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

Abstract

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.

Phenological shift along an elevational gradient and dispersal of pollen and seeds maintain a hybrid zone between two cherry tree species

“…Cerasus, hybrid index, life-cycle stage, nuclear microsatellite, staggered phenology 1 | INTRODUCTION Hybrid zones are areas where genetically differentiated taxa meet and produce their hybrids (Arnold, 1997;Hewitt, 1988). In hybrid zones, the parental taxa often occupy the opposite sides of an environmental gradient with a certain range of overlap, and their hybrids usually appear in the intermediate range of the gradient (Abbott, 2017;Abbott & Brennan, 2014;Aizawa & Iwaizumi, 2020). Tension zones, one of the typical types of hybrid zones, persist under the balance between production of hybrids and selection against hybrids (Barton & Hewitt, 1985;Curry, 2015).…”

A DNA barcode database for the woody plants of Japan

“…Rosales and Fagales have high species diversity but are also characterised by families where recurrent past or ongoing gene flow via hybridisation and/or hybrid species is widespread in Japan (Iwatsuki et al, 1999(Iwatsuki et al, , 2001(Iwatsuki et al, , 2006. In contrast, Pinales have lower number of species but undergo extensive chloroplast sharing resulting in a lower identification rate in some genera (Aizawa & Iwaizumi, 2020;Watano et al, 2004). Similar identification rates have been observed in European Pinus (Celiński et al, 2017).…”

Asymmetric Reproductive Barriers and Gene Flow Promote the Rise of a Stable Hybrid Zone in the Mediterranean High Mountain

“…However, when searching for hybrid contact zones on altitudinal gradients, the approach must be more local: first, because the area at the mountain peaks is reduced with the elevation, and second, because the changes of kilometers in latitude correspond to the changes of meters in elevation for the environmental conditions affecting the plants (Jump et al, 2009 ). The interest for studying hybrid zones in elevation gradients is increasing in recent years (Aizawa and Iwaizumi, 2020 ; Mimura and Suga, 2020 ; Tamaki and Yamada, 2020 ), although some of these hybrid zones are already in the central body knowledge of hybridization in plants (Aguilar et al, 1999 ; James and Abbott, 2005 ).…”

“…kilometers in latitude correspond to the changes of meters in elevation for the environmental conditions affecting the plants (Jump et al, 2009). The interest for studying hybrid zones in elevation gradients is increasing in recent years (Aizawa and Iwaizumi, 2020;Mimura and Suga, 2020;Tamaki and Yamada, 2020), although some of these hybrid zones are already in the central body knowledge of hybridization in plants (Aguilar et al, 1999;James and Abbott, 2005).…”