Host-associated genetic differentiation in pecan leaf phylloxera

Host-associated differentiation (HAD) is the formation of genetically distinct host-associated populations. One of the genotypic signatures of HAD is that populations exhibit stronger differentiation by host-plant species than by geographic isolation. HAD, as a mechanism promoting ecological speciation, has been invoked to explain phytophagous insect diversity. Two traits proposed to promote HAD are endophagy and parthenogenesis. Using amplified fragment length polymorphisms (AFLPs), we tested for the presence of HAD in pecan leaf phylloxera, Phylloxera notabilis Pergande (Hemiptera: Phylloxeridae), an endophagous, gall inducing, and cyclically parthenogenetic insect on sympatric pecan and water hickory at a geographic mesoscale. This species shows strong HAD. Whereas the effect of collecting site was significant, accounting for 7.3% of molecular variation, host-plant species identity accounted for 63.5%. In addition, a choice test indicated that pecan leaf phylloxera originating from water hickory showed weak but significant preference for leaflets of the natal host, whereas pecan leaf phylloxera originating from pecan did not. This is the first such study of a species of arboreal Phylloxeridae, a poorly known insect group. This is also the first endophage and the second parthenogen shared by these two hickory species to show evidence of HAD. This hickory system could be a good parthenogen-rich counterpoint to the goldenrod system in the study of HAD in insect communities.