Molecular divergence and gene flow in cold ocean tunicates: insights from cytochrome oxidase 1

Gaps in our ability to identify species and understand factors influencing the evolution of species ranges, limit the extent to which we can diagnose changes to ocean ecosystems that occur in response to anthropogenic stressors. DNA barcodes offer one method to facilitate our assessment and understanding of diversity. Tunicates are a group of marine invertebrates for which researchers have used the typical animal barcoding region of the cytochrome oxidase 1 (CO1) gene to identify cryptic species complexes and elucidate patterns of gene flow in non-indigenous ascidians. Waters surrounding Newfoundland contain several species of invasive ascidians and native appendicularian tunicates. My research utilized the CO1 animal barcoding region to explore the genetic structure of two cold ocean tunicates in Newfoundland waters, the native appendicularian tunicate, Oikopleura vanhoeffeni and the non-native ascidian Botryllus schlosseri. Specifically, I provide the first report of a partial CO1 sequence for O. vanhoeffeni and report on gene flow of B. schlosseri collected from Newfoundland waters. The CO1 gene proved unsuitable for further evaluation as a marker for species level identification in O. vanhoeffeni, because the presence of tandem repeat regions would require multiple primer pairs to amplify the CO1 barcoding region. Across Newfoundland harbours, considerable genetic structure among the 7 Botryllus schlosseri populations sampled indicates increasing genetic diversity from west to east. All of the B. schlosseri CO1 haplotypes identified in Newfoundland waters were recorded previously in Nova Scotian harbours, and a few in the Mediterranean Sea. Thus, regional boating traffic from mainland Atlantic Canada likely provided an important vector of introduction for this species in Newfoundland, but transoceanic and/or local boating traffic may play a role