Intraspecific phylogenetic congruence among multiple symbiont genomes

Eukaryotes often form intimate endosymbioses with prokaryotic organisms. Ca ses in which these symbionts are transmitted cytoplasmically to host progen y create the potential for co-speciation or congruent evolution among the d istinct genomes of these partners. If symbionts do not move horizontally be tween different eukaryotic hosts, strict phylogenetic congruence of their g enomes is predicted and should extend to relationships within a sing le hos t species. Conversely, even rare 'host shifts' among closely related lineag es should yield conflicting tree topologies at the intraspecific level. Her e, we investigate the historical associations among four symbiotic genomes residing within an aphid host: the mitochondrial DNA of Uroleucon ambrosiae aphids, the bacterial chromosome of their Buchnera bacterial endosymbionts , and two plasmids associated with Buchnera. DNA sequence polymorphisms pro vided a significant phylogenetic signal and no homoplasy for each data set, yielding completely and significantly congruent phylogenies for these four genomes and no evidence of horizontal transmission. This study thus provid es the first evidence for strictly vertical transmission and 'co-speciation ' of symbiotic organisms at the intraspecific level, and represents the low est phylogenetic level at which such coevolution has been demonstrated. The se results may reflect the obligate nature of tl-lis intimate mutualism and indicate opportunities for adaptive coevolution among linked symbiont geno mes.