VIRTA, Valerie C.*; COOPER, Mark S.; University of Washington; University of Washington: Ontogeny and Phylogeny of the Yolk Extension in Cypriniform Fishes

At the beginning of the Mesozoic era, Actinopterygian fish transformed their mode of gastrulation. During this transition, yolky endoderm was a morphological hotspot for rapid evolutionary change. As holoblastic cleavage patterns were modified into meroblastic cleavage, marginal blastomeres in the midblastula became progenitors of nuclei that populate a syncytial cortical layer, known as the yolk syncytial layer (YSL). All known teleost embryos possess a YSL, suggesting that YSL morphogenesis was a key cytological innovation involved in the evolution of teleost gastrulation.

In zebrafish, one of the most dramatic morphogenetic events in the YSL is the formation of the yolk extension, which begins around the 15-somite stage. A sustained contractile event reshapes the posterior region of the YSL, forcing yolk to flow both anteriorly and posteriorly as the contractile band constricts in a ventral-to-dorsal direction. We have found that a mechanical transection of the neural rod and notochord does not inhibit yolk extension formation or body straightening, suggesting that the yolk extension can be formed in zebrafish embryos without mechanical forces contributed by overlying axial tissues.

The yolk extension appears to be a conserved character within the order of Cypriniformes, which diverged from other Teleostean fish about 40-60 million years ago (mya). The Cypriniform yolk extension shows that the yolk cell has continued to evolve its morphology since teleostean gastrulation first appeared 200 mya. Remarkably, the yolk extension serves as a prominent cladistic character that sets apart Cypriniform embryos from other vertebrate embryos at the �phylotypic stage.�