Reduced fitness under drought stress in F1 hybrids of Antirrhinum majus varieties with divergent flower colors

Premise: What maintains trait divergence in the face of gene flow? Two varieties of wild snapdragon (Antirrhinum majus) characterized by divergent flower color hybridize in their native range. Selection on flower color genes is indicated by sharp clines, but the selective agents have not been demonstrated. Although previous work has focused on pollinators, pigmentation genes can also contribute to abiotic stress tolerance. We hypothesized that pigmentation in A. majus mediates stress tolerance, which could contribute to hybrid zone maintenance through parental niche divergence or hybrid maladaptation. Specifically, we tested whether morphotype mediates drought tolerance in an experiment comparing magenta-flowered var. pseudomajus, yellow-flowered var. striatum, and their pink-flowered hybrid cross. Methods: We experimentally compared drought tolerance of each morphotype from allopatric crosses within and between varieties using three greenhouse treatments. Control plants were watered as needed, while drought-treated plants were watered half as often, either from the transplant stage (“early” drought), or from flowering onset (“late” drought). Results: Parental morphotypes responded identically to drought in fitness and most phenotypic traits. However, hybrids had lower survival (14%) under late drought stress than parental morphotypes (70%). All hybrids that flowered in the late drought treatment died, compared to ~20% of flowering parental morphotypes. Conclusions: Hybrid maladaptation to abiotic stress could potentially contribute to flower color divergence in the face of gene flow in A. majus. Further research should test the relevance of our results to field conditions and explicitly probe the role of flower color genes in drought tolerance.