Parallel acceleration of evolutionary rates in symbiont genes underlying host nutrition

(1)The overproduction of essential amino acids by Buchnera aphidicola, the primary bacterial mutualist of aphids, is considered an adaptation for incr eased production of nutrients that are lacking in aphids' diet of plant sap . Given their shared role in host nutrition, amino acid biosynthetic genes of Buchnera are expected to experience parallel changes in selection that d epend on host diet quality, growth rate, and population structure. This stu dy evaluates the hypothesis of parallel selection across biosynthetic pathw ays by testing for correlated changes in evolutionary rates at biosynthetic genes of Buchnera. Previous studies show fast evolutionary rates at trypto phan biosynthetic genes among Buchnera associated with the aphid genus Urol eucon and suggest reduced purifying selection on symbiont nutritional funct ions in this aphid group. Here, we test for parallel rate acceleration at o ther amino acid biosynthetic genes of Buchnera-Uroleucon, including those f or leucine (leuABC) and isoleucine/valine biosynthesis (ilvC). Ratios of no nsynonymous to synonymous substitutions (d(N)/d(S)) were estimated using co don-based maximum-likelihood methods that account for the extreme AT compos itional bias of Buchnera sequences. A significant elevation in d(N)/d(S) at biosynthetic loci but not at two housekeeping genes sampled (dnaN and tuf) suggests reduced host-level selection on biosynthetic capabilities of Buch nera-Uroleucon. In addition, the discovery of trpEG pseudogenes in Buchnera -U. obscurum further supports reduced selection on amino acid biosynthesis. (C) 2001 Academic Press.