Germination timing of Arabidopsis thaliana displays strong plasticity to geographic location and seasonal conditions experienced by seeds. We identified which plastic responses were adaptive using recombinant inbred lines in a field manipulation of geographic location (Kentucky, KY; Rhode Island, RI), maternal photoperiod (14-h and 10-h days), and season of dispersal (June and November). Transgressive segregation created novel genotypes that had either higher fitness or lower fitness in certain environments than either parent. Natural selection on germination timing and its variation explained 72% of the variance in fitness among genotypes in KY, 30% in June-dispersed seeds in RI, but only 4% in November-dispersed seeds in RI. Therefore, natural selection on germination timing is an extremely efficient sieve that can determine which genotypes can persist in some locations, and its efficiency is geographically variable and depends on other aspects of life history. We found no evidence for adaptive responses to maternal photoperiod during seed maturation. We did find adaptive plasticity to season of seed dispersal in RI. Seeds dispersed in June postponed germination, which was adaptive, while seeds dispersed in November accelerated germination, which was also adaptive. We also found maladaptive plasticity to geographic location for seeds dispersed in June, such that seeds dispersed in KY germinated much sooner than the optimum time. Consequently, bet hedging in germination timing was favorable in KY; genotypes with more variation in germination timing had higher fitness because greater variation was associated with postponed germination. Selection on germination timing varied across geographic location, indicating that germination timing can be a critical stage in the establishment of genotypes in new locations. The rate of evolution of germination timing may therefore strongly influence the rate at which species can expand their range.
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