Evolution of eyes: Pax, gene duplications & morphology

Abstract

During the course of evolution animal complexity and diversity is generated. The last couple of years it has become increasingly clear that morphologically diverse animals share a great deal of their genomic contents, and it must be the expression of regulatory genes in new setting, rather than the generation of new genes, that are fundamental for the generation of diversity. Basal phyla hold a key position for explaining the generation of diversity. About 10 years ago it was realized that eyes that had previously been believed to constitute classical examples of convergent evolution, share the genes patterning their development. We have here studied one of these patterning genes (pax), and the morphology of evolutionary basal animals. Cnidarians have nervous systems and simple ocelli despite being overall morphologically simple. We isolated three pax genes from Aurelia, Polyorchis and Chiropsalmus. With a re-evaluation of the early evolution of the Pax family, we concluded that the last ancestor to cnidarians and Bilateria had a repertoire of at least 4 pax genes. Pax genes are recognized by a highly conserved paired domain, which binds to conserved DNA sequences. We showed that the coral PaxD paired domain does not bind to these conserved sequences in vitro. This is surprising considering the fact that PaxD is closely related to the Bilateria Pax3/7 subfamily - members of which all bind to conserved DNA sites. With mutagenesis we identified two adjacent residues to be primarily responsible for the observed lack of binding. Cnidarian larvae are of the planula type. The larva’s main purpose is to find the optimal settling location for metamorphosis into a sessile polyp. We have showed that cubozoan larvae, although morphologically simple, have distinct ocelli that might aid in settling. Finally we have studied vertebrate evolution. Vertebrate rods and cones provide night and day vision respectively. Not only opsins, but also several of the phototransduction proteins differ between the two cell types. It is believed that the vertebrates underwent two whole-genome duplications after the divergence from early chordates. We have showed that the generation of rod and cone specific phototransduction paralogs took place at the time of the early vertebrate tertaploidizations.