Figure 1. Polyclad flatworms are dorsoventrally flattened as adults, some species develop directly into this body plan, others develop indirectly through larval forms with transient features such as lobes and ciliary tufts and bands. (a) Adult polyclad, Yungia sp. (b–d) Hatchlings of direct developing species; Euplana gracilis (b), Notocomplana sp. (c) and Echinoplana celerrima (d). (e–k) Hatchlings of indirect developing taxa; newly hatched Müller's larva of Cycloporus gabriellae, (e), newly hatched Müller's larva of Prosthecereaus crozieri (f); Prosthiostomum acroporae shows ‘intermediate development’ with the embryo developing eight larval lobes inside the egg capsule (g (i)), but most individuals undergo intra-capsular metamorphosis (g (ii)); Müller's larva of Prosthiostomum siphunculus (h), four-lobed Goette's larva of Stylochus ellipticus (i), ventral (j (i)) and dorsal (j (ii)) view of 10-lobed larva collected from the plankton, species unknown; and lateral view of 10-lobed larva collected from the plankton (k). B-K scale 50 µm.
I, along with my colleagues Kate Rawlinson and Bernhard Egger and my former advisors Michael Cummings and Allen Collins, published a new paper on the phylogenomics of polyclad flatworms and larval evolution in the clade in Royal Society Open Science (https://royalsocietypublishing.org/doi/full/10.1098/rsos.220939#d1e2064)! It took a bit of time to finish (7 years!), but we're excited to see it out.
Our transcriptome phylogeny provides much stronger support for deeper nodes than previous single- or multi-gene trees, and we recover a new monophyletic clade of early branching cotyleans. We then used ancestral state reconstructions to investigate ancestral modes of development within Polycladida and more broadly within flatworms. In polyclads, we were unable to reconstruct the ancestral state of deeper nodes with significant support because early branching clades show diverse modes of development. This suggests a complex history of larval evolution in polyclads that likely includes multiple losses and/or multiple gains. Our ancestral state reconstruction across a previously published platyhelminth phylogeny does support a direct developing prorhynchid/polyclad ancestor though. This suggests that a larval stage in the life cycle evolved along the polyclad stem lineage or within polyclads.