Evolutionary Changes in Proneural Gene Expression - atonal and ASH in Daphnia magna

We show here that the D. magna UAS-atonal construct shows similar phenotypes in the over-expression and rescue experiments as compared to the D. melanogaster atonal gene. However, the partial co-expression of atonal and ASH in D. magna embryos indicates that the molecular mechanisms in sense organ determination have changed and/or that sense organs are formed that show characteristics distinct from insect sense organs. There is indeed evidence for the latter scenario. Crustaceans possess sensory organs that contain scolopidial structures in their dendrites and can perceive internal and external mechanosensory stimuli. The combinatorial expression of ASH and atonal might specify these sense organs. In insects such sense organs have not been described and scolopidial structures are confined to the internal chordotonal organs. If we assume that atonal specifies sensory organs with scolopidial structures, the separate origin of internal (mechanosensory) chordotonal organs and external mechanosensory organs in insects could account for the non-overlapping expression of atonal and achaete-scute described in D. melanogaster.