Major quantitative trait loci control divergence in critical photoperiod for flowering between selfing and outcrossing species of monkeyflower (Mimulus).

• Divergence in flowering time is a key contributor to reproductive isolation between incipient species, as it enforces habitat specialization and causes assortative mating even in sympatry. Understanding the genetic basis of flowering time divergence illuminates the origins and maintenance of species barriers. • We investigated the genetics of divergence in critical photoperiod for flowering between yellow monkeyflowers Mimulus guttatus (outcrosser, summer flowering) and Mimulus nasutus (selfer, spring flowering). We used quantitative trait locus (QTL) mapping of F2 hybrids and fine-mapping in nearly isogenic lines to characterize the genomic regions underlying a > 2 h critical photoperiod difference between allopatric populations, and then tested whether the same QTLs control flowering time in sympatry. • We identified two major QTLs that almost completely explain M. nasutus's ability to flower in early spring; they are shared by allopatric and sympatric population pairs. The smaller QTL is coincident with one that differentiates ecotypes within M. guttatus, but the larger effect QTL appears unique to M. nasutus. • Unlike floral traits associated with mating system divergence, large interspecific differences in flowering phenology depend on only a few loci. Major critical photoperiod QTLs may be 'speciation genes' and also restrict interspecific gene flow in secondary sympatry.