in C. elegans, SHE-1 does not bind GLD-1 in C. briggsae. Thus, both species recruited F box genes produced by recent duplication events into the sex-determination pathway to control hermaphrodite development, but these genes have distinct activities. This result implies that some gene families are more likely to give rise to novel regulatory genes than other families. Finally, we note that null mutations of
she-1 are temperature sensitive, so C. briggsae might once have been a facultative hermaphrodite.