Regulative development in a spiralian embryo as shown by cell deletion experiments on the acoel, <i>Childia</i>

Abstract

Abstract Spiral cleavage and mosaicism are closely associated in the Annelida, Mollusca, and probably the polyclad Turbellaria, but the evolutionary history of these phenomena remains uncertain. The Acoela, the most primitive order of turbellarians, exhibit spiral cleavage but with the formation of duets of micromeres rather than the more typical quartets. The regulative versus mosaic capacities of the acoel, Childia groenlandica , were investigated by deleting various combinations of blastomeres from the two‐cell stage through the third duet. The acoel embryo showed extensive regulative ability since all experiments yielded normal animals except those in which both macromeres were deleted. These abnormal animals were ciliated spheres with a central cavity and indicated that the macromeres are necessary for form and organ differentiation. An inductive interaction between the micromeres and macromeres is suggested. Since mosaicism is probably specialized and a derivative of the indeterminate type of development, the regulative capacity of the acoel embryo suggests that this order lies near the ancestral line of the spiralia and represents an evolutionary stage where the very defined pattern of spiral cleavage is not yet associated with early programming of the cell's fate.