New Molecular Tools for Dendroctonus frontalis (Coleoptera: Curculionidae: Scolytinae) Reveal an East–West Genetic Subdivision of Early Pleistocene Origin

Abstract

Southern pine beetle, Dendroctonus frontalis Zimmermann, is a major native pest of pine trees in the southeastern United States, Mexico, and Central America. The species' range has recently expanded north for the first time in recorded history. Accordingly, information about the timing of population divergence and past geographic range occupancy may provide an important yardstick for understanding rapid range expansions. Using 16 new and eight existing microsatellite loci, together with mitochondrial (COI) and nuclear (EF-1α and 28S) DNA sequence data, we characterized broad-scale patterns of genetic variation in D. frontalis, and estimated divergence times for the entire D. frontalis species complex. Molecular dating suggested a middle Miocene (ca. 12 million years ago [Mya]) origin of the D. frontalis species complex, and an early Pleistocene (ca. 2.2 Mya) divergence between eastern and western D. frontalis populations that are separated by the Tamaulipan mezquital ecoregion in southern Texas and northeastern Mexico. In the western D. frontalis group, there was additional differentiation between populations from Michoacan versus Arizona, suggesting that additional genetic structure could be uncovered in this region. In the eastern group, there was high genetic diversity, but little structure. There was no pattern of isolation by distance, and only weak population differentiation that distinguished populations from Georgia and Florida from the other eastern populations. Overall, our results suggest that eastern D. frontalis originated as a distinct group well before the last glacial period, but additional markers may be necessary to fully describe its contemporary rapid range expansion.