Strong Differentiation in Mitochondrial Dna of Dendroctonus brevicomis (Coleoptera: Scolytidae) on Different Subspecies of Ponderosa Pine

Earlier phylogenetic analysis of the bark beetle genus Dendroctonus based on mitochondrial DNA (mtDNA) uncovered large genetic distances between 2 populations of Dendroctonus brevicomis, 1 in California and 1 in Colorado. The genetic distance between the 2 populations was equivalent to that observed between fully recognized sister species of Dendroctonus and suggested the presence of 2 cryptic species. To test this hypothesis, we sampled D. brevicomis from 12 populations across its range. Using polymerase chain reaction, we amplified a 1,250-bp region of the mtDNA gene cytochrome oxidase I (COX I). A restriction site survey using 8 enzymes revealed 8 fixed differences between western (California, Oregon, Idaho, and British Columbia) and eastern (Colorado, Utah, Arizona, New Mexico) populations, suggesting that D. brevicomis is composed of 2 cryptic species. Moreover, the distribution of these 2 groups corresponds with the distributions of Pinus ponderosa variety scopulorum and variety ponderosa, the principal hosts of D. brevicomis over the majority of its range. We suggest that these populations of D. brevicomis may have become reproductively isolated as a consequence of the geographic separation of the host varieties.