Gastrulation in Cnidaria: The key to an understanding of phylogeny or the chaos of secondary modifications?

Abstract

The data revealed by comparative embryology of basal (diploblastic) metazoans are traditionally considered a valuable potential source of information on the origin and early evolution of the animal kingdom and its major clades. Special attention is paid to the fundamental morphogenetic process of gastrulation, during which the cells of the early embryo differentiate into germ layers and the primary body plan is formed. Comparative analysis of gastrulation in different cnidarian taxa reveals a high level of intergroup, intragroup, and individual variation. With few exceptions, there is no robust correlation between the gastrulation type and the taxon. Current data do not support the idea that the morphogenetic processes underlying cnidarian gastrulation can be divided into several distinct types. Rather, there is a continuum of equifinal ontogenetic trajectories. In cnidarians, the mode of gastrulation apparently depends less on the macroevolutionary history of the species than on various evolutionary plastic features, such as the oocyte size, amount of yolk, number of cells at the blastula (or morula) stage, presence of phototrophic symbionts, or larval ecology. Thus, in cnidarians, the morphogenetic basis of gastrulation contains only a very weak phylogenetic signal and can have only limited application in phylogenetic reconstructions. On the other hand, comparative studies of the ontogeny of basal metazoans shed light on the general rules of the evolution of morphogenetic processes, which is crucial for an understanding of the early history of the animal kingdom.