臺灣檫樹開花行為與微衛星體基因座之特性分析

Abstract

特有種的臺灣檫樹為臺灣闊葉一級木，亦為珍貴保育類動物寬尾鳳蝶幼蟲的唯一食草植物，在經濟與生態上都具有重要的地位，但是臺灣檫樹只零星分布在臺灣中海拔山區，數量稀少且族群間相互隔離，被認定為易受害物種，需要人為介入經營保育。臺灣林業界長期以來對臺灣檫樹的研究不曾間斷，但由於臺灣檫樹種子休眠期長、小苗死亡率高，復育工作尚待進一步突破。另一方面，臺灣檫樹的生殖生物學在過去缺乏研究，花部形態沒有一致性的論述，Chung et al. (Ann. Missouri Bot. Gard. 97: 1-10. 2010) 在最近的研究中指出，臺灣檫樹實為雌先熟的兩性花，且可能具有同調雌雄異熟的開花行為。本研究於 2009 年與 2010 年對棲蘭山區及太平山的 4 株臺灣檫樹進行觀察與記錄，追蹤時間包含整個花季，共紀錄花芽 165 枚，並以定時連續攝影的方式拍攝 3 枚不同植株的花芽，記錄花芽從萌發到凋謝的過程。研究發現，臺灣檫樹雖然為雌先熟的雌雄異熟完全花，但是並非原先預期的同調雌雄異熟，而為異調雌雄異熟，臺灣檫樹花芽剛萌發出的花為柱頭溼潤，花藥閉合的雌功能花，但是雌功能花最多維持 4 天，便開始轉為柱頭萎縮、花藥開裂的雄功能花。本研究觀察時間最長為 14 天，8–13 天內都有雌花出現，雄花比例在約 7 天後會快速上升且超越雌花比例，顯示檫樹的柱頭與花粉可以同時存在同一株植物上，本研究清楚地表示檫樹為異調雌雄異熟，並顯示其有同株異花受粉的可能性。影像紀錄不僅記錄了花朵形態，亦捕捉到許多動物在花上活動的畫面，最常出現者為雙翅目昆蟲，夜間另有鱗翅目昆蟲。另一方面，為了進一步探討臺灣檫樹族群的遺傳多樣性與遺傳結構，並探討遺傳多樣性與生殖生物學間的關聯，本研究著手開發篩選臺灣檫樹的多型性微衛星體基因座作為遺傳標記，自臺灣北部採集到的 53 個個體中，開發出 14 組具有多型性的微衛星體序列，其中 8 組顯著偏離哈溫平衡，對偶基因數在 2–7 之間，平均為 4.07。預估異型合子比例 (HE) 在 0.743–0.126 之間，平均值為 0.475，實際異型合子比例 (HO) 在 1.000–0.081 之間，平均值為 0.483。比較臺灣其他維管束植物族群的微衛星體分析結果後，本研究認為臺灣檫樹族群的遺傳多樣性並不高，形成原因可能是因為瓶頸效應或是近親交配所造成。

Sassafras randaiense (Lauraceae), an ecologically and economically important tree noted as the sole host plant of the highly endangered broad-tailed swallowtail butterfly (Agehana maraho), is a rare and vulnerable endemic species with scattered and isolated populations in the mid elevation forest of Taiwan. Although considerable research efforts had been invested to study and conserve this precious species, silviculture of S. randaiense remains challenging due to its low and variable seed production, deep seed dormancy, and high mortality of seedlings. Recently Chung et al. (Ann. Missouri Bot. Gard. 97: 1-10. 2010) reported that S. randaiense is hermaphroditic and protogynous with a possible sexual system of synchronous dichogamy, rejecting previous accounts claiming polygamy for the species. To test the proposition of synchronous dichogamy that may partially explain its unpredictable seed yield, the reproductive biology of S. randaiense was investigated during its flowering seasons in 2009 and 2010. With the aid of three scaffold observatories in Chilanshan and Taipingshan areas, the sexual phases of 165 inflorescence buds of four trees were recorded and tracked for their entire flowering periods. Photographs were also taken at a one hour interval for selected inflorescences to record the changes of sexual phase and flower visitors. Surprisingly, although our observations confirm the protogyny in S. randaiense, flowers within an individual tree did not change their sexual phase synchronously. Instead the bright and flesh white stigma, signifying the female phase of each flower, lasted for less than 4 days and then became brown and wilted, following by the male phase. For each tree, female phase flowers were present for 8 to 13 days, and at the 7th day, portion of male phase flowers exceeded those of female phase. Based on field observations and interval timing shooting, diptera were the most frequent flower visitors, with moths also being recorded by camera during the night time. Our data clearly show that asynchronous dichogamy better characterizes the sexual system of S. randaiense and geitonogamy is possible with an individual tree. In addition to the study of reproductive biology, polymorphic microsatellite loci were also isolated and characterized. Of the 14 polymorphic microsatellite loci developed, 8 deviate significantly from Hardy-Weinberg equilibrium. The number of alleles range from 2 to 7 and the average number is 4.07 alleles per locus. Expect heterozygosity ranges from 0.743–0.126 and observe heterozygosity is from 1.000–0.081. Compared to microsatellite genetic variation of other vascular plant species in Taiwan, S. randaiense has much lower low genetic diversity, likely resulted from bottleneck effect or inbreeding commonly observed in tree species of small and isolated population size.