The phenomenon of parallel evolution, whereby similar genomic and phenotypic changes occur across replicated pairs of populations or species, is widely studied. Nevertheless, the determining factors of parallel evolution remain poorly understood. Theoretical studies have proposed that pleiotropy, the influence of a single gene on multiple traits, is an important factor. In order to gain a deeper insight into the role of pleiotropy for parallel evolution from standing genetic variation, we characterized the interplay between parallelism, polymorphism, and pleiotropy. The present study examined the parallel gene expression evolution in 10 replicated populations of Drosophila simulans, which were adapted from standing variation to the same new temperature regime. The data demonstrate that the parallel evolution of gene expression from standing genetic variation is positively correlated with the strength of pleiotropic effects. The ancestral variation in gene expression is, however, negatively correlated with parallelism. Given that pleiotropy is also negatively correlated with gene expression variation, we conducted a causal analysis to distinguish cause and correlation and evaluate the role of pleiotropy. The causal analysis indicated that both direct (causative) and indirect (correlational) effects of pleiotropy contribute to parallel evolution. The indirect effect is mediated by historic selective constraint in response to pleiotropy. This results in parallel selection responses due to the reduced standing variation of pleiotropic genes. The direct effect of pleiotropy is likely to reflect a genetic correlation among adaptive traits, which in turn gives rise to synergistic effects and higher parallelism.

Wolbachia are maternally transmitted bacterial symbionts that are ubiquitous among arthropods. They can hijack host reproduction in various ways, including male-killing (MK), where the sons of infected mothers are killed during development. The recent discovery of MK-associated Wolbachia genes, i.e., oscar in Ostrinia moths and wmk in Drosophila flies, stimulates our interest in the diversity and commonality of MK mechanisms, which remain largely unclear. We recently discovered that a Wolbachia symbiont of the moth Homona magnanima carries an MK-associated prophage region encoding homologs of oscar (Hm-oscar) and wmk (wmk-1–4). Here, we investigated the effects of these genes in the native host. Upon transient overexpression, Hm-oscar, but not wmk, induced male lethality in H. magnanima, in contrast to our observations in Drosophila, where the wmk homologs, but not Hm-oscar, killed the males. Hm-oscar disrupted sex determination in male embryos by inducing a female-type doublesex splicing and impaired dosage compensation, recapitulating the Wolbachia phenotype. Cell-based transfection assays confirmed that Hm-oscar suppressed the function of masculinizer, the primary male sex determinant involved in lepidopteran dosage compensation. Our study highlights the conserved roles of oscar homologs in Wolbachia-induced lepidopteran MK and argues that Wolbachia have evolved multiple MK mechanisms in insects.