Evolutionarily stable infection by a male-killing endosymbiont in Drosophila innubila: molecular evidence from the host and parasite genomes

Abstract

Maternally inherited microbes that spread via male-killing are common pathogens of insects, yet very little is known about the evolutionary duration of these associations. The few examples to date indicate very recent, and thus potentially transient, infections. A male-killing strain of Wolbachia has recently been discovered in natural populations of Drosophila innubila. The population-level effects of this infection are significant: approximately 35% of females are infected, infected females produce very strongly female-biased sex ratios, and the resulting population-level sex ratio is significantly female biased. Using data on infection prevalence and Wolbachia transmission rates, infected cytoplasmic lineages are estimated to experience a approximately 5% selective advantage relative to uninfected lineages. The evolutionary history of this infection was explored by surveying patterns of polymorphism in both the host and parasite genomes, comparing the Wolbachia wsp gene and the host mtDNA COI gene to five host nuclear genes. Molecular data suggest that this male-killing infection is evolutionarily old, a conclusion supported with a simple model of parasite and mtDNA transmission dynamics. Despite a large effective population size of the host species and strong selection to evolve resistance, the D. innubila-Wolbachia association is likely at a stable equilibrium that is maintained by imperfect maternal transmission of the bacteria rather than partial resistance in the host species.

Figure 1.—

Relation between A group Wolbachia infection (as determined by PCR) and offspring sex ratio in wild-caught females of D. innubila. Light and dark shading indicates uninfected and infected females, respectively. Based on 369 flies collected from the Chiricahua Mountains of Arizona in August-September of 2001–2003, each of which produced at least 10 offspring.

Figure 2.—

Median-joining network of mtDNA COI haplotypes that occur in the Chiricahua Mountains population of D. innubila. The area of each circle is proportional to the relative frequency of that haplotype in the overall sample. The shading within each haplotype represents the proportion of individuals of each infection status: dark shading indicates infection with male-killing A group Wolbachia, light shading indicates infection with the B group Wolbachia, and no shading represents uninfected individuals. The arrow indicates where the D. falleni haplotype connects to the network.

Figure 3.—

Maximum-likelihood estimation of the mtDNA scalar relative to four nuclear loci (Adhr, cp36, tpi, and v). The gray line indicates the 2-unit likelihood bounds (95% confidence interval). The dotted lines show the scalar expectations based on (a) 35% infection frequency with a male-killing endosymbiont, (b) 35% infection frequency and a 66% female population sex ratio, and (c) neutrality. Results were obtained using the MLHKA program of S. Wright.

Figure 4.—

Most credible Wolbachia phylogeny based on Bayesian analysis of wsp. Wolbachia strains are shown by host name and host strain, when relevant. Those that are known to induce male-killing are underlined. The tree is rooted with the C group of Wolbachia, which infects filarial nematodes. Branch lengths are the mean of the posterior estimates for each branch, and the numbers at the nodes represent the proportion of sample trees containing particular bipartitions.

Figure 5.—

Distribution of the pairwise distances among the 17 haplotypes identified from 115 D. innubila and among the 15 haplotypes identified from 15 D. falleni. D. innubila and D. falleni are shown in dark and light shading, respectively. Pairwise distances are in units of base pairs and are Jukes-Cantor corrected.