Incomplete barriers to mitochondrial gene flow between pheromone races of the North American pine engraver, Ips pini (Say) (Coleoptera: Scolytidae)

The pine engraver Ips pini (Say) is known to include three pheromone races, but gene flow between these races has not been investigated. We used maternally inherited mitochondrial DNA (mtDNA) variation to infer gene flow between 22 widely distributed North American populations of I. pini for a total of 217 individuals, based on 354 bp of the cytochrome oxidase I gene. Gene how was estimated cladistically as migrants per generation (Nm) and as haplotype variation between populations (N-st). Three distinct mtDNA haplotype lineages, generally corresponding to eastern (I), Rocky Mountain (II) and western (III) regions of North America, were resolved with a total of 34 distinct I. pini haplotypes. The distributions of these lineages were largely congruent with the geographical ranges of the 'New York', 'California' and 'Idaho-Montana' pheromone races. Only individuals with lineage I mtDNA were observed among eastern populations, whereas individuals with lineage II or III mtDNA predominated among western populations. Gene flow (Nm and N-st) was generally moderate between all populations. However, the presence of lineage I mtDNA on the eastern side of western North America and the absence of lineage II and III mtDNA in eastern North America suggest directional gene flow from cast to west. These results indicate that female-controlled assortative mating among pheromone races may disrupt gene flow between conspecifics, reflecting incomplete pre-mating barriers.