Herbivore host-associated genetic differentiation depends on the scale of plant genetic variation examined

Herbivore adaptation to plant genetic variation can lead to reproductive isolation and the formation of host-associated lineages (host-associated differentiation, or HAD). Plant genetic variation exists along a scale, ranging from variation among individual plant genotypes to variation among plant species. Along this scale, herbivores may adapt and diverge at any level, yet few studies have examined whether herbivore differentiation exhibits scaling with respect to host variation (e.g., from genotypes to species). Determining at which level(s) herbivore differentiation occurs can provide insight into the importance of plant genetic variation on herbivore evolution. Previous studies have found strong genetic differentiation in the eriophyid mite, Aceria parapopuli, between hybrid Populus hosts and parental Populus species, but minimal neutral-locus differentiation among individual trees of the same species. We tested whether genetic differentiation in A. parapopuli scales with genetic variation in its Populus hosts. Using mite ITS1 sequence data collected among host species and among host populations, two key patterns emerged. (1) We found strong differentiation of A. parapopuli among Populus species, supporting the hypothesis that plant species differences drive reproductive isolation and HAD. (2) We did not find evidence of host-driven genetic differentiation in mites at the level of plant populations, suggesting that this level of plant variation is insufficiently strong to drive differentiation at a neutral locus. In combination with previous studies, we found that HAD occurs at the higher levels of plant genetic variation, but not at lower levels, and conclude that HAD depends on the scale of plant genetic variation examined.