Ontogeny and Phylogeny of the Yolk Extension in Embryonic Cypriniform Fishes

The validity of defining a common phylotypic stage for all vertebrates has been questioned because of variations in embryonic morphological traits between vertebrate classes, as well as variations in embryonic phenotypes between species of the same vertebrate class. To evaluate the evolutionary lability of phylotypic features in vertebrate embryos, we have examined the phylogenetic and ontogenetic origins of the yolk extension-a distinctive morphological trait that is found in the ventrolateral trunk region of cypriniform fish embryos. This posterior axial protrusion, extending from the embryonic yolk ball, is formed in cypriniform fishes by a ventrolateral constriction of the yolk mass during the phylotypic period of development. Using a functional definition of the phylotypic period, a comparative analysis of published literature on developing actinoptyerygian (ray-finned) fishes reveals that the yolk extension is a shared embryonic trait of the clade Cypriniformes. The yolk extension also appears in several species in two other basal teleostean clades, Characiformes and Anguilliformes. The conservation of the yolk extension in the clade Cypriniformes, as well as its presence in two other basal teleostean clades, supports the hypothesis that the yolk extension is a product of evolutionary transformation. Besides exhibiting evolutionary transformation, the process of yolk extension formation satisfies five other defined criteria for developmental modularity. Thus, it appears that yolk extension ontogenesis is a novel evolutionary, developmental module that has been incorporated into the phylotypic period of certain teleostean lineages. J. Exp. Zool. (Mol. Dev. Evol.) 312B:196-223, 2009. (C) 2009 Wiley-Liss, Inc.