Anural development of the ascidian Molgula pacifica (Huntsman)

Abstract

Molgula pacifica embryos exhibit anural development in which embryogenesis proceeds directly to the development of a juvenile, without the development of a tailed larva. The purpose of this study was to investigate the key events that are responsible for the development of M. pacifica juveniles. The results of the present investigation indicate that the timing and spatial rearrangements of the egg cytoplasm that occur after fertilization (termed ooplasmic segregation) are similar in M. pacifica eggs as compared with those that occur in typical urodele species. These observations suggest that the mechanism that is responsible for the urodele pattern of ooplasmic segregation was conserved during the evolution of anural species. The cleavage patterns displayed by M. pacifica embryos up to the eight-cell stage were similar to those exhibited by urodele embryos. However, gastrulation in M. pacifica embryos differed from the typical urodele mode of gastrulation. The mode of gastrulation exhibited by M. pacifica embryos is likely a consequence of their eggs containing a greater quantity of yolk than the less yolky eggs of species more commonly studied. In the second part of this investigation, ampullar development, structure, and function were studied. Two conclusions were made from these studies. First, the extracellular matrix materials comprising the tunic are secreted by the epidermal ampullar cells. Second, a shift in the timing of ampullar rudiment development in M. pacifica embryos suggests the possibility of a heterochronic mechanism of evolutionary change within the epidermal cell lineage.