A test of genomic modularity among life-history adaptations promoting speciation with gene flow

Speciation with gene flow may require adaptive divergence of multiple traits to generate strong ecologically based reproductive isolation. Extensive negative pleiotropy or physical linkage of genes in the wrong phase affecting these diverging traits may therefore hinder speciation, while genetic independence or "modularity" among phenotypic traits may reduce constraints and facilitate divergence. Here, we test whether the genetics underlying two components of diapause life history, initial diapause intensity and diapause termination timing, constrain differentiation between sympatric hawthorn and apple-infesting host races of the fly Rhagoletis pomonella through analysis of 10,256 SNPs measured via genotyping-by-sequencing (GBS). Loci genetically associated with diapause termination timing were mainly observed for SNPs mapping to chromosomes 1-3 in the genome, most notably for SNPs displaying higher levels of linkage disequilibrium (LD), likely due to inversions. In contrast, selection on initial diapause intensity affected loci on all five major chromosomes of the genome, specifically those showing low levels of LD. This lack of overlap in genetically associated loci suggests that the two diapause phenotypes are largely modular. On chromosome 2, however, intermediate level LD loci and a subgroup of high LD loci displayed significant negative relationships between initial diapause intensity and diapause termination time. These gene regions on chromosome 2 therefore affected both traits, while most regions were largely independent. Moreover, loci associated with both measured traits also tended to exhibit highly divergent allele frequencies between the host races. Thus, the presence of nonover-lapping genetic modules likely facilitates simultaneous, adaptive divergence for the measured life-history components.