東亞特有種苦苣苔親緣地理、花發育及花對稱性基因自然選汰

Abstract

研究歷史事件如何影響現今物種分布模式與分子調控機制如何控制發育過程是生物學上重要的研究議題。在本論文中，苦苣苔(Conandron ramondioides) 被用來研究這兩個議題。理由有三。第一，自第四紀以來，東中國海海床曾因冰期循環造成海平面下降形成陸橋而多次連結亞洲陸塊與鄰近島嶼。現今零散且間斷分布的苦苣苔族群(包含日本、東南中國、台灣與西表島)是否曾經透過東中國海陸橋傳播是第一個研究目標。第二，花對稱性基因在兩側對稱花侷限表現於背側。花對稱性基因在輻射對稱花的苦苣苔花部表現是否有所改變為第二個研究議題。第三，多條花對稱性調控基因(GCYCs)在同屬於Asiatic Trichosporeae族的苦苣苔與兩側對稱花物種(如角桐草、石吊蘭)中被分離。Asiatic Trichosporeae族物種保留多條花對稱性調控基因的可能機制與多樣的花對稱性調控基因表現模式的關連是第三個研究議題。 重建多基因座親緣樹顯示現今零散且間斷分布的苦苣苔族群形成三個與地理區域相符合的分群，分別為本州-四國群、東南中國群與台灣-西表島群。三群間並沒有群間共享單套型且各群間缺乏基因交流顯示即使東中國海陸橋有多次連結亞洲陸塊與鄰近島嶼，苦苣苔族群仍無法跨越東中國海陸橋進行基因交流。結果顯示東中國海陸橋對苦苣苔是無法跨越的障礙(過濾屏障)。第二，檢視苦苣苔花部發育過程發現苦苣苔的雄蕊輪與花瓣輪自花原基分化後維持等速生長直到開花且花對稱性調控基因(GCYCs)不表現於苦苣苔的雄蕊輪與花瓣輪。因此推論苦苣苔發育為輻射對稱花是透過失去花對稱性調控基因在背側表現而達成。第三，由重建的Asiatic Trichosporeae GCYCs親緣樹發現 GCYCs基因形成GCYC1C與GCYC1D兩群。分析天擇訊號(ω)在重複事件發生前後的改變，結果顯示ω由重複事件發生前的0.2819上升至發生後的0.3985，呈現由遺傳限制釋放的訊號(relaxation)。由於Asiatic Trichosporeae 族中兩側對稱花或是輻射對稱花物種的GCYCs基因都呈現多樣的表現模式。我們推論在Asiatic Trichosporeae 族中GCYCs重複事件發生後的GCYC1C與GCYC1D拷貝由遺傳限制釋放創造了 ” 演化的彈性窗戶(evolutionary window of flexibility)” 使得多條GCYC1C與GCYC1D拷貝能被保留且各自獲得在花部多樣的基因表現。

Studying how historical events shaping current species distribution pattern and molecular regulatory mechanism in controlling developmental process are important issues in biology. In this dissertation, Conandron ramondioides was used to study these two issues. The reasons are 1. East China Sea (ECS) seafloor connected Asia continent and adjacent islands repeatedly due to sea level lowering during glacial period since Quaternary. First, whether current scatter and disjunct distributed C. ramondioides populations (distributed in Honshu, Shikoku, Kyushu, Southeast China, Taiwan and Iriomote) disperse through ECS landbridge in the past? Second, floral symmetry determining gene expressed in dorsal region in zygomorphic flower species. Whether floral symmetry determining genes alter their expression pattern in C. ramondioides, which have actinomorphic flower? Third, multiple floral symmetry determining genes (GCYCs) were isolated from Asiatic Trichosporeae species, including actinomorphic flower species (e.g. C. ramondioides) and zygomorphic flower species (e.g. Hemiboea bicornuta and Lysionotus pauciflorus). To trace what kind of selection force in maintaining multiple GCYCs and find association between divergent expression pattern of duplicated GCYCs in both actinomorphic and zygomorphic flower species in Asiatic Trichosporeae species are the third research aim in this dissertation. First, reconstructed multiloci phylogeny showed that current scatter and disjunct distributed C. ramondioides populations form three geographical corresponding groups, including Honshu-Shikoku, Southeast China and Taiwan-Iriomote group. Lack of shared haplotypes and restricted gene flow among groups suggested migration of C. ramondioides populations through ECS landbridge is unlikely. My results suggested ECS landbridge serves as filter instead of dispersal corridor for C. ramondioides populations. Second, RT-PCR results showed there is no GCYCs expression in flower in C. ramondioides. Thus, loss of dorsal region specific floral symmetry determining genes (GCYCs) expression in C. ramondioides has evolved to switch zygomorphy to actinomorphy. Third, GCYCs isolated from Asiatic Trichosporeae species with divergent expression pattern in flower formed GCYC1 clade, comprising GCYC1C subclade and GCYC1D subclade. Relaxation from selection right after the GCYC1 duplication (ω pre-duplication = 0.2819, ω post-duplication = 0.3985) was detected among GCYC1C and GCYC1D. Expression pattern of GCYC genes of selected zygomorphic flower species (Hemiboea bicornuta and Lysionotus pauciflorus) exhibits dorsal restricted and copy specific pattern (GCYC1C for H. bicornuta and GCYC1D for L. pauciflorus). Together with previously published data, it appeared that GCYC1C and GCYC1D copies diversified their expression in a distinct species-specific pattern. I propose that the selection relaxation after the GCYC1 duplication created an 'evolutionary window of flexibility' in which multiple copies were retained with randomly diverged roles for dorsal-specific expressions in association with floral symmetry changes.