Abstract
This study gathered evidence from principal component analysis (PCA) of morphometric data and molecular analyses of nucleotide sequence data for four nuclear genes (28S, TpI, CAD1, and Wg) and two mitochondrial genes (COI and 16S), using parsimony, maximum likelihood, and Bayesian methods. This evidence was combined with morphological and chorological data to re-evaluate the taxonomic status of Nebria lacustris Casey sensu lato. PCA demonstrated that both body size and one conspicuous aspect of pronotal shape vary simultaneously with elevation, latitude, and longitude and served to distinguish populations from the southern Appalachian highlands, south of the French Broad, from all other populations. Molecular analyses revealed surprisingly low overall genetic diversity within Nebria lacustris sensu lato, with only 0.39% of 4605 bp varied in the concatenated dataset. Evaluation of patterns observed in morphological and genetic variation and distribution led to the following taxonomic conclusions: (1) Nebria lacustris Casey and Nebria bellorum Kavanaugh should be considered distinct species, which is a NEW STATUS for Nebria bellorum. (2) No other distinct taxonomic subunits could be distinguished with the evidence at hand, but samples from northeastern Iowa, in part of the region known as the “Driftless Zone”, have unique genetic markers for two genes that hint at descent from a local population surviving at least the last glacial advance. (3) No morphometric or molecular evidence supports taxonomic distinction between lowland populations on the shores of Lake Champlain and upland populations in the adjacent Green Mountains of Vermont, despite evident size and pronotal shape differences between many of their members.
Highlights
IntroductionHe included these two species, along with Nebria pallipes Say (1823: 78), in his “Group pallipes”
Thomas Lincoln Casey described Nebria lacustis in 1913, based on specimens from Wisconsin and Minnesota
Subtle individual and interpopulational differences are apparent in the length, width, and shape of the anterior pronotal angles, the shape of the posterior pronotal angles, and in the width of the lateral explanation of the pronotum, but we have found no clear pattern to this variation
Summary
He included these two species, along with Nebria pallipes Say (1823: 78), in his “Group pallipes” He distinguished N. lacustris from N. expansa on the basis of overall size (N. lacustris adults being smaller than those of N. expansa), relative size of the pronotum (which he described as larger and wider, especially anteriorly, in N. expansa adults than in N. lacustris), and distinctness of the punctures of the elytral striae (which he described as more deeply and distinctly punctate in N. lacustris than N. expansa). Bell (1955) suggested that Bänninger’s omission indicated that “he considered expansa to be a synonym” of N. lacustris and added that, based on his review of “the materials in the United States National Museum, including the types of both species, I believe that expansa is at most a poorly defined geographic race of lacustris.” He further suggested that “typical lacustris is from more northern localities, but typical expansa and intermediates are represented from both southern and northern localities Bänninger (1925) listed N. lacustris in his treatment of the Nebriini but did not mention N. expansa. Bell (1955) suggested that Bänninger’s omission indicated that “he considered expansa to be a synonym” of N. lacustris and added that, based on his review of “the materials in the United States National Museum, including the types of both species, I believe that expansa is at most a poorly defined geographic race of lacustris.” He further suggested that “typical lacustris is from more northern localities, but typical expansa and intermediates are represented from both southern and northern localities
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