Abstract
Gene flow between populations can allow the spread of beneficial alleles and genetic diversity between populations, with importance to conservation, invasion biology, and agriculture. Levels of gene flow between populations vary not only with distance, but also with divergence in reproductive phenology. Since phenology is often locally adapted, arriving migrants may be reproductively out of synch with residents, which can depress realized gene flow. In flowering plants, the potential impact of phenological divergence on hybridization between populations can be predicted from overlap in flowering schedules—the daily count of flowers capable of pollen exchange—between a resident and migrant population. The accuracy of this prospective hybridization estimate, based on parental phenotypes, rests upon the assumptions of unbiased pollen transfer between resident and migrant active flowers. We tested the impact of phenological divergence on resident–migrant mating frequencies in experiments that mimicked a single large gene flow event. We first prospectively estimated mating frequencies two lines of Brassica rapaselected or early and late flowering. We then estimated realized mating frequencies retrospectively through progeny testing. The two estimates strongly agreed in a greenhouse experiment, where procedures ensured saturating, unbiased pollination. Under natural pollination in the field, the rate of resident–migrant mating, was lower than estimated by phenological divergence alone, although prospective and retrospective estimates were correlated. In both experiments, differences between residents and migrants in flowering schedule shape led to asymmetric hybridization. Results suggest that a prospective estimate of hybridization based on mating schedules can be a useful, although imperfect, tool for evaluating potential gene flow. They also illustrate the impact of mating phenology on the magnitude and symmetry of reproductive isolation.
Highlights
Gene flow between populations has important consequences for the evolution, survival, and spread of species
One conservation measure—Assisted Gene Flow (AGF), has been proposed to affect genetic rescue of populations threatened by global change by introducing adaptive alleles from related population in situ (Aitken & Whitlock, 2013)
Prospective and retrospective estimates were correlated under nat‐ ural pollination in the field, the rate of resident–migrant mating was lower than predicted by phenological divergence alone
Summary
Gene flow between populations has important consequences for the evolution, survival, and spread of species It can introduce adap‐ tive alleles into a declining resident populations, potentially saving it from extinction through genetic rescue (Frankham, 2015; Tallmon, Luikart, & Waples, 2004). We performed a pair of two‐generation experiments to mimic the translocation of phenologically mismatched migrants into a resi‐ dent population These experiments were designed to mimic an AGF program, whereby a single massive gene flow event is imposed upon a resident population in need of genetic rescue. In both experiments, the potential for hybridization was estimated in the first generation from overlap in flowering schedules of the phenologically divergent lines—the prospective estimate. Prospective and retrospective estimates were correlated under nat‐ ural pollination in the field, the rate of resident–migrant mating was lower than predicted by phenological divergence alone
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