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Origin and diversification of the cryptic ant genus Stenamma Westwood (Hymenoptera: Formicidae), inferred from multilocus molecular data, biogeography and natural history

Authors


Michael G. Branstetter, Department of Entomology, One Shields Ave., University of California, Davis, CA 95616, U.S.A. E-mail: mgbranstetter@ucdavis.edu

Abstract

The genus Stenamma Westwood comprises a group of cryptic, cold tolerant ants that occur throughout the Holarctic and Middle American regions. Traditional approaches to taxonomy and phylogeny are confounded by multiple factors, including the conservative and often convergent morphology of workers and the rarity of reproductive castes in collections. Monophyly of Stenamma and relationships within the genus are uncertain as nearly all previous taxonomic work has been regional in scope. Furthermore, the sister group to Stenamma has not been well established. Here an extensive molecular dataset consisting of ten genes (∼8 kb of data), 48 ingroup taxa (20 Nearctic, 6 Palaearctic and 22 Neotropical) and 8 outgroup taxa (6 closely related non-Stenamma and 2 additional myrmicines) is used to investigate the broad-scale phylogeny and evolutionary history of Stenamma. Phylogenetic analysis is performed under maximum likelihood and Bayesian frameworks on individual genes and several alternate concatenated datasets, which are used to investigate the effects of inclusion or exclusion of COI and intronic regions. The timing of Stenamma evolution is inferred in beast and ancestral areas are reconstructed using both the s-diva and DEC methods, as implemented in the programs rasp and lagrange, respectively. Stenamma is revealed as monophyletic with high support and tentatively is sister to a group of New World species placed currently in Aphaenogaster Mayr and Messor Forel. Within Stenamma, two major clades are recovered: a ‘Holarctic clade’ (HOC) and a ‘Middle American clade’ (MAC). The HOC consists of the European S. striatulum Emery sister to two well-supported groups, the informal ‘debile’ and ‘brevicorne’ clades. The ‘brevicorne’ clade is entirely Nearctic, whereas the ‘debile’ clade includes both Nearctic and Palaearctic representatives. The MAC occurs from the southern United States to northern South America and, with the exception of S. huachucanum Smith, is almost completely isolated geographically from the HOC. It includes a depauperate northern clade and the ‘MAC core’, which is a diverse assemblage of wet forest adapted species found throughout Central America. Divergence dating and biogeographic reconstruction show that Stenamma is most likely to have originated in the Nearctic at the Eocene–Oligocene boundary (∼35 Ma) and diversified more rapidly at 16 and 8 Ma for the HOC and MAC, respectively. Potential environmental factors affecting the evolution of Stenamma include the intense global cooling of the late Eocene combined with aridification and mountain building. The phylogenetic results are discussed in relation to the current Stenamma species groups and several new morphological characters are presented to help in identification.

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