Cospeciation Between Bacterial Endosymbionts (Buchnera) and a Recent Radiation of Aphids (Uroleucon) and Pitfalls of Testing for Phylogenetic Congruence

Abstract

Abstract. Previous studies of phylogenetic congruence between aphids and their symbiotic bacteria (Buchnera) supported long-term vertical transmission of symbionts. However, those studies were based on distantly related aphids and would not have revealed horizontal transfer of symbionts among closely related hosts. Aphid species of the genus Uroleucon are closely related phylogenetically and overlap in geographic ranges, habitats, and parasitoids. To examine support for congruence of phylogenies of Buchnera and Uroleucon, sequences from four mitochondrial, one nuclear, and one endosymbiont gene (trpB) were obtained. Congruence of phylogenies based on pooled aphid genes with phylogenies 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 both trees. Two kinds of analyses testing the null hypothesis of perfect congruence between pairwise combinations of datasets and tree topologies were performed: the Kishino-Hasegawa test and the likelihood-ratio test. Both tests indicated significant disagreement among most pairwise combinations of mitochondrial, nuclear, and symbiont datasets. Because rampant recombination among mitochondrial genomes of different aphid species is unlikely, inaccurate assumptions in the evolutionary models underlying these tests appear to 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 the same phylogeny as the aphids. Overall, analyses support the interpretation that symbionts and aphids have undergone strict cospeciation, with no horizontal transmission of symbionts even among closely related, ecologically similar aphid hosts.

Vertical Transmission of Biosynthetic Plasmids in Aphid Endosymbionts ( Buchnera )

“…2). The relationships at each gene generally agree with relationships among the Uroleucon hosts (14).…”

‘Candidatus Streptomyces philanthi’, an endosymbiotic streptomycete in the antennae of Philanthus digger wasps

“…The high degree of 16S rRNA gene sequence similarity among Philanthus endosymbionts suggests that they are transmitted vertically from mother to offspring, as has been described for many other endosymbiotic bacteria (Aksoy et al, 1997;Clark et al, 2000Clark et al, , 2001Ishikawa, 2003;Sauer et al, 2000). Alternatively, the bacteria may be taken up from the environment with certain mechanisms preventing the uptake of non-symbiotic bacteria, a transmission route that has been demonstrated for the symbionts of the squid Euprymna scolopes (McFallNgai & Ruby, 1991;Nishiguchi, 2002;Nyholm et al, 2000;Nyholm & McFall-Ngai, 2004).…”

Burkholderia bacteria infectiously induce the proto-farming symbiosis of Dictyostelium amoebae and food bacteria

“…The trait of secondary bacterial carriage can be beneficial for the amoebas because they can replenish their food supply when dispersed to food-impoverished environments. This trait serves as an unusual example of how symbiotic associations can endow novel functionality to host organisms (44). If we consider that Burkholderia colonization may confer adaptive traits to D. discoideum, such as the carriage of bacterial food, it may be reasonable to imagine that these adaptive traits may translate into an even broader form of phenotypic plasticity.…”