ISOLATION AND DEVELOPMENTAL EXPRESSION OF THE AMPHIOXUS PAX-6 GENE (AMPHIPAX-6) - INSIGHTS INTO EYE AND PHOTORECEPTOR EVOLUTION

Pax-6 genes have been identified from a broad range of invertebrate an d vertebrate animals and shown to be always involved in early eye deve lopment. Therefore, it has been proposed that the various types of eye s evolved from a single eye prototype, by a Pax-6-dependent mechanism. Here we describe the characterization of a cephalochordate Pax-6 gene . The single amphioxus Pax-6 gene (AmphiPax-6) can produce several alt ernatively spliced transcripts, resulting in proteins with markedly di fferent amino and carboxy termini, The amphioxus Pax-6 proteins are 92 % identical to mammalian Pax-6 proteins in the paired domain and 100% identical in the homeodomain. Expression of AmphiPax-6 in the anterior epidermis of embryos may be related to development of an olfactory ep ithelium. Expression is also detectable in Hatschek's left diverticulu m as it forms the preoral ciliated pit, part of which gives rise to th e homolog of the vertebrate anterior pituitary, A zone of expression i n the anterior neural plate of early embryos is carried into the cereb ral vesicle (a probable diencephalic homolog) during neurulation, This zone includes cells that will differentiate into the lamellar body, a presumed homolog of the vertebrate pineal eye, In neurulae, AmphiPax- 6 is also expressed in ventral cells at the anterior tip of the nerve cord; these cells are precursors of the photoreceptive neurons of the frontal eye, the presumed homolog of the vertebrate paired eyes. Howev er, AmphiPax-6 expression was not detected in two additional types of photoreceptors, the Joseph cells or the organs of Hesse, which are evi dently relatively recent adaptations (ganglionic photoreceptors) and a ppear to be rare exceptions to the general rule that animal photorecep tors develop from a genetic program triggered by Pax-6.