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The 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 present in all species.
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