Fraxinus mandshurica is a key species in the succession of climax community in cool-temperate forests and an important timber tree in Northeast China (NEC). The natural forest resource of F. mandshurica is increasingly threatened, due to over exploitation and widespread deforestation during the past century. Thus, it was designated as an endangered tree species in need of national priority protection in China. However, the levels of genetic diversity and population genetic structure of F. mandshurica in NEC remain unknown.

The thesis attempted to understand the magnitudes of genetic diversity of F. mandshurica using nuclear microsatellites. It focused on clarifying the factors affecting the structure of genetic distribution at various spatial and temporal scales in NEC, the species' central range.

The thesis was divided into five chapters. Chapter 1 and 2 were the general introduction and review, respectively. In chapter 3, the wide-scale genetic variation and phylogeographical structure of F. mandshurica adult populations in NEC were primarily investigated. At a comparable level, a phylogeographical evaluation of the variety in Japan (var. japonica) was also conducted. Chapter 4 was concerned on understanding the impacts of landscape heterogeneities, recent regeneration processes and silvicultural practices on genetic diversity of F. mandshurica in two typical landscapes in NEC. Ultimately, the genetic information overall the thesis would be useful of proposal for management and conservation of F. mandshurica in NEC, which was thoroughly demonstrated in the general discussion of chapter 5.

Chapter 1 generally introduced the status of forest resources in NEC, importance of genetic information in forest management and ecology, biology and silviculture of the study species. Then, chapter 2 presented a retrospect of the previous work of population genetic studies of forest tree species using molecular markers, focusing on the factors affecting the patterns of population genetic structure of forest trees. Population genetic knowledge of related Fraxinus species in Europe and tree species in NEC was also summarized in this review chapter.

In chapter 3, a wide-scale phylogeographical investigation indicates the possibility of F. mandshurica maintaining a high level of genetic diversity within extant adult populations across NEC. The intra-population genetic diversity (both allelic richness and expected heterozygosity) were significantly negatively correlated to latitude. These results were possibly associated with the systematically northward shifts of forest biomes in eastern China during the mid-Holocene. Conversely, a low level of genetic differentiation and a lack of modern genetic structure were detected among these populations across NEC (with the exception of Jianxin). These results might be elucidated by the abundant (historical) gene flow and the presence of a wide refuge rather than multiple small refugia in NEC during the last glacial maximum. In contrast, the most northerly population in NEC (Jianxin) was significantly diverged from the continuous gene pool in NEC and exhibited the lowest allelic diversity. It proposes that Jianxin population requires priority conservation efforts to prevent it from being extirpated from its native habitat, such as the establishment of a distinct management unit and in the combination of in situ and ex situ conservation strategies.

In this chapter, the phylogeographical structure of F. mandshurica var. japonica was also investigated on a wide-scale in Japan - one of the species' traditionally marginal ranges. Contrasting patterns of genetic variation and population structure were identified between northern (Hokkaido and Shimokita) and southern (Honshu) populations in Japan. Based on the common usage of microsatellites, similar sampling scheme and balanced analysis algorithm however, populations in NEC showed significantly lower levels of allelic richness and expected heterozygosity than those in populations of var. japonica in Japan. These results seemed to disagree with the classical central-peripheral hypothesis that genetic diversity within peripheral populations is normally reduced as compared to more central populations. Bayesian clustering algorithm further identified all individuals into the NEC cluster and Japan cluster. This suggests that long-term interglacial isolation might have led to aggravation of population differentiation and speciation of intraspecific species (var. japonica). The opposite patterns of genetic variation and phylogeography of F. mandshurica between NEC and Japan allowed the development of effective conservation strategies for this species.

In chapter 4, spatial analysis of molecular variance and principal coordinate analysis consistently revealed that F. mandshurica exhibited landscape genetic structure across a riparian-mountain system, despite the overall low level of genetic differentiation between populations. This is probably due to the different effects of landscape heterogeneities (riparian corridors and mountain obstacles) on seed dispersal. Conversely, there was no spatial genetic structure among populations within the riparian system, suggesting that the Hydrochory corridors might enhance genetic connectivity among riparian populations. A significant accumulation of genetic diversity within downstream populations was not detected, probably because gene flow in F. mandshurica is prevailed by the primary, bidirectional wind-dispersal.

In a severe anthropogenic landscape, similar magnitudes of intra-population genetic diversity of F. mandshurica were detected in the recently colonized populations (seedlings and juveniles) naturally regenerated in the L. gmelinii plantation forests or open farmlands, when they were compared to adjacent adult populations. Spatial autocorrelation analysis failed to detect a significant fine-scale spatial genetic structure in any of the recent generations or natural populations. On the contrary, two seedling cohorts in the adult forests exhibited significantly positive fine-scale spatial genetic structure over short distances and also demonstrated a significant trend of lower allelic richness but higher inbreeding coefficient than those in the adult forests. These results suggest that the seedling cohorts were probably half-siblings and largely originated from a few related parents within their respective adult forests. In addition, this study also suggests that the negative impacts of recent anthropogenic practices (selective thinning and artificial regeneration) on the genetic diversity and/or fine-scale spatial genetic structure of F. mandshurica are negligible.

In the light of the overall genetic information of F. mandshurica, the fundamental proposal on conservation and management practices for this species in NEC was ultimately formulated in chapter 5. In general, the present neutral genetic resources of F. mandshurica in NEC may not be seriously threatened; thus it dose not necessitate specific conservation programmes for solely safeguarding intra-population genetic diversity. The properly applied silvicultural treatments and recent regeneration processes on F. mandshurica may not lead to genetic degradation in the future generations. However, it requires effective efforts to maintain or enhance genetic connectivity among present populations on landscapes and among future generations at the whole scale of NEC.

本論文は、中国東北地方の重要樹種であるヤチダモ(Fraxinus mandshurica)を対象として、9つの核マイクロサテライト遺伝子座を用いた集団遺伝学的研究により、その遺伝的多様性の実態把握と保全への提言を行ったもので、5章からなる。

第1章では、中国東北地方(以下中国)の森林の現状、森林管理における遺伝的情報の重要性ならびに対象樹種の種特性について記述した。第2章では、森林樹木における遺伝的多様性研究をレビューし、中国における研究の必要性を示した。

第3章では、集団の遺伝的多様性解析に基づき、中国および日本における系統地理学的構造を明らかにした。まず、中国の広域にわたる研究から、現存の成木集団の遺伝的多様性は十分高く維持されていること、また、集団間の遺伝的分化の程度は低く、1集団を除き全域において明確な遺伝構造は認められないことを明らかにした。一方、集団内の対立遺伝子多様度およびヘテロ接合度の期待値には、経度と有意な負の相関が認められたが、これは、完新世中期における、本種を含む森林帯の北進に伴って形成されたものと考えられた。これらの結果は、これまで十分な集団間の遺伝子流動が起こってきたこと、また最終氷期時のレフュージアが、複数の小さな集団からなるものでなく、比較的広域に及ぶ単一のものであったことを示している。例外的に、最北に位置する1集団は、他集団から遺伝的に大きく離れており、独自の保全が必要と指摘された。

次いで、日本のヤチダモ(F. mandshurica var. japonica)について、分布域を網羅する研究を行った。その結果、北海道と下北半島の集団を含む北方クラスターとそれより南の本州クラスターの二つが抽出された。中国と日本の集団を合わせた解析の結果、両者は明確に区別され、長期間の地理的隔離が遺伝的分化を導いたことが示された。一方、日本のヤチダモ集団の遺伝的多様性は、中国集団に比べて有意に高かった。このことは、分布の端の集団は、中心集団に比べて遺伝的多様性が低いという一般仮説に反する。これは、日本における本種の分布における複雑な歴史を反映したものと考えられた。

第4章では、中国の一地域において、ランドスケープレベルの遺伝的多様性について、河畔域と山地の集団の比較および人為的影響の強い景観での更新や施業の影響の検討を行った。まず、河畔域と山地の集団の比較から、前者では集団間の分化の程度は低く、集団内遺伝構造が見られないこと、後者では集団間の分化の程度が大きく、明瞭な遺伝構造が認められることを明らかにした。これらは、比較的狭いランドスケープ内でも、種子の散布様式や遺伝子流動パターンの違いに起因して、異なる遺伝構造が形成されることを示している。

次いで、強度の人為的景観改変を受けた地域において、グイマツ(Lalix gmelinii)植林地と開けた農地に天然更新した若齢木集団、天然林および林内に更新した実生集団の遺伝的多様性を調べた。若齢木集団では周囲の成木集団と同程度の高い値に保たれており、いずれも集団内の空間的遺伝構造は認められなかった。また、若齢木集団、成木集団共に、間伐による遺伝構造の変化は認められなかった。さらに、人工造林地でも天然林と同程度の遺伝的多様性が確認された。これらのことから、現在のところ、人為は本種の遺伝的多様性や集団内の遺伝構造に大きな影響を及ぼしていないと推察された。一方、実生集団では、近距離における空間的遺伝構造が認められること、成木集団に比べて対立遺伝子多様度は有意に低く、近交係数は有意に高い事が明らかになった。これらは、実生更新時に寄与した成木の偏りによるものと考えられ、ヤチダモ林分の更新において、今後注意を払うべき点であると考えられた。

第5章では、以上の結果を踏まえて、中国東北地方におけるヤチダモ林の管理を提案した。現在の中国東北部におけるヤチダモについては、遺伝的な劣化はそれほど深刻ではないことが示された。したがって、遺伝的変異の保全上、本種のみに限った特別な保護対策は必要ないと考えられる。しかし、将来世代の遺伝的多様性の維持に対しては、景観内および世代間での遺伝的連結性を維持するため、更新プロセスの維持や適切な森林施業が不可欠であると結論された。

以上、本論文は、ヤチダモについて系統地理学的な観点からその特徴を明らかにし、さらに、ランドスケールレベルでの遺伝的多様性の実態を明らかにし、それらに基づいて具体的な森林の保全・管理方法を提案にしたものであり、学術上、応用上寄与するところが少なくない、よって審査委員一同は本論文が博士(農学)の学位論文として価値あるものと認めた。