李 悦

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博士学位論文の要旨及び審査結果の要旨 (Summary of Thesis(DR))othe

李, 悦

Tohoku University Repository (TOUR) / 東北大学機関リポジトリ

Variation in frequency and extent ofhybridization in the populations of a hybridzone between Leucosceptrum japonicum and L.stellipilum (Lamiaceae) in central Japanesemainland著者 李 悦学位授与機関 Tohoku University学位授与番号 生博第287号URL http://hdl.handle.net/10097/58234   氏名（本籍地） 学 位 の 種 類 学 位 記 番 号 学位授与年月日 学位授与の要件 研 究 科 ， 専 攻  論 文 題 目      博士論文審査委員   り  ゆえ 李 悦  博士（生命科学） 生博第２８７号 平成２６年９月２４日 学位規則第４条第１項該当 東北大学大学院生命科学研究科 （博士課程）生態システム生命科学専攻 Variation in frequency and extent of hybridization in the populations of a hybrid zone between Leucosceptrum japonicum and L. stellipilum (Lamiaceae) in central Japanese mainland (ミカエリソウとテンニンソウ（シソ科）の交雑集団における交雑頻度と交雑程度の変異に関する研究)  （主査） 教 授 牧 雅之     教 授 千葉 聡      准教授 酒井 聡樹   論文内容の要旨 Comparative studies will qualify the total and individual components of reproductive isolation between species. However, the relative contribution of each isolating mechanism in reducing interspecific gene flow is largely unknown. It is difficult to determine reproductive isolation involved in the initial reduction in gene flow between species by examining present day reproductive isolation because present day reproductive isolation also contains some isolation mechanisms evolved after species divergence. Contact sites with various level of isolation breakdown might represent different levels of differentiations between species pairs along the gradual speciation continuum. When intrinsic reproductive isolation barrier is absent or similar among contact sites, examination of the differences in isolation barrier between unimodal contact site (with weaker isolation barrier) and bimodal contact site (with stronger isolation barrier) could help us to pinpoint extrinsic reproductive isolating mechanisms that limit gene flow between species pair and thus promote speciation.    Morphological and genetic variations in nine populations in a putative hybrid zone between L. japonicum and L. stellipilum were investigated. I used multivariate morphometric analyses, sequence variation in internal transcribed spacer (nrITS), sequence variation in a gene spacer in chloroplast DNA (trnL-F) and pollen fertility to examine whether natural hybridization occurred between two species and preliminarily investigated morphological and genetic variations in nine putative hybrid populations. Limited chloroplast introgression between L. japonicum and L. stellipilum in the hybrid zone indicated that no historical introgression occurred between the two species when they come into contact. The morphological intermediacy, ITS additivity and reduced pollen fertility confirmed the occurrence of contemporary hybridization between L. japonicum and L. stellipilum. Hybrid individuals appear to be not only first generation hybrids but also advanced generation hybrids and /or backcross. Thus, morphology of the putative early hybrid generation was too variable to distinguish all these hybrids from the parental species. Putative hybrid populations exhibited varying levels of morphological and genetic variation. The character variation in hybrid populations between L. japonicum and L. stellipilum may have been caused by heterogenous local condition.  I developed 10 new primers for amplifying microsatellite loci for both Leucosceptrum japonicum and L. stellipilum to study the genetic structure of hybrid zones between these two species. I isolated DNA fragments containing microsatellites from an enriched library and designed the 10 primer pairs amplifying the fragments. These newly developed markers will be useful in investigating genetic structure and hybridization patterns in hybrid zones between L. japonicum and L. stellipilum.    Variations in the frequency and extent of hybridization among mixed populations located in the same hybrid zone provide natural laboratories for the study of extrinsic reproductive isolation maintaining species integrity. I examined the pattern of hybridization between L. japonicum and L. stellipilum among mixed populations in different localities of a hybrid zone. The genetic structures of three sympatric populations and six mixed populations in the hybrid zone, and five reference populations far from the hybrid zone, were characterized using 10 neutral nuclear microsatellite markers. Evidence from genetic distance-based clustering analysis, the frequency distribution of admixture proportion values, and the hybrid category assignment approaches indicated that the frequency and extent of hybridization varied considerably among populations in the contact zone between L. japonicum and L. stellipilum. Differences in frequency and extent of hybridization may be caused by variation in exogenous (ecological) selection among populations. The results suggested that extrinsic reproductive isolation mechanisms limited gene flow between L. japonicum and L. stellipilum. The present study will facilitate future research exploring the evolution of reproductive isolation between L. japonicum and L. stellipilum.  論文審査結果の要旨  李悦氏は，本研究科の博士課程において，シソ科のミカエリソウとテンニンソウの交雑現象を対象に研究を行った．野生植物における交雑現象はここ 20 年ほど進化生物学の大きなトピックとなっており，多くの研究者の関心を引いている．李悦氏は，交雑の影響を受けていない母種（ミカエリソウとテンニンソウ）の集団と交雑によってさまざまな形態の個体が存在する中部地方の集団から多数の個体を採取し，形態学的，分子分類学的，および分子集団遺伝学的解析を行った．  複数の形態形質の計測，花粉稔性の計測，葉緑体 DNAおよびリボゾーマル DNA変異に基づき，交雑集団の解析を行った．その結果，交雑集団の個体は確かにミカエリソウとテンニンソウの交雑から生じた個体を含んでいることが確かめられた．一方，形態の変異は交雑集団ごとに異なっており，一様ではなかった．交雑集団のリボゾーマル DNA変異と葉緑体 DNA変異によって，単なる 1代雑種のみならず戻し交雑や 2代雑種以降が存在することが示唆された．  リボゾーマル DNA変異と葉緑体 DNA変異だけでは，さまざまな雑種後代の解析を行うのに情報が限られるため，李悦氏は引き続き，濃縮ライブラリ法によって超変異性のあるマイクロサテライト領域を単離して，プライマーを設計した．  前段で開発したマイクロサテライトプライマーのうち，ミカエリソウとテンニンソウの両方で増幅するプライマーを用いて，集団遺伝学的解析を行った．交雑個体の構成は集団によって変異が見られることがわかり，また，戻し交雑個体や 2代雑種以降の個体が存在することも明らかとなった．集団間で交雑個体の構成が変化しているのは，集団によって生態的隔離の程度が異なっているためであると結論づけた．  すでに本博士論文の一部は Application in Plant Sciences誌に掲載されているほか， Plant Systematics and Evolution に受理されており，残りの部分も投稿中である．これらのことは，李悦氏は，自立して研究活動を行うに必要な高度の研究能力と学識を有することを示している。したがって，李悦氏提出の論文は，博士（生命科学）の博士論文として合格と認める。         

Variation in frequency and extent of hybridization in the populations of a hybrid zone between Leucosceptrum japonicum and L. stellipilum (Lamiaceae) in central Japanese mainland

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ミカエリソウとテンニンソウ(シソ科)の交雑集団における交雑頻度と交雑程度の変異に関する研究

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