Limited effective gene flow between two interfertile red oak species

Abstract

Key message Highly elevated differentiation in differ- ent life stages between two interfertile oak species at a CONSTANS-like gene suggests a role of this gene in pre- zygotic isolation and adaptive divergence between species. Abstract Genome-wide differentiation patterns among oak species suggest that divergent selection can maintain species-specific adaptations and morphological integrity by reducing effective interspecific gene flow. While there is evidence for both pre- and post-zygotic isolation mechan- isms in oaks (e.g., differences in flowering time, selection against hybrids), these mechanisms are rarely studied at each life stage from acorns to adult trees within the same forest. To assess the reproductive isolation mechanisms between two ecologically divergent species, we (1) quan- tified the number of hybrids in different life stages in Quercus rubra and Quercus ellipsoidalis, two interfertile red oaks with different adaptations to drought, and (2) assessed the timing of bud burst in both natural populations and in a seedling common garden trial. The low number of hybrids in all life stages suggested pre-zygotic isolation between species or selection in very early life stages that have not been sampled (e.g., early seed abortion). Sig- nificant differences in bud burst were found in two con- secutive years between species in a common garden seedling trial but not in natural populations of the same provenance. In addition, we found evidence for divergent selection on several gene loci between species in each life stage. In particular, an SSR repeat located within the coding sequence of a CONSTANS-like gene, a locus in- volved in the photoperiodic regulation of flowering time and development, showed very high interspecific differ- entiation between species in all life stages (mean FST = 0.83), compared to the average neutral differen- tiation of 3.7 %.