Investigating How the Color Allele in Ipomoea cordatotriloba Resists Introgression via Epistatic Selection

Abstract

When species hybridize, they often exchange alleles through introgression. When rates of genetic migration are high, alleles that resist introgression are presumed to be maintained by selection. While most prior research has focused on characterizing direct selection at these sites, alleles could alternatively be maintained via forms of indirect selection. In particular, correlational selection could act to jointly favor resistance to introgression. Alternatively, if variation at one locus modifies the pattern of selection on other traits, then it is also possible that that locus may resist introgression. Here I examine whether resistance to introgression at a locus affecting floral limb color is due primarily due to direct selection or to epistatic selection in Ipomoea cordatotriloba. To do so, I conducted field experiments on recombinant inbred lines that varied in limb color, flower size and nectar sugar concentration. To test for the effect of direct selection on flower color, I estimated the effect of color genotype on four fitness components – germination, survival, fecundity, and siring success. To test for the effect of epistatic selection, I ask if either floral size or nectar sugar concentration is correlated with fecundity in lines that are pink (consistent with I. cordatotriloba) but not white (consistent with I. lacunosa). To test for the effects of pollinator behavior on fitness, I observed patterns of pollinator behavior in the same plants in which I estimated fitness. I find no evidence for direct selection on flower color in any of the four fitness components. Instead, I found that flower color modulates selection on other floral traits; both flower size and sugar concentration significantly correlate with fecundity in pink but not white limbed lines. I use evolutionary models to show that at low rates of migration (<3% per generation), modulation of selection on flower size and sugar concentration traits can cause resistance to introgression at the flower color locus via epistatic selection. Together, these results suggest flower color is maintained via epistatic, not direct selection. Pollinator behavior recapitulated selection results for direct selection, but not epistatic selection. Color did not significantly impact any aspect of pollinator behavior, suggesting that the modulation of selection is mediated by non-pollinator agents. Jointly, these results suggest that binary traits may resist introgression through indirect, epistatic selection, but that the pattern of epistasis is likely not caused by pollinators.