Molecular reevaluation of the Nebria gregaria infragroup and the implications for the existence of an ice age refugium on the Queen Charlotte Islands
Date
1998
Authors
Clarke, Thomas E.
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Abstract
Discussion on the origins of many of the endemic and disjunct organisms on the north western coast of North America have centered on the role of the Queen Charlotte Islands as a refugium during the Fraser glaciation. Extended periods of isolation on the archipelago has been proposed as an explanation for the morphologically divergent island varieties of mammals, birds, insects, and plants. However, many of the endemic subspecies are differentiated from the mainland forms on the basis of small morphological changes that may instead be the result of rapid post-glacial evolution. One group of animals that has been used as evidence for the existence of a glacial refugium is the Nebria gregaria infragroup. Two members of this species group, N. charlottae and N. louisea, are restricted to cobble beaches on the Queen Charlotte Islands. A third, N. haida, is found only in alpine regions of the archipelago and the adjacent mainland. The remaining two species of the gregaria infragroup, N. lituyea and N. gregaria, show highly restricted distributions in the mountains of the Alaskan panhandle and on the beaches of the Aleutian Islands, respectively. Because of the morphological similarity among the five species, a morphometric study was performed on body length and pronotal shape measurements in order to determine if each species was distinct. The results of a discriminant analysis on the measurements indicate instead that the five species grade into each other, forming a morphological continuum. To determine the relationships of the five species, a phylogenetic analysis was conducted on 1856 bp from five regions of the mitochondrial DNA, comprising the NADH subunit 1 gene, the cytochrome oxidase subunit 1 gene, the cytochrome B gene, and two regions of the cytochrome oxidase II gene. The results of the phylogenetic analysis indicated that one species, N. lituyea, did not belong in the gregaria infragroup and that a very close relationship existed among the four remaining species. Only seven mutations separated the two most divergent species, N. louisea and N. haida. To further explore the relationships of the three Queen Charlotte Island members of the infragroup, a neighbour-joining tree was constructed from the DNA fingerprint patterns of N haida, N. charlottae, and N. louisea. Genetic diversity was highest in the N haida populations and lowest in N. charlottae and the eastern populations of N. louisea. In addition, the tree indicated that the latter two populations were sisters, derived from the western N. louisea populations, which were in tum derived from N. haida. The molecular data indicate that the morphological differences between the three Queen Charlotte species may be post-glacial in origin and that together with N gregaria, these four should be considered as local variations of a single species. The implications for a glacial refugium on the Northwest coast are reevaluated in light of these new findings.