Flower color locus resists introgression due to correlational selection with other floral traits in Ipomoea cordatotriloba

When species hybridize, resistance to introgression is presumably due to selection against hybridizing alleles. While many studies have characterized direct selection at these sites, alleles may resist introgression through correlational selection. Here we investigate the role of direct and correlational selection in reducing introgression at the color locus in Ipomoea cordatotriloba .

We used recombinant inbred lines that varied in limb color, flower size and sugar concentration to estimate the fitness advantage of the flower color allele via direct and correlational selection. To assess the effect of correlational selection on fitness, we ask if floral size or nectar sugar concentration is correlated with fecundity in pink- but not white-limbed lines.

We find no evidence for direct selection on flower color across four fitness components – germination, survival, fecundity, and siring success. Instead, both flower size and sugar concentration significantly correlate with fecundity in pink, but not white limbed lines. As a result, correlational selection on the color allele opposes introgression when recurrent migration is low (<3%).

These results demonstrate that correlational, rather than direct, selection is sufficient to resist introgression via hybridization and suggest that correlational selection is an underexplored mechanism to generate resistance to introgression across multiple loci.