DUAL ORIGINS OF MESODERM IN A BASAL SPIRALIAN - CELL LINEAGE ANALYSESIN THE POLYCLAD TURBELLARIAN HOPLOPLANA-INQUILINA

Evolutionary modifications in the origins and organization of the meso derm represent significant events in the diversification of metazoan b ody plans. Within the Spiralia, mesoderm comprises ectomesoderm, which typically is generated by derivatives of the second and third quartet s of micromeres, and endomesoderm, which is formed from the fourth qua rtet micromere of the D quadrant (4d). It has been held that endomesod erm generates the majority of adult mesodermal derivatives, while larv al mesoderm is formed primarily from ectomesoderm . The evolutionary h istory of these mesodermal sources could be clarified by examining bas al members of the Spiralia such as the polyclad turbellarians, whose e mbryos exhibit canonical quartet spiral cleavage. Using the fluorescen t lineage tracer DiI, we show that larval mesoderm is derived from onl y two cells, one from the ventral embryonic quadrant (2b, the ''mesect oblast'' cell), and the other from the dorsal quadrant (4d, the mesent oblast cell). We compare these results with mesodermal origins in othe r spiralian phyla and conclude that a dual origin of mesoderm is a pri mitive feature of spiralian development. We also argue that ectomesode rm and endomesoderm should trot be considered as the exclusive precurs ors of larval and adult mesoderm, respectively. (C) 1996 Academic Pres s, Inc.