The unique developmental program of the acoel flatworm, Neochildia fusca

Acoel embryos exhibit a unique form of development that some investigators argue is related to that found in polyclad turbellarians and coelomate spir alians, which display typical quartet spiral cleavage. We generated the fir st cell-lineage fate map for an acoel flatworm, Neochildia fusca, using mod ern intracellular lineage tracers to assess the degree of similarity betwee n these distinct developmental programs. N. fusca develops via a "duet" cle avage pattern in which second cleavage occurs in a leiotropically oblique p lane relative to the animal-vegetal axis. At the four-cell stage, the plane of first cleavage corresponds to the plane of bilateral symmetry. All rema ining cleavages are symmetrical across the sagittal plane. No ectomesoderm is formed; the first three micromere duets generate only ectodermal derivat ives. Endomesoderm, including the complex assemblage of circular, longitudi nal, and oblique muscle fibers, as well as the peripheral and central paren chyma, is generated by both third duet macromeres. The cleavage pattern, fa te map, and origins of mesoderm in N. fusca share little similarity to that exhibited by other spiralians, including the Platyhelminthes (e.g., polycl ad turbellarians). These findings are considered in light of the possible e volutionary origins of the acoel duet cleavage program versus the more typi cal quartet spiral cleavage program. Finally, an understanding of the cell- lineage fate map allows us to interpret the results of earlier cell deletio n studies examining the specification of cell fates within these embryos an d reveals the existence of cell-cell inductive interactions in these embryo s. (C) 2000 Academic Press.