Gastrulation in the white sturgeon, Acipenser transmontanus, involves many of the same processes as in the amphibian Xenopus laevis, but the timing and relative importance of these processes are altered so that they function appropriately in a different type of egg. In both species, convergence and extension result from a combination of radial and mediolateral cell intercalation. In sturgeons, where the blastopore lip forms at the equator, an early phase of thinning and extension of the animal cap moves the marginal zone below the equator during late blastula and early gastrula stages. This early extension without convergence is followed by convergent extension of the dorsal marginal zone after its displacement vegetally. When the animal cap is removed before gastrulation, precluding the initial extension that moves the marginal zone below the equator, autonomous convergence of the lower marginal zone produces an equatorially constricted embryo. Dorsal explants of sturgeon embryos undergo convergent extension similar to that documented in Xenopus (Keller and Danilchik: Development, 103:193-209, 1988), with distinct zones of extension in the involuting and non-involuting marginal zone regions. The extension of cultured explants demonstrates that this morphogenetic behavior is intrinsic to the dorsal tissue. These results show that normal gastrulation depends not only on the function of these independent morphogenetic mechanisms, but also on their mechanical context in the embryo. Experimental analyses and comparison of gastrulation in similar embryos, such as those of Xenopus and sturgeons, reveal both common developmental mechanisms, and variation in their roles.
