金毛杜鵑族群分化、族群波動及在地適應性演化
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2019
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闡明現今森林樹種分布曾歷經何種演化過程是演化生物學的核心主題，金毛杜鵑(Rhododendron oldhamii)是一種在台灣廣泛但不連續分布的特有種闊葉樹種，闊葉植物過去深受第四紀冰河期與間冰期的氣候交替而分別地限縮和擴張其棲地，並在近期受到全球氣候暖化和人為活動干擾的影響而呈現棲地破碎化。棲地擴張導致的遺傳歧異度上升可以增加族群的適應能力，而族群在面臨不同棲地的天擇壓力時，棲地破碎化造成的基因交流減少更能促使族群產生在地適應性。本研究使用18個金毛杜鵑族群及26個位於表現序列標籤中的簡單重複序列(expressed sequence tag-simple sequence repeat，EST-SSR)基因座、六個開花相關基因和六個非開花基因來研究金毛杜鵑的遺傳歧異度、族群結構、族群波動歷史和基因交流，並檢測金毛杜鵑是否已演化出因應當地環境的在地適應性。貝氏分群(Bayesian clustering)的分析結果顯示金毛杜鵑族群可被分為四個區域性群組，包含北部、中部、南部和東南部，而多種族群波動歷史的分析結果均顯示金毛杜鵑過去經歷族群擴張，而且錯位分布(mismatch distribution)的分析結果顯示金毛杜鵑符合空間擴張模型(spatial expansion model)的預期分布，此外，四個金毛杜鵑群組長期以來均維持頻繁的基因交流，但大部分的近期基因交流則只發生在各個區域性群組內，而本研究也找到兩個EST-SSR基因座及開花相關基因SPA1的遺傳變異與溫度、水分的可獲得性、地表植被、空間變異、地形因子和風速具有顯著的關聯性，以及開花相關基因AMP1是受到弱淨化天擇的作用。本研究的結果顯示環境異質性已造成金毛杜鵑族群產生在地適應性，金毛杜鵑在冰河期過後的棲地擴張可能造成遺傳歧異度上升並增加金毛杜鵑對環境異質性的適應能力，而金毛杜鵑族群在受到不同天擇壓力的情況下，棲地破碎化反而減少不適合的等位基因進入族群，促使金毛杜鵑演化出在地適應性。
Elucidation of the patterns and processes shaping current distributions of forest species is a central theme in evolutionary biology. Rhododendron oldhamii, an endemic broadleaf tree species, is presently widespread but discontinuously distributed in Taiwan. Cyclic glacial/interglacial events in the Quaternary have influenced distributions and demography of broadleaf species. Moreover, global warming and recent anthropogenic disturbances altering distributions of native forest lead to habitat fragmentation for plants. An increase of genetic variation caused by postglacial range expansion can enhance the adaptive potential of populations. Reduced gene flow among populations due to habitat fragmentation can facilitate local adaptations of populations under different selective pressures. Here, we use 26 expressed sequence tag-simple sequence repeat (EST-SSR) loci, six non-flowering-related genes, and six flowering-related genes from 18 R. oldhamii populations to investigate patterns of genetic diversity, population structure, demographic history, gene flow, and local adaptation. Bayesian clustering analysis revealed four regional population groups of north, central, south and southeast. Results of demographic analyses indicated R. oldhamii has undergone a demographic increase. Range expansion of R. oldhamii was suggested by a goodness-of-fit of the observed mismatch distributions to the expectation under the spatial expansion model. Large estimates of long-term immigrants and limited recent migration between regional groups was found. Moreover, two EST-SSR loci and the flowering-related gene SPA1 displayed a pattern of adaptive divergence in response to temperature, the water availability, vegetation, spatial variation, topographic factors, and wind speed, and the flowering-related gene AMP1 was subjected to weak purifying selection. Our results suggested that environmental factors and spatial variation have played a prominent role in driving local adaptation in R. oldhamii. An increase of genetic diversity due to demographic expansion and frequent gene flow during postglacial range expansion can enhance the adaptive potential of populations in response to local selection pressures. Besides, a reduction of maladapted gene flow due to habitat fragmentation can facilitate adaptations of populations under different selective pressures.
Elucidation of the patterns and processes shaping current distributions of forest species is a central theme in evolutionary biology. Rhododendron oldhamii, an endemic broadleaf tree species, is presently widespread but discontinuously distributed in Taiwan. Cyclic glacial/interglacial events in the Quaternary have influenced distributions and demography of broadleaf species. Moreover, global warming and recent anthropogenic disturbances altering distributions of native forest lead to habitat fragmentation for plants. An increase of genetic variation caused by postglacial range expansion can enhance the adaptive potential of populations. Reduced gene flow among populations due to habitat fragmentation can facilitate local adaptations of populations under different selective pressures. Here, we use 26 expressed sequence tag-simple sequence repeat (EST-SSR) loci, six non-flowering-related genes, and six flowering-related genes from 18 R. oldhamii populations to investigate patterns of genetic diversity, population structure, demographic history, gene flow, and local adaptation. Bayesian clustering analysis revealed four regional population groups of north, central, south and southeast. Results of demographic analyses indicated R. oldhamii has undergone a demographic increase. Range expansion of R. oldhamii was suggested by a goodness-of-fit of the observed mismatch distributions to the expectation under the spatial expansion model. Large estimates of long-term immigrants and limited recent migration between regional groups was found. Moreover, two EST-SSR loci and the flowering-related gene SPA1 displayed a pattern of adaptive divergence in response to temperature, the water availability, vegetation, spatial variation, topographic factors, and wind speed, and the flowering-related gene AMP1 was subjected to weak purifying selection. Our results suggested that environmental factors and spatial variation have played a prominent role in driving local adaptation in R. oldhamii. An increase of genetic diversity due to demographic expansion and frequent gene flow during postglacial range expansion can enhance the adaptive potential of populations in response to local selection pressures. Besides, a reduction of maladapted gene flow due to habitat fragmentation can facilitate adaptations of populations under different selective pressures.
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金毛杜鵑, 族群分化, 族群波動, 在地適應性, Rhododendron oldhamii, population differentiation, population demography, local adaptation