Recent speciation and hybridization in Icelandic deep-sea isopods: An integrative approach using genomics and proteomics.

The crustacean marine isopod species Haploniscus bicuspis (G.O. Sars,
1877) shows circum-Icelandic distribution in a wide range of environmental
conditions and along well-known geographic barriers, such as the
Greenland-Iceland-Faroe (GIF) Ridge. We wanted to explore population
genetics, phylogeography and cryptic speciation as well as to investigate
whether previously described, but unaccepted subspecies have any merit.
Using the same set of specimens, we combined mitochondrial COI sequences,
thousands of nuclear loci (ddRAD), and proteomic profiles, plus selected
morphological characters using Confocal Laser Scanning Microscopy (CLSM).
Five divergent genetic lineages were identified by COI and ddRAD, two
south and three north of the GIF Ridge. Assignment of populations to the
three northern lineages varied and detailed analyses revealed
hybridization and gene flow between them, suggesting a single northern
species with a complex phylogeographic history. No apparent hybridization
was observed among lineages south of the Ridge, inferring the existence of
two more species. Differences in proteomic profiles between the three
putative species were minimal, implying an ongoing or recent speciation
process. Population differentiation was high, even among closely
associated populations, and higher in mitochondrial COI than nuclear ddRAD
loci. Gene flow is apparently male-biased, leading to hybrid zones and
instances of complete exchange of the local nuclear genome through
immigrating males. This study did not confirm the existence of subspecies
defined by male characters, which probably characterize different male
developmental stages.