薜荔和愛玉子及其授粉小蜂之遺傳分化

Abstract

榕屬植物 (Ficus spp.) 及其授粉榕小蜂間之共生關係是探討共演化的經典例子。榕屬植物倚賴榕果小蜂為其授粉，同時提供生長發育的營養及環境做為回報。兩者間的關係向來被認為具物種的專一性，並一起共演化。台灣原生種之榕屬植物：愛玉子 (Ficus pumila L. var. awkeotsang (Makino) Corner) 在分類上被歸為薜荔 (Ficus pumila L. var. pumila) 之變種，且這兩種榕屬植物的授粉小蜂被認為是同一種，Wiebesia pumilae (Hill)。但是薜荔與愛玉子的棲地環境、榕果果型、以及生化特性都不相同。因此推測它們在遺傳上，可能已經有分化的情形，而且榕屬植物的分化可能也一併造成其授粉榕小蜂的分歧。為探討榕屬植物以及其授粉榕小蜂的遺傳分化情形，本研究使用榕屬植物之3個DNA片段，葉綠體trnT-trnL基因區間 (IGS1)、核內基因nitrate reductase的第三個內含子 (NIA-i3) 以及核糖核酸內轉錄區II (ITS II)。授粉榕小蜂部分選取一個基因，粒線體cytochrome oxidase I (mtCOI) 基因，進行定序及分析。在榕屬植物間，NIA-i3以及ITS II沒有固定變異存在，而trnT-trnL基因區間的遺傳分化程度僅 0.3%。而授粉榕小蜂間之粒線體COI基因，存在著高達 11.9% 的遺傳分化。本研究的結果顯示，授粉榕小蜂在宿主植物分化之前已經有很大的遺傳分歧，並且暗示兩種榕屬植物可能處於種化的初期。在不同的授粉小蜂間，愛玉子授粉小蜂的雌性有效族群數約為薜荔授粉小蜂的 2.3 倍。由Bayesian skyline plot與mismatch distribution的結果顯示，前者在最近的冰河期歷經了族群擴張。親緣地理的分析結果發現，採自離島與高雄的薜荔授粉小蜂與台灣其他地區的授粉小蜂有明顯的區隔。在與外群比較後發現，薜荔之授粉榕小蜂與日本珍珠蓮之授粉榕小蜂Wiebesia callida之關係非常接近，暗示了宿主轉移的可能性，但其真正的機制仍待進一步研究。

The obligate mutualism between pollinating fig wasps in the family Agaonidae (Hymenoptera: Chalcidoidea) and Ficus species (Moraceae) is often regarded as a classic example of co-evolution. Each fig species is generally pollinated by an unique species of fig wasps. Figs depend on its own specific species of wasp for pollination and the larvae of these wasps develop in their specific host. This relationship between figs and their pollinators is considered to be species-specific and may lead to co-speciation. The endemic jelly fig (Ficus pumila L. var. awkeotsang (Makino) Corner; awkeotsang) is a variety of creeping fig (F. pumila L. var. pumila; pumila) and both of them are thought to share a common pollinator wasp, Wiebesia pumilae (Hill). Since jelly and creeping figs are different in their habitats, forms of figs, and biochemical component of figs, it is reasonable to suggest the existence of genetic differentiation in between. Moreover, the differences between host figs may also have promoted genetic differentiation between their pollinators. According to our hypothesis, three DNA segments from figs, including chloroplast trnT-trnL intergenic spacer (IGS1), third intron of nitrate reductase (NIA-i3), and ribosomal intergenic transcript spacer II (ITS II), and one gene from wasps, mitochondrial cytochrome oxidase I (mtCOI), were sequenced and analyzed. Between two figs, no fixed difference was found in NIA-i3 and ITS II, and genetic distance recovered from IGS1 was only 0.3%. The nucleus gene flow between two fig populations is evident. In contrast, genetic distance derived from mtCOI gene was 11.9% between two pollinators. These results insinuate that the pollinator fig wasps may have been differentiated before the divergence of their host, and different varieties of F. pumila may be in the early stage of speciation. Within different pollinators, the long-term female effective population size of awkeotsang pollinator is estimated to be 2.3 times larger than that of pumila pollinator. Results derived from Bayesian skyline plot and mismatch distribution suggest the former have underwent population expansion during the last glacial epoch. Phylogeographic analysis indicates that populations of pumila pollinator can be divided into two groups, samples from Kaohsiung and off-shore islands are distinct from samples from the rest of Taiwan Island. Comparing with the outgroups, the indistinguishable relationships between pumila pollinator and the pollinator of F. sarmentosa, Wiebesia callida, was observed, implying the possibility of recent host transfer. The detailed mechanism may need further investigation.