Sex determination has long intrigued evolutionists,
geneticists and developmental biologist. A large body of
evidence indicates that the process evolves rapidly (for
reviews, see REFS 64,65). In Drosophila and C. elegans, sex
determination is triggered by differences in the ratio of
the set of X chromosomes to autosomes. Animals with
two X chromosomes develop as females or, in the case of
C. elegans, as self-fertilizing hermaphrodites, but
develop as males if only one X chromosome is present.
Genetic and molecular studies revealed that C. elegans
sex determination relies on a signalling pathway.
Genetically, it involves a cascade of negatively acting factors
that, at a molecular level, consists of a signalling
pathway that triggers the zinc-finger transcription factor
transformer-1 (TRA-1)66 (FIG. 4).