Patch size of gall-forming aphids: deme formation revisited

Plants of the genus Pistacia (Anacardiaceae) serve as obligate hosts for a group of specialized gall-forming aphids (Homoptera: Fordinae). The aphids regularly migrate between the Pistacia (primary) host plants and the roots of non-specific grasses and cereals (secondary hosts). Gall density varies considerably between trees and sites. The intimate relationships between the aphids and their primary host, the natural variation of host susceptibility, and the heterogeneous geographical environment may promote local adaptation and deme formation in the aphid populations. Indeed, previous analyses of the genetic structure of the aphid Baizongia pistaciae, which forms large galls on the deciduous P. palaestina trees, suggested deme formation (Martinez et al. 2005). In this study, we analyzed the genetic structure of the B. pistaciae population at eight sites and 78 trees throughout Israel and a single population in Turkey, using two molecular markers (AFLP fingerprints and COI sequencing). The genetic distance between the Israeli populations was found to be low (D = 0.01-0.02), and there was no genetic differentiation found between any population pairs. In five of the Israeli populations, we also compared the genetic identity between aphids forming galls on the same tree and between galls on neighboring trees. The analysis indicated that the genetic identity of different galls within a tree resembles the correspondence between trees within a population. Our results showed no indication of deme formation or any hierarchical genetic substructuring within B. pistaciae populations in Israel. The extensive gene flow between aphid colonies may be explained by their dispersal abilities and the potential bridging role of the secondary hosts.