Tadpole development is eliminated in the life cycle of the ascidian Molgula pacifica. The elimination of a tailed larva is termed anural development, in contrast to urodele development which is exhibited by most ascidian species. In the present study, transmission electron microscopy and histochemistry were used to gain a better understanding of anural development in M. pacifica. The fine structure of M. pacifica oocytes and fertilized eggs was similar to urodele oocytes and eggs, except that a perivitelline space and test cells were absent. M. pacifica embryos exhibited the typical cleavage pattern of urodele embryos. Gastrulation was initiated at the vegetal pole, as in urodeles, and occurred at the same time as in two urodele species (Molgula manhattensis and Pyura haustor). However, changes in cell shapes and cell movements of the vegetal pole cells that participate in gastrulation were highly modified compared to commonly studied ascidians. The changes in shapes and movements of the vegetal pole cells were minimal and resulted in embryos having a very small archenteron and blastopore. The presence of large, yolky cells in the interior of the embryo likely restricted vegetal cell movements. Two ultrastructurally distinct types of epidermal cells were evident at the gastrula stage. When gastrulae were manually dechorionated from their surrounding mucous-follicular envelope layers, the embryos were already surrounded by a thin tunic. When day 1 juveniles in the process of hatching were sectioned along the anterior-posterior axis, regional differences in cell types were evident. Differentiated muscle cells in the posterior region were not evident. Day 1 M. pacifica juveniles, anural-developing M. provisionalis juveniles and tadpoles from three urodele species were tested for their abilities to express AchE activity. The highest levels of AchE activity were detected in the larval tail muscle cells of urodeles, low levels of activity were detected in the posterior region of M. provisionalis juveniles, whereas M. pacifica juveniles did not exhibit AchE activity. The results are discussed in terms of evolutionary mechanisms responsible for anural development in ascidians.

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.

No abstract

No abstract

No abstract