The cell lineage of a polyclad turbellarian embryo reveals close similarity to coelomate spiralians

Recent molecular evidence suggests the turbellarian Platyhelminthes may rep resent the extant basal members of the Spiralia and therefore probably exhi bit ancient features of the spiralian developmental program. The stereotypi c quartet spiral cleavage pattern of the polyclad turbellarian embryo, amon g other features, indicates that this group may be closely related to the a ncestral flatworm; however, polyclad embryos have been the subject of few e xperimental studies. Here we report the results of a cell lineage analysis of the embryo of the polyclad Hoploplana inquilina based on microinjection of DiI into cleavage-stage blastomeres following formation of each of the f our quartets of micromeres. The first quartet gives rise to most of the lat eral and anterior ectoderm of the Muller's larva; the second quartet forms largely dorsal and ventral ectoderm as well as the circular muscles; the th ird quartet forms only small clones of ectoderm; and only the 4d cell of th e fourth quartet contributes to larval structure, forming the longitudinal muscles, mesenchyme, and probably endoderm. Our results demonstrate a strik ing similarity between the cell lineages of polyclad and higher spiralian e mbryos, in which the four quadrants also bear the same relationships to the larval axes and give rise to comparable larval structures, including deriv ation of mesoderm from both ectodermal (2b) and endodermal precursors (4d). (C) 1998 Academic Press.