Meeting Abstract
Flatworms are among the most diverse invertebrate phyla, with over 100,000 parasitic and free-living species. Polycladida, an order of predatory marine flatworms, exhibits direct, intermediate, and indirect development and a diversity of larval morphologies. Because life history strategies are important components of fitness, understanding the evolution of modes of development can help us comprehend the complexities of adaptation. Polycladida has traditionally been divided into two sub-orders based on, in part, the presence (Cotylea, ~350 species) or absence (Acotylea, ~450 species) of a ventral adhesive structure. Due to morphological homogeneity and insufficient molecular data, deep divergences among taxa within this order remain poorly understood. To improve phylogeny inference for this clade, we generated RNA-Seq data for 20 species of polyclads and combined these data with transcriptomes from fifteen additional in- and out-group taxa from the NCBI for phylogeneny inference. We next reconstructed ancestral life history characters among the lineages in our tree to discern where particular development modes and larval forms originated. Our results provide a well-supported preliminary hypothesis for early divergences within Polycladida, including support for the two sub-orders Cotylea and Acotylea. Further, this phylogenetic hypothesis indicates that taxa from Cotylea, and many from Acoylea, possess indirect development (although with different larval forms). However, there appears to be a transition to direct development with Acotylea. This work represents an important step in our comprehension of life history evolution among free-living flatworm clades.
