A new molecular phylogeny of salps (Tunicata: Thalicea: Salpida) and the evolutionary history of their colonial architecture

Salps are marine pelagic urochordates with a complex life cycle including a solitary and colonial stage composed of asexually-budded individuals. These colonies develop into species-specific architectures with distinct zooid orientations, including transversal, oblique, linear, helical, and bipinnate chains; as well as whorls, and clusters. The evolutionary history of salp colony architecture has remained obscured due to the lack of a homology-based ontology to characterize architectures, as well as a lack of phylogenetic taxon sampling and resolution of critical nodes. We (1) collected and first-time sequenced eight species of salps, (2) inferred the phylogenetic relationships among salps, and (3) reconstructed the evolutionary history of salp colony architecture. We collected salp specimens via offshore SCUBA diving, dissected tissue samples, extracted their DNA, amplified their 18S gene, and sequenced them using Sanger technology. We inferred a new molecular phylogeny using both Maximum Likelihood and Bayesian approaches. Using this phylogeny, we reconstructed the ancestral states of colony architecture using a Bayesian ordered Markov model informed by the presence and absence of specific developmental mechanisms that lead to each architecture. We find that the ancestral salp architecture is either oblique or linear, with every other state being derived. Moreover, linear chains have evolved independently at least three times. While transversal chains are developmentally basal and hypothesized to be ancestral, our phylogenetic topology and reconstructions strongly indicate that they are evolutionarily derived through the loss of zooid torsion. These traits are likely critical to multijet locomotory performance and evolving under natural selection. Our work showcases the need to study the broader diversity of salp species in order to gain a comprehensive understanding of their organismal biology, evolutionary history, and ecological roles in pelagic ecosystems.