Dach1, a vertebrate homologue of Drosophila dachshund, is expressed in the developing eye and ear of both chick and mouse and is regulated independently of Pax and Eya genes.

We have cloned a chick homologue of Drosophila dachshund (dac), termed Dach1. Dach1 is the orthologue of mouse and human Dac/Dach (hereafter referred to as Dach1). We show that chick Dach1 is expressed in a variety of sites during embryonic development, including the eye and ear. Previous work has demonstrated the existence of a functional network and genetic regulatory hierarchy in Drosophila in which eyeless (ey, the Pax6 orthologue), eyes absent (eya), and dac operate together to regulate Drosophila eye development, and that ey regulates the expression of eya and dac. We find that in the developing eye of both chick and mouse, expression domains of Dach1 overlap with those of Pax6, a gene required for normal eye development. Similarly, in the developing ear of both mouse and chick, Dach1 expression overlaps with the expression of another Pax gene, Pax2. In the mouse, Dach1 expression in the developing ear also overlaps with the expression of Eya1 (an eya homologue). Both Pax2 and Eya1 are required for normal ear development. Our expression studies suggest that the Drosophila Pax-eya-dac regulatory network may be evolutionarily conserved such that Pax genes, Eya1, and Dach1 may function together in vertebrates to regulate neural development. To address the further possibility that a regulatory hierarchy exists between Pax, Eya, and Dach genes, we have examined the expression of mouse Dach1 in Pax6, Pax2 and Eya1 mutant backgrounds. Our results indicate that Pax6, Pax2, and Eya1 do not regulate Dach1 expression through a simple linear hierarchy.     

Role of Pax genes in eye evolution: a cnidarian PaxB gene uniting Pax2 and Pax6 functions

PaxB from Tripedalia cystophora, a cubomedusan jellyfish possessing complex eyes (ocelli), was characterized. PaxB, the only Pax gene found in this cnidarian, is expressed in the larva, retina, lens, and statocyst. PaxB contains a Pax2/5/8-type paired domain and octapeptide, but a Pax6 prd-type homeodomain. Pax2/5/8-like properties of PaxB include a DNA binding specificity of the paired domain, activation and inhibitory domains, and the ability to rescue spa(pol), a Drosophila Pax2 eye mutant. Like Pax6, PaxB activates jellyfish crystallin and Drosophila rhodopsin rh6 promoters and induces small ectopic eyes in Drosophila. Pax6 has been considered a "master" control gene for eye development. Our data suggest that the ancestor of jellyfish PaxB, a PaxB-like protein, was the primordial Pax protein in eye evolution and that Pax6-like genes evolved in triploblasts after separation from Cnidaria, raising the possibility that cnidarian and sophisticated triploblastic eyes arose independently.     

Cre-loxp fate-mapping of Pax6 enhancer active retinal and pancreatic progenitors

Pax6 plays important roles in the control of ocular and pancreatic development. We identified a 450 bp Pax6 enhancer that contains two interacting sequences: a 274 bp fragment sufficient for expression in retinal progenitors and an adjacent 156 bp fragment required for expression in pancreatic progenitors. Since this enhancer is only transiently expressed during embryogenesis, a Cre-loxP fate-mapping strategy was used to investigate the developmental potential of these progenitors. Surprisingly, the labeled retinal precursors predominantly gave rise to horizontal cells, indicating a cell lineage role in horizontal cell differentiation. In the pancreas, all enhancer-specific cells were restricted to endocrine and ductal cell lineages. This result lends support to a model whereby Pax6-expressing progenitors contribute to the adult pancreatic islets and ducts. The progenitor cell-specificity of this enhancer will be useful in studies that require either cell-specific expression or conditional gene inactivation in these cell populations.     

Cubozoan jellyfish: an Evo/Devo model for eyes and other sensory systems

Cnidaria are the most basal phylum containing a well-developed visual system located on specialized sensory structures (rhopalia) with eyes and statocyts. We have been exploring the cubozoan jellyfish, Tripedalia cystophora. In addition to containing simple photoreceptive ocelli, each rhopalium in Tridedalia has a large and small complex, camera-type eye with a cellular lens containing three distinct families of crystallins which apparently serve non-lenticular functions. Thus, Tridpedalia recruited crystallins by a gene sharing strategy as have mollusks and vertebrates. Tripedalia has a single Pax gene, PaxB, which encodes a structural and functional Pax 2/5/8-like paired domain as well as an octapeptide and Pax6-like homeodomain. PaxB binds to and activates Tripedalia crystallin promoters (especially J3-crystallin) and the Drosophila rhodopsin rh6 gene in transfection tests and induces ectopic eyes in Drosophila. In situ hybridization showed that PaxB and crystallin genes are expressed in the lens, retina and statocysts. We suggest from these results that an ancestral PaxB gene was a primordial gene in eye evolution and that eyes and ears (mechanoreceptors) may have had a common evolutionary origin. Thus, the numerous structural and molecular features of Tridpalia rhopalia indicate that ancient cubozoan jellyfish are fascinating models for evo/devo insights into eyes and other sensory systems.     

The functional conservation of proteins in evolutionary alleles and the dominant role of enhancers in evolution.

Drosophila paired- embryos can be rescued to viable adults by the evolutionary alleles prd-Gsb and prd-Pax3, which express the Drosophila Gooseberry and mouse Pax3 proteins under the control of the paired cis-regulatory region. As prd-Gsb uncovers a prd function involved in the proper abdominal segmentation of adults, evolutionary alleles, defined and constructed in this manner, may often be weak and thus serve to discover hitherto unknown functions of a gene. Our findings show that the Gooseberry and Pax3 proteins have conserved most or all functions of the related Drosophila Paired protein although their C-terminal halves appear unrelated in sequence but not in 3-D structure essential for function. It follows that the acquisition of new cis-regulatory regions rather than the divergence of the C-terminal coding regions has been the primary device for the functional diversification of the Drosophila genes paired and gooseberry and the mouse Pax3 gene. The operation of this mechanism in insects as well as vertebrates suggests a major role in evolution.     

Genetic and biochemical diversity in the Pax gene family.

The mammalian Pax gene family comprises nine members that are characterized by a conserved DNA-binding motif, the paired domain, which was originally described in the Drosophila protein paired. Both loss- and gain-of-function studies reveal that Pax genes carry out essential roles during embryogenesis, and in some instances, may function as master regulatory genes. This review focuses on both genetic and biochemical aspects of the Pax family, and emphasizes important differences in the activity of individual Pax genes and their protein products.