Proximal fate marker homothorax marks the lateral extension of stalk-eyed fly Cyrtodopsis whitei.

The placement of eyes on insect head is an important evolutionary trait. The stalk-eyed fly, Cyrtodopsis whitei, exhibits a hypercephaly phenotype where compound eyes are located on lateral extension from the head while the antennal segments are placed inwardly on this stalk. This stalk-eyed phenotype is characteristic of the family Diopsidae in the Diptera order and dramatically deviates from other dipterans, such as Drosophila. Like other insects, the adult eye and antenna of stalk-eyed fly develop from a complex eye-antennal imaginal disc. We analyzed the markers involved in proximo-distal (PD) axis of the developing eye imaginal disc of the stalk-eyed flies. We used homothorax (hth) and distalless (dll), two highly conserved genes as the marker for proximal and distal fate, respectively. We found that lateral extensions between eye and antennal field of the stalk-eyed fly's eye-antennal imaginal disc exhibit robust Hth expression. Hth marks the head specific fate in the eye- and proximal fate in the antenna-disc. Thus, the proximal fate marker Hth expression evolves in the stalk-eyed flies to generate lateral extensions for the placement of the eye on the head. Moreover, during pupal eye metamorphosis, the lateral extension folds back on itself to place the antenna inside and the adult compound eye on the distal tip. Interestingly, the compound eye in other insects does not have a prominent PD axis as observed in the stalk-eyed fly.