Hybrid zone Exome capture
Although interspecific hybridization may reduce fitness, it can also promote rapid adaptation to novel environments. A genomic region acquired from Pinus parviflora inhabiting higher temperatures could increase the fitness of a recipient alpine species Pinus pumila adapted to lower temperatures, particularly in the context of global warming. However, the large genome size of Pinus species has made the genome wide studies impractical. Therefore, we applied exome capture and investigated the genome-wide hybridization patterns of P. pumila and P. parviflora in the Hakkoda Mountains, Japan. Hybrid individuals occurred predominantly at mid-elevations and possessed P. pumila-like chloroplasts, suggesting that P. pumila is the most recent pollen parent. Genome-wide patterns of introgression indicated that more frequent backcrossing with P. pumila and limited backcrossing with P. parviflora. These patterns may result from geographic isolation, phenological differences in flowering time, and natural selection favoring hybrids in specific environmental contexts. Specifically, in terms of natural selection, tall hybrids may be advantageous for light competition at mid-elevations compared to the surrounding moorland vegetation, while smaller, pure P. pumila individuals may be favored in the high-altitude wind-exposed alpine fellfields, and hybrids with reduced height compared to those of pure P. parviflora may be disadvantaged in low-elevation broadleaved deciduous forests. Future functional analysis of introgressed genes could elucidate the adaptive significance of these hybridization patterns along elevational and environmental gradients.