Evolution of Otx paralogue usages in early patterning of the vertebrate head.

To assess evolutional changes in the expression pattern of Otx paralogues, expression analyses were undertaken in fugu, bichir, skate and lamprey. Together with those in model vertebrates, the comparison suggested that a gnathostome ancestor would have utilized all of Otx1, Otx2 and Otx5 paralogues in organizer and anterior mesendoderm for head development. In this animal, Otx1 and Otx2 would have also functioned in specification of the anterior neuroectoderm at presomite stage and subsequent development of forebrain/midbrain at somite stage, while Otx5 expression would have already been specialized in epiphysis and eyes. Otx1 and Otx2 functions in anterior neuroectoderm and brain of the gnathostome ancestor would have been differentially maintained by Otx1 in a basal actinopterygian and by Otx2 in a basal sarcopterygian. Otx5 expression in head organizer and anterior mesendoderm seems to have been lost in the teleost lineage after divergence of bichir, and also from the amniotes after divergence of amphibians as independent events. Otx1 expression was lost from the organizer in the tetrapod lineage. In contrast, in a teleost ancestor prior to whole genome duplication, Otx1 and Otx2 would have both been expressed in the dorsal margin of blastoderm, embryonic shield, anterior mesendoderm, anterior neuroectoderm and forebrain/midbrain, at respective stages of head development. Subsequent whole genome duplication and the following genome changes would have caused different Otx paralogue usages in each teleost lineage. Lampreys also have three Otx paralogues; their sequences are highly diverged from gnathostome cognates, but their expression pattern is well related to those of skate Otx cognates.