The phylogenetic significance of maximum direct development in the ascidian, Molgula pacifica

Studies of Molgula pacifica, ascidians that live along the west coast of Vancouver Island, Canada, emphasize the need to look beyond the study of typical model species if we are to eventually construct accurate phylogenies of chordates. This paper reviews some of our recent results pertaining to the ecology and embryology of a maximum direct developer and what this information may be telling us about chordate evolutionary mechanisms. First, some of the early life history parameters of this direct developer are described suggesting that egg size, in contrast to echinoderms, is not correlated with direct development. Second, a detailed examination of gastrulation suggests that embryonic cell movements in M. pacifica are highly modified and likely contribute to the absence of all four basic ascidian larval structures: the dorsal neural tube, notochord, two rows of posterior muscle cells and anterior ectodermal papillae responsive to settlement cues. Third, the possible roles of maternally derived gene products in eggs and early embryonic cell interactions were examined using UV-irradiation and blastomere deletions, respectively. Surprisingly, these results suggest that early cell interactions are essential for normal juvenile development in striking contrast to the well studied determinant type of ascidian development that produces tailed larvae. Finally, some of the possible phylogenetic implications of these findings are briefly discussed.