Phylogenomics reveals deep relationships and diversification within phylactolaemate bryozoans

Bryozoans are mostly sessile colonial invertebrates that inhabit all kinds
of aquatic ecosystems. Extant bryozoan species fall into two clades with
one of them, Phylactolaemata, being the only exclusively freshwater clade.
Phylogenetic relationships within the class Phylactolaemata have long been
controversial owing to their limited distinguishable characteristics that
reflect evolutionary relationships. Here, we present the first
phylogenomic analysis of Phylactolaemata using transcriptomic data
combined with dense taxon sampling of six families to better resolve the
interrelationships and to estimate divergence time. Using maximum
likelihood and Bayesian inference approaches, we recovered a robust
phylogeny for Phylactolaemata in which the interfamilial relationships are
fully resolved. We show Stephanellidae is the sister taxon of all other
phylactolaemates and confirm that Lophopodidae represents the second
offshoot within the phylactolaemate tree. Plumatella fruticosa clearly
falls outside Plumatellidae as previous investigations have suggested, and
instead clusters with Pectinatellidae and Cristatellidae as the sister
taxon of Fredericellidae. Our results demonstrate that cryptic speciation
is very likely in F. sultana and in two species of Plumatella (P. repens
and P. casmiana). Divergence time estimates show that Phylactolaemata
appeared at the end of the Ediacaran and started to diverge in the
Silurian, although confidence intervals were large for most nodes. The
radiation of most extant phylactolaemate families occurred mainly in the
Paleogene and Neogene highlighting post-extinction diversification.