Common Transcriptional Mechanisms for Visual Photoreceptor Cell Differentiation among Pancrustaceans

Abstract

A hallmark of visual rhabdomeric photoreceptors is the expression of a rhabdomeric opsin and uniquely associated phototransduction molecules, which are incorporated into a specialized expanded apical membrane, the rhabdomere. Given the extensive utilization of rhabdomeric photoreceptors in the eyes of protostomes, here we address whether a common transcriptional mechanism exists for the differentiation of rhabdomeric photoreceptors. In Drosophila, the transcription factors Pph13 and Orthodenticle (Otd) direct both aspects of differentiation: rhabdomeric opsin transcription and rhabdomere morphogenesis. We demonstrate that the orthologs of both proteins are expressed in the visual systems of the distantly related arthropod species Tribolium castaneum and Daphnia magna and that their functional roles are similar in these species. In particular, we establish that the Pph13 homologs have the ability to bind a subset of Rhodopsin core sequence I sites and that these sites are present in key phototransduction genes of both Tribolium and Daphnia. Furthermore, Pph13 and Otd orthologs are capable of executing deeply conserved functions of photoreceptor differentiation as evidenced by the ability to rescue their respective Drosophila mutant phenotypes. Pph13 homologs are equivalent in their ability to direct both rhabdomere morphogenesis and opsin expression within Drosophila, whereas Otd paralogs demonstrate differential abilities to regulate photoreceptor differentiation. Finally, loss-of-function analyses in Tribolium confirm the conserved requirement of Pph13 and Otd in regulating both rhabdomeric opsin transcription and rhabdomere morphogenesis. Taken together, our data identify components of a regulatory framework for rhabdomeric photoreceptor differentiation in Pancrustaceans, providing a foundation for defining ancestral regulatory modules of rhabdomeric photoreceptor differentiation.

Two temporal functions of Glass: Ommatidium patterning and photoreceptor differentiation

“…Our studies in Drosophila have now described a key regulatory link between rhabdomeric cell type patterning and differentiation mediated by Glass and Pph13. Both of these molecules are conserved in Pancrustaceans (Liu and Friedrich, 2004; Rivera et al, 2010) and homologs can be found in other protostomes including Apylsia californica ((Mahato et al, 2014) and NCBI Ref. Seq.…”

“…As a first step in deciphering the potential evolutionary role of Glass, we investigated the function of Glass in Tribolium castaneum . Previous reports have demonstrated that both Glass and Pph13 are expressed in the developing photoreceptors of the Tribolium castaneum adult visual system (Liu and Friedrich, 2004; Mahato et al, 2014). Utilizing RNAi, SEM analysis of Glass RNAi knockdowns revealed an identical phenotype as compared to Drosophila ; there was an overall reduction of eye size, including a reduction in ommatidium number, as well as the presence of fused ommatidia (Fig.…”

“…Library construction and sequencing were performed as described (Mahato et al, 2014). HISeq read sequences were cleaned using Cutadapt version 1.2.1 (Rehm et al, 2011) to remove adapter sequences and perform quality trimming.…”

“…Also, Otd is required for the proper innervation of photoreceptors into the lamina and medulla (Mencarelli and Pichaud, 2015). Lastly, the function and relationship between Otd and Pph13 with respect to differentiation of photoreceptors is not limited to Drosophila , but conserved among Pancrustaceans (Mahato et al, 2014). …”

Development of functional ectopic compound eyes in scarabaeid beetles by knockdown of orthodenticle

“…Rather than being brought about through targeted overexpression of master regulator genes, this induction results from the knockdown of the transcription factor orthodenticle (otd) (8). This phenotype is surprising: otd function has been studied in detail in Tribolium beetles, where it is needed for photoreceptor development in bilateral compound eyes but plays no detectable role in specifying the external morphology of eyes or the head in general (8,9), and in Drosophila, where it is required for proper rhabdomere and photoreceptor development in compound eyes (10) and for the formation of medial ocelli (11) (i.e., single-lens eyes that differ from compound eyes in many fundamental aspects). However, all extant scarabaeids and almost all other beetles, including Tribolium, lack medial ocelli (12), and even though dorsalization of compound eye compartments has evolved in a subset of insect taxa (the turbinate eyes of some male mayflies being an extreme example), completely distinct compound eyes developing middorsally on the cephalic vertex are not known from any extant insect lineages (13).…”