Enigma variations: control of sexual fate in nematode germ cells

Years ago, French pop star Patrick Juvet raised a question that evolutionary biologists are pondering anew in the aftermath of a paper about nematode sex determination from Eric Haag, David Pilgrim and their colleagues published recently in Developmental Cell [1]: "Où sont les femmes?" - Where are the fems? By showing that the fem genes, which are essential for spermatogenesis in Caenorhabditis elegans, are dispensable in Caenorhabditis briggsae germ cells, they proved that the regulatory pathways in these species have undergone recent and dramatic change.During the 1980s and 1990s, researchers cloned many of the genes that control sexual development in nematodes and fruit flies, and found that none of them resembled each other (reviewed in [2]). Since many other regulatory pathways were conserved, these results took everyone by surprise. Later, Raymond et al. [3] found that a single downstream gene, mab-3/doublesex, had been conserved between nematodes and insects. Taken together, these data implied that the regulatory pathways that control sexual development are derived from a common ancestor, but have been evolving rapidly.As an example of just how rapid this process can be, consider the nematode family Rhabditidae. In most of its species, XO animals are male and XX animals are female. Some species, however, feature XO males and XX hermaphrodites. Furthermore, all these hermaphrodites are essentially female animals that make their own sperm during larval development, which they use for self-fertilization. Surprisingly, self-fertile hermaphrodites have evolved independently many times in the Rhabditidae [4]. Even during recent evolution, these mating systems have changed multiple times within a small subgroup of the genus Caenorhabditis [5,6]. Thus, these nematodes provide a terrific model for studying the rapid evolution of sexual traits, and the recent work by Hill et al. [1] is the first major advance in this developing field.Genetic analysis of C. elegans reve