WHAT STUDIES OF TURBELLARIAN EMBRYOS CAN TELL US ABOUT THE EVOLUTION OF DEVELOPMENTAL MECHANISMS

In spiralian embyros determination of the axes of bilateral symmetry i s associated with D quadrant specification. This can occur late throug h equal cleavage and cell interactions (conditional specification) or by the four-cell stage through unequal cleavage and cytoplasmic locali zation (autonomous specification). Freeman & Lundelius (1992) suggest that in spiralian coelomates the former method is ancestral and the la tter derived, with evolutionary pressure to shorten metamorphosis resu lting in early D quadrant determination through unequal cleavage and a ppearance of adult features in the larvae. Because of the key phylogen etic position of the turbellarian platyhelminthes, understanding the m ethod of axis specification in this group is important in evaluating t he hypothesis. Polyclad development, with equal quartet spiral cleavag e, is believed to represent the most primitive condition among living turbellarians and has been examined experimentally in Hoploplana inqui lina. Blastomere deletions at the two and four-cell stage produce larv ae that are abnormal in morphology and symmetry, indicating that early development is not regulative, and also establish that the embryo doe s not have an invariant cell lineage. Deletions of micromeres and macr omeres at the eight-cell stage indicate that cell interactions are inv olved in dorso-ventral axis determination, with cross-furrow macromere s playing a more significant role than non-cross-furrow cells. The res ults support the idea that conditional specification is the primitive developmental mode that characterized the common ancestor of the turbe llarians and spiralian coelomates. Evolutionary trends in development in polyclads and other turbellarian orders are discussed.