Natural hybridization and introgression of Abies firma and Abies homolepis along the altitudinal gradient and genetic insights into the origin of Abies umbellata

Two Japanese fir species (Pinaceae), Abies firma and A. homolepis, comprise the majority of the temperate forest vegetation zones of Japan. Their vertical distributions are exclusively segregated: A. firma occurs in low elevations and A. homolepis occurs in elevated regions; they contact each other at an altitude of around 1,000–1,100 m. Around the contact zone, A. umbellata is described as a tree with shoots of A. homolepis. It has the greenish cones of A. firma instead of the violet A. homolepis cones. An albinism hypothesis and a hybrid hypothesis were proposed to explain the origin of A. umbellata, provoking long debates. Therefore, we examined a total of 78 A. firma and A. homolepis trees (including an A. umbellata tree) along the altitudinal gradient in two study sites using 13 nuclear microsatellites and a species-specific chloroplast DNA (cpDNA) marker and examined morphologies of shoots and cone scales. Finally, we tested the two hypotheses. The genetic analyses revealed that shoot morphologies of A. homolepis and A. firma correspond mostly to the genetic identity of the species, whereas nine trees with intermediate shoot morphologies and two trees (including the A. umbellata tree) with A. homolepis shoots were genetically intermediate, indicating natural hybrids of A. firma and A. homolepis. These 11 hybrids occur in the narrow altitude ranges of their contact zones. The genetic analyses also suggested that hybrid backcrosses with parental species occur. These results support the hybrid hypothesis and suggest that the natural hybrids should taxonomically be classified as A. × umbellata.