A single, stepwise origin of ectolecithality in Platyhelminthes

Meeting Abstract

64.5  Friday, Jan. 6  A single, stepwise origin of ectolecithality in Platyhelminthes? LAUMER, C.E.*; GIRIBET, G.; Harvard Museum of Comparative Zoology; Harvard Museum of Comparative Zoology claumer@oeb.harvard.edu

Ectolecithality is a process of functionally and (usually) spatially divided oogenesis that effects distinct populations of vitelline cells and nearly-yolkless germ cells. The origin of ectolecithality may have had pervasive impacts on the embryology and diversification of its beneficiaries, but its evolutionary roots remain obscure. Early classifications group all ectolecithal flatworms in a single clade, Neoophora. However, some have proposed multiple origins of the phenomenon and/or reversals to an entolecithal state, and Neoophora has so far seen no molecular support. We address this quandary using perhaps the largest molecular phylogenetic study of deep Platyhelminth relationships to date, with nearly complete 18S and 28S rRNA and two mtDNA fragments from 83 terminals. Taxon sampling was directed to span the diversity of each major group, including several “Problematica”, as well as the enigmatic Lecithoepitheliata, hitherto underrepresented in molecular phylogenies. Conventional analytical methods consistently validate the monophyly of nearly all the classical orders, and largely resolve a clade possessing spatially distinct ovaria and vitellaria. In addition, implied-weights parsimony as well as Bayesian analyses under a mixture model both strongly support the monophyly of Neoophora. These data thus corroborate the classical view of lecithoepitheliate germovitellaria as preserving an early step on the road to complete ectolecithality. Certain questions, however, including the identity of the earliest-branching Rhabditophora and the monophyly of Lecithoepitheliata, remain open. We propose that the phylogenetic utility of familiar markers to solve such persistent problems has effectively been exhausted, and that future work must embrace a different paradigm of gene sampling in order to resolve the deepest splits in Platyhelminthes.

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