Results

Two of the three species clades (Clades I and II) possessed sufficient variation in the ITS to test whether the Alaskan populations were genetically differentiated from non-Alaskan populations. The third, Clade III, was only represented by ten individuals forming three haplotypes with minimal variation. In this species, all Alaskan specimens carried the most frequent haplotype that was shared with Asian specimens as well. The datasets consisted of 653 and 640 characters for Clade I and Clade II, respectively, including gaps.

In Clade I, the 'North American' clade, 15 haplotypes were found, of which only one, haplotype D, was found in Alaska (Tables 1 and 2). MDIV showed no evidence of gene flow (M~0) and estimated a divergence time of T~0.9 between the Alaskan and non-Alaskan populations (Fig. 1a). Therefore, in simulations using Genetree, we assumed subdivided population structure and a very low level of migration (close to 0) for the starting backward migration matrix. The coalescent-based genealogy was informative with respect to the resolution of the splitting event between the Alaskan and non-Alaskan populations and inference of the mutational history with respect to variation between and within populations (Fig. 2a).

In Clade II, the 'Eurasian' clade, 14 haplotypes were found, six of which occur in Alaska (Tables 1-2). Of these six, five were found only in Alaska (haplotypes C, D, G, H, I), while one (haplotype A) represented the most abundant haplotype in the sample, spanning Eurasia and Alaska. It must be noted, that haplotype D (or subclade II/A in Geml et al. 2006), a lineage endemic to the maritime rainforests of the Pacific Northwest, was considered 'Alaskan' as opposed to 'Eurasian', despite not being strictly restricted by the political boundaries and occurring as far South as Northwestern Washington state. Apart from this, no other Clade II haplotypes have been found in North America, outside Alaska. Despite the very similar values for Theta (Fig. 1), the population structure in Clade II proved to be very different from

Table 1 (continued)

Haplotypes

Specimen

Geographic origin

GAL3643 RET149-1 RET149-2 N8

GAL18122 GAL2814 GAL4810 31452

Glacier Hwy, mile 30, Juneau, Alaska, USA Southern Highlands Prov., Tanzania Southern Highlands Prov., Tanzania Magadan district, Magadan region, Russia Bonanza Creek LTER site, Alaska, USA Dalton Hwy, mile 122, Alaska, USA Denali National Park, Alaska, USA Hampshire, England, UK Gdansk, Poland Baden-Württemberg, Germany Bavaria, Germany Aomori-shi, Aomori, Japan Queenstown, New Zealand Ola district, Magadan region, Russia Tenka district, Magadan region, Russia

30965

RET152-6 RET152-8 30961

30987

that of Clade I discussed above. MDIV showed evidence for intermediate levels of gene flow (M~2.5) between Alaskan and Eurasian populations and estimated no population divergence (7~0) (Fig. 1b). Our analyses using MIGRATE suggested that migration between Alaska and Eurasia has been asymmetrically bidirectional with Alaska receiving more migrants (M12=9,482.98 and M21=33.53; M12 referring to the migration from Eurasia to Alaska, and M21 vice versa). Therefore, in simulations using Genetree, we assumed panmictic population structure. As expected, the coalescent-based genealogy did not show any historical population division (Fig. 2b). However, based on the fact that the specimens from the maritime rainforest ecoregion of the Pacific Northwest of North America formed a well-supported sister group (II/A) to the rest of Clade II in our previous multi-locus phylogenetic analyses (Geml et al. 2006, 2008), we tested in a separate analysis whether or not there was any gene flow between this group inhabiting a special habitat with a restricted distribution and the rest of Clade II, this latter including boreal Alaskan and Eurasian specimens. In this case, MDIV showed no migration (M~0) and estimated a divergence time of 7~1.5 between the two populations, i.e. subclade II/A vs. Clade II (Fig. 1c). Nonetheless, the coalescent-based genealogy estimated under an assumption of population subdivision did not differ significantly from the one generated assuming panmixia (Fig. 2c).

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