Cospeciation between bacterial endosymbionts (Buchnera) and a recent radiation of aphids (Uroleucon) and pitfalls of testing for phylogenetic congruence

Previous studies of phylogenetic congruence between aphids and their symbio tic bacteria (Buchnera) supported long-term vertical transmission of symbio nts. However, those studies were based on distantly related aphids and woul d not have revealed horizontal transfer of symbionts among closely related hosts. Aphid species of the genus Uroleucon are closely related phylogeneti cally and overlap in geographic ranges, habitats, and parasitoids. To exami ne support for congruence of phylogenies of Buchnera and Uroleucon, sequenc es from four mitochondrial, one nuclear, and one endosymbiont gene (trpB) w ere obtained. Congruence of phylogenies based on pooled aphid genes with ph ylogenies based on trpB was highly significant: Most nodes resolved by trpB corresponded to nodes resolved by the pooled aphid genes. Furthermore, no nodes were both inconsistent between the trees and strongly supported in bo th trees. Two kinds of analyses testing the null hypothesis of perfect cong ruence between pairwise combinations of datasets and tree topologies were p erformed: the Kishino-Hasegawa test and the likelihood-ratio test. Both tes ts indicated significant disagreement among most pairwise combinations of m itochondrial, nuclear, and symbiont datasets. Because rampant recombination among mitochondrial genomes of different aphid species is unlikely, inaccu rate assumptions in the evolutionary models underlying these tests appear t o be causing the hypothesis of a shared history to be incorrectly rejected. Moreover, trpB was more consistent with the aphid genes as a set than any single aphid gene was with the others, suggesting that the symbionts show t he same phylogeny as the aphids. Overall, analyses support the interpretati on that symbionts and aphids have undergone strict cospeciation, with no ho rizontal transmission of symbionts even among closely related, ecologically similar aphid hosts.