Insights into the species evolution of Calanus copepods in the northern seas revealed by de novo transcriptome sequencing

Copepods of the zooplankton genus Calanus play a key role in marine ecosystems in the northern seas. Although being among the most studied organisms on Earth, due to their ecological importance, genomic resources for Calanus spp. remain scarce, mostly due to their large genome size (from 6 to 12 Gbps). As an alternative to whole‐genome sequencing in Calanus spp., we sequenced and de novo assembled transcriptomes of five Calanus species: Calanus glacialis, C. hyperboreus, C. marshallae, C. pacificus, and C. helgolandicus. Functional assignment of protein families based on clusters of orthologous genes (COG) and gene ontology (GO) annotations showed analogous patterns of protein functions across species. Phylogenetic analyses using maximum likelihood (ML) of 191 protein‐coding genes mined from RNA‐seq data fully resolved evolutionary relationships among seven Calanus species investigated (five species sequenced for this study and two species with published datasets), with gene and site concordance factors showing that 109 out of 191 protein‐coding genes support a separation between three groups: the C. finmarchicus group (including C. finmarchicus, C. glacialis, and C. marshallae), the C. helgolandicus group (including C. helgolandicus, C. sinicus, and C. pacificus) and the monophyletic C. hyperboreus group. The tree topology obtained in ML analyses was similar to a previously proposed phylogeny based on morphological criteria and cleared certain ambiguities from past studies on evolutionary relationships among Calanus species.