Genomic analysis of a hybrid zone between spruce budworm species in western Canada

Hybrid zones are important “natural laboratories” that provide evolutionary biologists with a glimpse into the processes involved in the divergence of species. Hybrids between three closely related spruce budworm species whose ranges meet in the Canadian Rocky Mountains, Choristoneura fumiferana, C. occidentalis, and C. biennis, are readily obtained ex situ owing to weak reproductive barriers, but knowledge of their hybridization in nature remains limited. Here, we employ high-throughput genotyping-by-sequencing to characterize genetic variation across a three-way hybrid zone, delimit hybrid zone boundaries, and determine the extent of gene flow between these species. Contrary to expectations, distinction between C. biennis and C. occidentalis populations was obscured by low genetic differentiation among western populations. Evidence for recent hybridization between these species and C. fumiferana was scarce but the presence of advanced generation backcrosses and high migration rates, especially between C. biennis and C. occidentalis, suggests recurrent and widespread gene flow between these species.