Abstract
Species delimitations by morphological and by genetic markers are not always congruent. Magnolia kobus consists of two morphologically different varieties, kobus and borealis. The latter variety is characterized by larger leaves than the former. For the conservation of M. kobus genetic resources in natural forests, the relationships between morphological and genetic variation should be clarified. We investigated variations in nuclear microsatellites, chloroplast DNA (cpDNA) sequences and leaf morphological traits in 23 populations of M. kobus over the range of species. Two genetically divergent lineages, northern and southern were detected and their geographical boundary was estimated to be at 39°N. The northern lineage consisted of two genetic clusters and a single cpDNA haplotype, while the southern one had multiple genetic clusters and cpDNA haplotypes. The northern lineage showed significantly lower genetic diversity than the southern. Approximate Bayesian computation indicated that the northern and southern lineages had experienced, respectively, population expansion and long-term stable population size. The divergence time between the two lineages was estimated to be 565,000 years ago and no signature of migration between the two lineages after divergence was detected. Ecological niche modeling showed that the potential distribution area in northern Japan at the last glacial maximum was very small. It is thus considered that the two lineages have experienced different population histories over several glacial-inter-glacial cycles. Individuals of populations in the central to northern part of Honshu on the Sea of Japan side and in Hokkaido had large leaf width and area. These leaf characteristics corresponded with those of variety borealis. However, the delimitation of the northern and southern lineages detected by genetic markers (39°N) was not congruent with that detected by leaf morphologies (36°N). It is therefore suggested that variety borealis is not supported genetically and the northern and southern lineages should be considered separately when identifying conservation units based not on morphology but on genetic markers.
Highlights
Tree species that are widely distributed along the Japanese archipelago show significant genetic differentiation1 3 Vol.:(0123456789)Journal of Plant Research (2019) 132:741–758 for neutral genetic markers between the Sea of Japan and Pacific Ocean sides (Fagus crenata, Hiraoka and Tomaru 2009; Cryptomeria japonica, Tsumura et al 2014) and/or between the north and south (Kalopanax septemlobus, Sakaguchi et al 2011; Quercus aliena, San Jose-Maldia et al 2017; Magnolia salicifolia, Tamaki et al 2018)
Broad-leaved tree species growing along the Sea of Japan side of the Japanese archipelago are often characterized by large-wide-thin leaves, while related species growing in the Pacific Ocean side are characterized by small-narrowthick leaves (Hotta 1974)
Delimitation based on morphological traits and that determined from genetic structure do not always accord (Duminil and Di Michele 2009)
Summary
Tree species that are widely distributed along the Japanese archipelago show significant genetic differentiation1 3 Vol.:(0123456789)Journal of Plant Research (2019) 132:741–758 for neutral genetic markers between the Sea of Japan and Pacific Ocean sides (Fagus crenata, Hiraoka and Tomaru 2009; Cryptomeria japonica, Tsumura et al 2014) and/or between the north and south (Kalopanax septemlobus, Sakaguchi et al 2011; Quercus aliena, San Jose-Maldia et al 2017; Magnolia salicifolia, Tamaki et al 2018). Tree species that are widely distributed along the Japanese archipelago show significant genetic differentiation. The boundaries of genetic differentiation are not always the same among species This may be mainly due to the differences in locations of refugia during the glacial period among species. Broad-leaved tree species growing along the Sea of Japan side of the Japanese archipelago are often characterized by large-wide-thin leaves, while related species growing in the Pacific Ocean side are characterized by small-narrowthick leaves (Hotta 1974). It is necessary to clarify the relationships between morphological traits reflecting physiological adaptation to environment, and genetic structure, when considering conservation of genetic resources of forest trees that are broadly distributed along the Japanese archipelago
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