Inclusive taxon sampling suggests a single, stepwise origin of ectolecithality in Platyhelminthes

Ectolecithality is a form of oogenesis unique within Metazoa but common in Platyhelminthes, in which nearly-yolkless oocytes and tightly associated yolk cells are deposited together in egg capsules. Despite profound impacts on the embryogenesis and morphology of its beneficiaries, the origins of this developmental phenomenon remain obscure. Traditionally, all ectolecithal flatworms were grouped in a clade called Neoophora. However, there are also morphological arguments for multiple origins of ectolecithality, and Neoophora has to date seen little support from molecular phylogenetic research, largely due to gaps in taxon sampling. Accordingly, we present a molecular phylogeny focused on resolving the deepest divergences among the free-living Platyhelminthes. Species were chosen to completely span the diversity of all major endo- and ectolecithal clades, including several aberrant species of uncertain systematic affinity, and additionally a thorough sampling of the “lecithoepitheliate” h...