A phylogenomic approach to resolving interrelationships of polyclad flatworms, with implications for life history evolution

Platyhelminthes (flatworms) are a diverse invertebrate phylum that are
useful for exploring life history evolution. Within Platyhelminthes, only
two clades develop through a larval stage: free-living polyclads and
parasitic neodermatans. Neodermatan larvae are considered evolutionarily
derived, whereas polyclad larvae are hypothesized to be retained from the
last common ancestor of Platyhelminthes – and Spiralia – due to ciliary
band similarities among polyclad and other spiralian larvae. However,
larval evolution has been challenging to investigate within polyclads due
to low support for deeper phylogenetic relationships. To investigate
polyclad life history evolution, we generated transcriptomic data for 21
species of polyclads to build a well-supported phylogeny for the group. We
then used ancestral state reconstruction to investigate ancestral modes of
development (direct vs indirect) within Polycladida, and flatworms in
general. The resulting tree provides strong support for deeper nodes and
we recover a new monophyletic clade of early branching cotyleans. Early
branching clades of acotyleans and cotyleans possess diverse modes of
development, suggesting a complex history of larval evolution in polyclads
that likely includes multiple losses and/or multiple gains. Our ancestral
state reconstructions in previous platyhelminth phylogenies also suggest
that larval similarities between flatworms and other phyla are most likely
convergently evolved.