S2-1.6 Jan. 4 Heads or tails? Amphioxus and the evolution of axial patterning in chordates YU, J-K.; ONAI, T.; HOLLAND, L. Z.*; California Institute of Technology , Univ. California, San Diego; Univ. California, San Diego; Univ. of California, San Diego email@example.com
Within the vertebrates, the mode of gastrulation is highly variable. Consequently, the evolutionary origin of the gastrula organizer, which induces the head and patterns the embryo along the anterior/posterior axis, remains unclear. In contrast to vertebrate embryos, the amphioxus embryo gastrulates by simple invagination. As a result, the relative positions of the ectoderm and mesendoderm remain constant throughout the gastrula stage, facilitating an understanding of embryonic patterning. We have found that the amphioxus gastrula expresses orthologs of organizer genes in patterns reminiscent of their deployment in vertebrates. Genes involved in dorso/ventral (D/V) patterning include a dorsally expressed group of secreted proteins (Nodal, Lefty, Chordin, and ADMP) and transcription factors (e.g. Goosecoid and Lim-1) as well as a ventrally expressed group of secreted proteins (including the BMP-signaling modulators BAMBI and Tolloid-like) and transcription factors (Vent and Evx). Genes involved in anterior/posterior (A/P) patterning (e.g. several Wnts, Wnt antagonists and the anterior endodermal marker Hex) are also expressed in comparable patterns as their vertebrate orthologs, consistent with a role for Wnt-signaling in A/P patterning. These results together with those from functional experiments suggest that amphioxus and vertebrates use homologous gene networks for both D/V and A/P patterning. In light of recent phylogenetic analyses placing cephalochordates basal in the chordate lineage, we conclude that separate D/V and A/P signaling centers, as proposed for chick and mouse, may already have been present in ancestral chordates.