In Drosophila, early eye development is governed by a set of DNA binding proteins and transcriptional coactivators that collectively are termed the retinal determination (RD) network. The core factors are encoded by the Pax6 genes eyeless (ey, Quiring et al.,1994; and twin of eyeless [toy], Czerny et al.,1999), the SIX class transcription factor sine oculis (so, Cheyette et al.,1994; Serikaku and O'Tousa,1994), the transcriptional coactivator and protein tyrosine phosphatase eyes absent (eya, Bonini et al.,1993) and a distant relative of the Ski/Sno protooncogene dachshund (dac, Mardon et al.,1994). Removal of any of these genes leads to the complete elimination of the adult compound eye (reviewed in Kumar,2008). Remarkably, forced expression of any member within nonretinal tissues such as the antennae, legs, halters, and wings can lead to the formation of ectopic eyes (Halder et al.,1995; Bonini et al.,1997; Shen and Mardon,1997; Czerny et al.,1999; Weasner et al.,2007). As a cassette, these factors function during eye development in a broad range of organisms including vertebrates (reviewed in Treisman,1999; Wawersik and Maas,2000; Hanson,2001). In addition to this core network of genes, there are several other factors that interact with and modulate the activities of these genes, which are also required for early eye development or can induce ectopic eye formation. These genes include the Pax6(5a) genes eyegone (eyg, Jun et al.,1998) and twin of eyegone (toe, Jang et al.,2003), the SIX transcription factor optix (Seimiya and Gehring,2000), the Meis1 homolog homothorax (hth, Pai et al.,1997), the serine-threonine kinase nemo (nmo, Braid and Verheyen,2008), the pipsqueak motif containing DNA binding proteins distal antenna (dan) and distal antenna related (danr, Curtiss et al.,2007) and the zinc finger transcription factor teashirt (tsh, Pan and Rubin,1998).
Tsh was first initially identified as a specifier of trunk identity and segmentation in the embryo, where loss of tsh function resulted in the transformation of the prothoracic segment to a labial identity (Fasano et al.,1991; Roder et al.,1992). A potential role for tsh during early eye development was first hinted at when it was recovered in an enhancer/promoter (EP) screen for genes that could force nonretinal tissues into adopting an eye fate (Pan and Rubin,1998). Tsh protein is distributed throughout the regions of the developing eye that lie anterior to the advancing morphogenetic furrow (MF). It functions to promote cell proliferation and depending upon the context can either promote or inhibit eye development (Bessa et al.,2002; Singh et al.,2002,2004; Bessa and Casares,2005). Retinal mosaic clones that completely lack tsh develop normally thereby prompting the suggestion that a second gene is acting in a functionally redundant manner to tsh in the eye (Pan and Rubin,1998).
The tsh paralog, tiptop (tio) was identified as a regulator of Drosophila embryogenesis (Laugier et al.,2005). tio null mutants are viable, fertile and display no retinal abnormalities. tsh and tio have different expression patterns until stage 17 of embryonic development, after which point they have shared expression domains. Interestingly, these genes are not completely redundant to each other as knockdown experiments with RNAi constructs can induce retinal defects. It appears that both genes are maintained, in part, because each gene is capable of repressing its own expression and that of its paralog (Laugier et al.,2005; Bessa et al.,2009). Thus, it seems that correct development of the retina is not dependent upon the absolute expression levels of each individual gene. Rather, it is suggested that the eye simply requires certain combined levels of this zinc finger subfamily (Bessa et al.,2009).
In this study, we have attempted to further elucidate the roles of Tsh and Tio in eye development. We find that both genes are expressed in identical patterns in the eye–antennal disc and that their expression levels are not significantly different from each other. We have also analyzed the ability of the duplicates to direct eye specification and demonstrate that while tsh and tio can induce ectopic eye formation (Pan and Rubin,1998; Bessa et al.,2009), this activity is limited in that both genes are capable of only redirecting the fate of the developing antennal imaginal disc. Additionally, we observe that tio is more effective than tsh in inducing ectopic eye formation.