上原 浩一

Geographic and environmental isolations of islands and the mainland offer excellent opportunity to investigate colonization and survival dynamics of island populations. We inferred and compared evolutionary processes and the demographic history of Rhododendron tsusiophyllum, in the Izu Islands and the much larger island Honshu, treated here as the mainland, using thousands of nuclear SNPs obtained by ddRAD-seq from eight populations of R. tsusiophyllum and three populations of R. tschonoskii as an outgroup. Phylogenetic relationships and their habitats suggest that R. tsusiophyllum had evolved and migrated from cold north to warm south regions. We detected clear genetic divergence among populations in three regions of Honshu and the Izu Islands, suggesting restricted migration between them due to isolated habitats on mountains even in the mainland. The three regions have different changes in effective population size, especially, genetic diversity and population size of the Izu Islands are small compared to the others. Further, habitats of populations in the Izu Islands are warmer than those in Honshu, suggesting that they have undergone adaptive evolution. Our study provides evidences of montane rather than insular isolation on genetic divergence, survival of populations and significance of adaptive evolution for island populations with small population size and low genetic diversity, despite close proximity to mainland populations.

Plants growing in similar climates with extensively overlapping distributions may exhibit similar phylogeographic patterns, due to similarities in range shifts during past climatic oscillations. We tested and exploited this expectation in a comparison of the evolutionary and demographic histories of two related and co-distributed species endemic to Japan-Rhododendron pentaphyllum and R. quinquefolium. Genetic variation in sequences of noncoding chloroplast DNA regions and hundreds of nuclear single nucleotide polymorphisms were used to investigate 191 individuals from 13 populations of R. pentaphyllum and 142 individuals from 10 populations of R. quinquefolium. We found that the two species were not each other's closest relative, and we found different times to the most recent common ancestor of extant populations based on both chloroplast DNA and nuclear SNPs. Rhododendron pentaphyllum exhibited higher genetic variation between populations than R. quinquefolium, while the two species had similar patterns of genetic divergence between regions with slightly different divergence times. These lines of evidence suggest that the similar ranges of the two species are the result of repeated range shifts since speciation during climatic oscillations.

<p>近年の比較系統地理学における興味の１つは、過去の第四紀気候変動に伴う分布変遷史の群集レベルでの類似性は各種の遺伝的変異に保存されているかという点である。温帯地域での化石記録と遺伝解析を併せた研究から、各地域で多くの種は似たような分布変遷を経験し、それが遺伝的変異に反映されている可能性が提示されている。ただし、必ずしもすべての種で共通した傾向が認められてはおらず、この議論は終結していない。そこで本研究では、分布域が大きく重複しており、生活史特性の類似性も高いツツジ属2種（ツクシアケボノツツジ、シロヤシオ）を対象として、種内遺伝的構造とその進化史を明らかにした。葉緑体DNA非コード4領域の塩基配列とMIG-seq法により取得したゲノムワイドSNPsを用いて遺伝解析を行った結果、ツクシアケボノツツジでは11葉緑体DNAハプロタイプ、シロヤシオでは3ハプロタイプを認識した。ツクシアケボノツツジでは葉緑体・核ともに集団間分化が著しく、シロヤシオでは集団間分化があまり明瞭ではない傾向が確認された。発表では、これら2種の種内変異の違いとそれを形成した歴史的過程を進化年代・個体群動態の両面から明らかにする。</p>

Ecological speciation is an important factor in the diversification of plants. The distribution of the woody species Rhododendron indicum, which grows along rivers and is able to withstand water flow when rivers flood (i.e. it is a rheophyte), is disjunct, in contrast to the widespread distribution of its relative, Rhododendron kaempferi. This study aimed to elucidate the phylogenetic relationships between R. indicum and R. kaempferi and the evolutionary processes that gave rise to them. The sequences of three non-coding chloroplast DNA regions (total length 1977 bp) were obtained from 21 populations covering the ranges of the two species. In addition, genome-wide SNPs were genotyped from 20 populations using a genotyping by sequencing method. Leaf morphologies were measured for eight representative populations. Two chloroplast DNA haplotypes, which were detected in R. indicum, were shared between the two species. Genome-wide SNPs identified two lineages in R. indicum and these lineages did not constitute a monophyletic group. Each of these two lineages was related to geographically close populations of R. kaempferi. Leaf morphology, which is a characteristic feature in rheophytes, was not differentiated between the two lineages in R. indicum. The morphological similarity between the two heterogeneous lineages may be a result of parallel evolution from R. kaempferi or of introgressive hybridization between the species due to strong selective pressure imposed by flooding.

Simple sequence repeat (SSR) markers were developed from expressed sequence tags (ESTs) for Rhododendron section Brachycalyx in order to elucidate its evolutionary processes and reproductive ecology. Nineteen polymorphic EST-SSR markers were developed from EST libraries of R. amagianum and R. hyugaense. Polymorphisms for these markers were assessed using four species of section Brachycalyx. The number of alleles ranged from 1 to 14, and the observed and expected heterozygosity ranged from 0.000 to 0.931 and 0.000 to 0.904, respectively. The EST-SSR markers developed in this study will be useful for elucidating population genetic structure and breeding systems in section Brachycalyx.

The genetic diversity of the species, Potamogeton lucens subsp. sinicus var. teganumensis, which is critically endangered in Japan, was investigated. This species now occurs in only two known localities in Japan. One is a native population (Oitoike population), but the other (Teganuma-Okahotto population) is found in a small artificial pond that was dug in 1998. It is considered that the Teganuma-Okahotto population grew from a soil seed bank. Based on RAPD variation, we compared the genetic diversity of the two populations of P lucens var. teganumensis in Japan and one population of P. lucens subsp. sinicus var. sinicus in China. The Teganuma-Okahotto population showed RAPD variation, suggesting that it may be derived from more than one seed buried in old sediments. This population also had the highest value of Shannon&apos;s Information Index among the three study populations. This finding suggests that seeds buried in sediments can contain genetic variability, and may be used to conserve the genetic diversity of rare and endangered plants. (c) 2006 Elsevier B.V. All rights reserved.