Independent phylogenetic origins of Xyleborus species (Coleoptera: Curculionidae: Scolytinae) native to the Nearctic

Xyleborini (Coleoptera: Curculionidae: Scolytinae) ambrosia beetles occur worldwide and are most abundant in tropical regions. Extreme inbreeding and the beetles’ symbiotic fungi make them well-suited for long-distance colonisation of new areas, with single pioneer females capable of creating new populations. Xyleborine colonisation events have been addressed for tropical locations but not for temperate regions. In this study, we focus on the seven endemic Nearctic Xyleborus Eichhoff species and test the hypothesis that these species are monophyletic. We reconstructed multigene phylogenies using Bayesian and parsimony analyses for these species and for a sample of Xyleborini species with worldwide distributions. Both analyses demonstrate that the Nearctic Xyleborus species are collectively polyphyletic and occur in four lineages: (1) X. celsus Eichhoff, (2) X. xylographus (Say), (3) X. planicollis Zimmermann + X. viduus Eichhoff + X. impressus Eichhoff, and (4) X. intrusus Blandford + X. pubescens Zimmermann. Three of these clades likely derived from Eastern Palearctic–Oriental or Nearctic species, whereas X. celsus ’s origin is ambiguous. Two clades, ( X. impressus + X. planicollis + X. viduus ) and ( X. intrusus + X. pubescens ), have radiations in the Nearctic that are limited compared to those of Xyleborini radiations in the tropics. Factors contributing to the low species diversity are discussed.