The genetic architecture of pheromone production between populations distant from the hybrid zone of the pine engraver, Ips pini

Across North America, populations of the pine engraver, Ips pini, differ in their expressed ratios of the two enantiomers of ipsdienol, the main component of its aggregation pheromone. We confirm previous studies, showing that the percentage of (+)-ipsdienol ranged from approximately 40 to 70% for New York (NY) males and less then 5% for California (CA) males. We performed line crosses including the F1, F2, and backcross generations between these populations. These line crosses showed that most F1 hybrids produced an intermediate enantiomeric blend of ipsdienol that was closer to the CA blend, with a frequency distribution peak near 15% (+)-ipsdienol. There was also strong segregation to either parental type in the F2 and backcross generations, but not in a pattern that could be clearly described by a single autosomal locus. There was also an X-linked effect that caused some individuals in the F1 to have phenotypes more characteristic of NY populations. Generation means analysis confirms this X-linked effect, and also suggests a complicated autosomal dominance by dominance epistatic interaction. Despite the appearance of segregation, these results suggest a more complicated system of pheromone blend control than the single major gene previously found between divergent lines for high and low (+)-ipsdienol blends from the hybrid zone in British Columbia.