The nature and evolution of the complementary sex determination pathway in honey bees. (A) In csd heterozygotes, csd (complementary sex determiner) gene products activate the downstream gene fem (feminizer), whose gene product is required for female development. In homo-/hemizygous animals, the csd gene product ensures male development by the default regulatory pathway, in which neither the csd nor fem gene products are activated. (B) The primary signal for sex determination, the csd gene, arose from the ancestral progenitor gene fem by gene duplication. The modern function of csd evolved by positive selection and fixation of new amino acid changes in the Csd protein.
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In humans, sex is determined by the presence or absence of X or Y sex chromosomes. In honeybees, however, evolution has resulted in a very different and unique sex determination system.
