COOPER, Mark S*; VIRTA, Valerie M; University of Washington; University of Washington: Evolution of Gastrulation in the Actinopterygian (Ray-Finned) Fishes

This presentation reviews evolutionary changes in the gastrulation of ancient lineages of Actinopterygian fish. Cellular features of zebrafish gastrulation will be interpreted from an evolutionary perspective, focusing on the ontogenetic innovations that resulted in the derivation of Teleostean gastrulation. The most ancestral form of Actinopterygian gastrulation is preserved in Chondrostean fish (sturgeons and paddlefish). Chondrosteans gastrulate in a manner similar to many amphibians and agnathans, suggesting that this conserved mode of gastrulation stems from an archaic ancestor that predates the Devonian. Bony ganoid fish (basal Neopterygians), such as the bowfin (Amia calva) and gar-pike (Lepidosteus), whose lineages first appeared in the early Mesozoic, represent a transitional group between Chondrostean and Teleostean (advanced Neopterygian) fish. Although the gar-pike is the most basal Neopterygian to develop a syncytial endodermal yolk cell, more prominent proto-features of Teleostean gastrulation are exemplified in the bowfin gastrula. Comparisons between the bowfin Organizer and zebrafish Organizer reveal striking similarities in the morphogenesis of their presumptive germ layers. In particular, the bowfin gastrula possesses a prominent group of enlarged cells (Cushion Cells of Ballard) in the superficial Organizer epithelium. Similar to homologous cells in the zebrafish Organizer (which later become Kupffer’s vesicle), bowfin Cushion Cells fail to involute during gastrulation, and come to lie beneath the chordoneural hinge. Evolution of several other prominent morphogenetic domains within the teleostean Organizer region will be discussed. A putative evolutionary sequence of gastrulative mechanics within the Neopterygian fishes will then be summarized.