Search

The obligate endosymbiotic bacterium Buchnera aphidicola shows elevated rates of sequence evolution compared to free-living relatives, particularly at nonsynonymous sites. Because Buchnera experiences population bottlenecks during transmission to the offspring of its aphid host, it is hypothesized that genetic drift and the accumulation of slightly deleterious mutations can explain this rate increase. Recent studies of intraspecific variation in Buchnera reveal patterns consistent with this hypothesis. In this study we examine inter- and intraspecific nucleotide variation in groEL, a highly conserved chaperonin gene that is constitutively overexpressed in Buchnera. Maximum-likelihood estimates of nonsynonymons substitution rates across Buchnera species are strikingly low at groEL compared to other loci. Despite this evidence for strong purifying selection on groEL, our intraspecific analysis of this gene documents reduced synonymous polymorphism, elevated nonsynonymous polymorphism, and an excess of rare alleles relative to the neutral expectation, as found in recent studies of other Buchnera loci. Comparisons with Escherichia coli generally show patterns predicted by their differences in [N.sub.e]. The sum of these observations is not expected under relaxed or balancing selection, selective sweeps, or increased mutation rate. Rather, they further support the hypothesis that drift is an important force driving accelerated protein evolution in this obligate mutualist.

Molecular systematic analysis of Calligarapha leaf beetles using sequences from mitochondrial and nuclear loci is conducted to investigate unisexual evolution of the species, within an otherwise bisexual group of insects. Findings indicate that each unisexual species represents an evolutionary independent Pleistocene origin, providing diverse evidence that these unisexuals originated through asymmetric interspecific hybridization and subsequent backcrossing with the paternal species.

Buchnera are maternally transmitted bacterial endosymbionts that synthesize amino acids that are limiting in the diet of their aphid hosts. Previous studies demonstrated accelerated sequence evolution in Buchnera compared to free-living bacteria, especially for nonsynonymous substitutions. Two mechanisms may explain this acceleration: relaxed purifying selection and increased fixation of slightly deleterious alleles under drift. Here, we test the divergent predictions of these hypotheses for intraspecific polymorphism using Buchnera associated with natural populations of the ragweed aphid, Uroleucon ambrosiae. Contrary to expectations under relaxed selection, U. ambrosiae from across the United States yielded strikingly low sequence diversity at three Buchnera loci (dnaN, trpBC, trpEG), revealing polymorphism three orders of magnitude lower than in enteric bacteria. An excess of nonsynonymous polymorphism and of rare alleles was also observed. Local sampling of additional dnaN sequences revealed similar patterns of polymorphism and no evidence of food plant-associated genetic structure. Aphid mitochondrial sequences further suggested that host bottlenecks and large-scale dispersal may contribute to genetic homogenization of aphids and symbionts. Together, our results support reduced [N.sub.e] as a primary cause of accelerated sequence evolution in Buchnera. However, our study cannot rule out the possibility that mechanisms other than bottlenecks also contribute to reduced [N.sub.e] at aphid and endosymbiont loci.

1. Outstanding issues in evolutionary ecology include: (1) the prevalence of host-associated ecological specialisation among herbivorous insects, and (2) the speciation processes contributing to the tremendous species-level diversity of herbivores. 2. Neochlamisus leaf beetle species are generally monophagous, but cumulatively employ a disparate array of host plants. These patterns occur in microcosm within the putatively oligophagous Neochlamisus bebbianae, which uses specific host species within each of six plant genera representing five families. I refer to all beetle populations associated with any one of these hosts as a particular ‘host form’. 3. This paper describes aspects of Neochlamisus biology and some experimental findings relevant to both host-use evolution and host specialisation in Neochlamisus, and to ecological divergence, reproductive isolation, and genetic differentiation among N. bebbianae host forms. It thereby reviews a research programme focused on ecological speciation. 4. With respect to these host forms, our findings include: (1) ubiquitous host specialisation, (2) constraints on ‘multitasking’ that select for host specialisation, (3) ubiquitous ecological divergence, (4) pre- and post-mating reproductive barriers, (5) genetic and environmental contributions to ecological divergence and reproductive isolation, (6) positive statistical associations between ecological divergence and reproductive isolation, (7) strongly differentiated loci despite gene flow, reflecting divergent host-related selection, and (8) possible host race formation. We additionally describe a unique example of intimate host adaptation in another species, Neochlamisus platani. 5. The intimate and discrete nature of herbivore–host associations may generate unusually strong (host-related) selection that could simultaneous explain pronounced rates of herbivore specialisation and speciation. [ABSTRACT FROM AUTHOR]