Harmothoe imbricata: species complex or complex species?

Abstract

Accurate estimates of species diversity are constrained by cryptic species complexes, in which multiple closely related species are grouped under a single species name due to the absence of clear morphological differences. Cryptic diversity is known to be prevalent in polychaete worms, a mostly marine group commonly known as bristle worms. A recent survey of polychaete diversity discovered that the widespread scale-worm Harmothoe imbricata comprises multiple distinct mitochondrial lineages based on analysis of the Cytochrome c oxidase I (COI) gene, which is often referred to as the 'barcoding' gene. Analyses based solely on DNA sequences from COI may overestimate the number of lineages comprising a cryptic species complex, so it has been recommended that cryptic species investigations incorporate nuclear gene sequences. The goal of this study was to determine whether the incorporation of DNA sequences from the nuclear genome corroborates the designation of H. imbricata as a cryptic species complex. I sequenced segments of COI and five nuclear genes: ITS1, ITS2, H3, and portions of the 18S and 28S genes of H. imbricata and analyzed them using distance measures, maximum likelihood, and Bayesian inference. I compared phylogenetic trees produced from mitochondrial and nuclear DNA sequences, as well as from a combined mitochondrial/nuclear dataset. Harmothoe imbricata was found to include five mitochondrial lineages, whereas the nuclear sequences only supported four well-defined lineages. These results corroborate previous reports showing COIbased cryptic species investigations find more lineages than nuclear DNA based investigations. These results provide additional lines of evidence that H. imbricata is a cryptic species complex. These divergent lineages likely arose after being separated during the last glacial maximum but they are now found in sympatry. A thorough morphological study of H. imbricata populations may reveal phenotypic differences correlated with the genetic lineages identified here.