What makes a man a man? Prenatal antennapedia expression is involved in the formation of the male phenotype in Daphnia.

Cyclic parthenogenetic organisms show a switch in reproductive strategy from asexual to sexual reproduction upon the occurrence of unfavourable environmental conditions. The sexual reproductive mode involves the production of ameiotic diploid males and the fertilization of meiotic haploid eggs. One beautiful example for this switch between parthenogenesis and sexual reproduction is Daphnia. Male and female Daphnia from the same clone are genetically identical. Morphological differences should therefore only be due to differential gene expression. This differential gene expression leads to sexually dimorphic phenotypes with elongated and moveable (i.e. leg-like) first antennae in males in comparison to females. For other arthropods, it has been demonstrated that the formation of differential morphology of legs and antennae involves the regulation of the Hox gene antennapedia (antp). Here, we show that antp is expressed during the embryogenesis of Daphnia, and that adults contain much lower amounts of antp mRNA than eggs. The eggs of mothers that were treated with the juvenile hormone methyl farnesoate (responsible for the production of male offspring) showed lower expression of antp than parthenogenetically produced female eggs. We therefore conclude that differential antp expression is involved in the molecular pathways inducing the male phenotype of Daphnia.