Evolution of gastrulation in the ray-finned (actinopterygian) fishes

Sometime before or during the early Mesozoic era, new lineages of actinopterygian (ray-finned) fishes radically transformed their mode of gastrulation. During this evolutionary transformation, yolky endoderm was a hotspot for ontogenetic change. As holoblastic cleavage patterns were modified into meroblastic cleavage patterns, major changes in cell identity specification occurred within the mesendodermal marginal zone, as well as in the superficial epithelium of the embryo. These cellular identity changes resulted in the appearance of two novel extra-embryonic tissues within the embryos of teleostean fishes: the enveloping layer (EVL) and the yolk syncytial layer (YSL). The generation of these extra-embryonic tissues prompted major morphogenetic changes within the Organizer Region. As these evolutionary changes occurred, the outermost cell layer of the Organizer (the Organizer Epithelium) was apparently retained as a signaling center necessary for the establishment of left-right embryonic asymmetry in the embryo. Conserved and derived features of Organizer morphogenesis and gastrulation within ancient lineages of ray-finned fishes provide important insights into how the genetically encoded cell behaviors of early morphogenesis can be altered during the course of evolution. In particular, a highly divergent form of actinopterygian gastrulation, which is found in the annual fishes of South America, demonstrates that no aspect of vertebrate gastrulation is inherently immutable to evolutionary change.