Functional genomics identifies regulators of the phototransduction machinery in the Drosophila larval eye and adult ocelli.

Sensory perception of light is mediated by specialized Photoreceptor neurons (PRs) in the eye. During development all PRs are genetically determined to express a specific Rhodopsin (Rh) gene and genes mediating a functional phototransduction pathway. While the genetic and molecular mechanisms of PR development is well described in the adult compound eye, it remains unclear how the expression of Rhodopsins and the phototransduction cascade is regulated in other visual organs in Drosophila, such as the larval eye and adult ocelli. Using transcriptome analysis of larval PR-subtypes and ocellar PRs we identify and study new regulators required during PR differentiation or necessary for the expression of specific signaling molecules of the functional phototransduction pathway. We found that the transcription factor Krüppel (Kr) is enriched in the larval eye and controls PR differentiation by promoting Rh5 and Rh6 expression. We also identified Camta, Lola, Dve and Hazy as key genes acting during ocellar PR differentiation. Further we show that these transcriptional regulators control gene expression of the phototransduction cascade in both larval eye and adult ocelli. Our results show that PR cell type-specific transcriptome profiling is a powerful tool to identify key transcriptional regulators involved during several aspects of PR development and differentiation. Our findings greatly contribute to the understanding of how combinatorial action of key transcriptional regulators control PR development and the regulation of a functional phototransduction pathway in both larval eye and adult ocelli.

Ultrastructure of the dorsal ocellus of Bittacus planus larvae (Mecoptera: Bittacidae) with evolutionary significance.

The Bittacidae are unique in holometabolous insects in that their larvae bear a dorsal ocellus on the frons. The fine structure of the dorsal ocellus, however, has not been investigated to date. Here, the ultrastructure of the larval dorsal ocellus was studied in the hangingfly Bittacus planus Cheng, 1949 using light, scanning, and transmission electron microscopy. The dorsal ocellus of the larvae comprises a cornea, corneagenous cells, and retinula cells. The cornea is a laminated structure. A layer of corneagenous cells is located below the cornea. Numerous retinula cells are arranged tightly beneath the corneagenous cells. The retinula cells modify their adjacent membranes into numerous linear microvilli, which form an analogue of the rhabdom among adjacent retinula cells. The results show that the dorsal ocellus of larval Bittacidae is a highly vestigial organ and appears to be degenerating during the postembryonic development. The presence of the vestigial dorsal ocellus is likely to represent an ancestral plesiomorphy of holometabolous insects, providing new evidence for exploring the evolutionary origin of holometabolous larvae.