MARTINDALE, M.Q.; FREEMAN, G.; Univ. of Hawaii, Honolulu; Univ. of Texase, Austin: The origin of mesoderm in phoronids.

A large number of metazoan phyla (e.g. mollusks, annelids, nemerteans, sipunculids, echiurans) display a highly stereotyped pattern of early development called spiral cleavage. The conservation of the spiralian developmental program allows embryologists to identify homologous cells in embryos from different phyla and follow their fate. In most spiralians, muscle cells are derived from two embryonic sources, ectomesoderm and endomesoderm. Early workers interpreted ectomesoderm to be larval mesoderm and endomesoderm to be adult mesoderm, although more detailed work has shown this not to be strictly true. Lophophorates (e.g. phoronids, brachiopods, bryozoans) are now thought to be the sister group to spiralians in a group called the Lophotrochozoa. We used intracellular cell labeling techniques to determine if there were any similarities in the development of the phoronid Phoronis vancouverensis to that of spiralian taxa. Our results confirm the absence of a stereotyped cleavage program in phoronids and showed that contrary to textbook accounts, the vast majority of larval muscle cells are derived from ectomesodermal sources at ectodermal-endodermal boundaries. The spatial positioning of mesoderm formation in phoronids will be discussed in comparison with other spiralian embryos.