REIS,, C.; BOLL, W.; NOLL, M.; OSSWALT, M.; WARNER, L.; HILL, M.; HILL, A.*; University of Z�rich; University of Z�rich; University of Z�rich; Boston College; University of Richmond; University of Richmond; University of Richmond: Isolation and analysis of Pax and Six genes in marine sponges

Sponges are true metazoans that lack a nervous system, true tissues or a defined body axis. These unique features provide a system in which to study the evolution of complex sensory systems. We are interested in genes involved in the developmental pathway leading to metazoan eyes. Here we report from the marine demosponge Haliclona sp. the isolation and analysis of two orthologs to genes known to be involved in metazoan eye development. It was determined that the sponge genome contains only one Pax gene (HlPaxB). Like other PaxB genes, HlPaxB contains a Pax2/5/8-like paired domain and a prd-like homeodomain with an unusual C-terminal part. The genomic structure of HlPaxB indicates conserved intron positions. HlPaxB is expressed at low levels in developing sponge larvae and is increased in expression in adult tissues. HlPaxB is able to substitute for Pax2 functions in the Drosophila eye, but is unable to induce ectopic eyes in Drosophila, even when the three crucial amino acids at position 42, 44, and 47 of the paired domain are substituted by those of Pax6-like proteins. This work supports the hypothesis that a PaxB-like rather than a Pax6 gene, was the ancestral gene responsible for development of complex eyes. The Sine oculis/Six class homeobox genes, like Pax6, have been suggested to have an ancestral role in photoreceptor cell development. We isolated a putative six1/2 ortholog (HlSix1/2) in Haliclona sp. Unlike HlPaxB, HlSix1/2 seems to be expressed throughout larval development/ and also in adult tissue. This work provides a foundation for future studies aimed at testing the hypothesis that a role for the development of primitive eyes already existed in the most basal animals.