食物利用驅動專食性與廣食性果蠅腸道菌相之分化

Abstract

微生物菌群普遍存在於動物的腸道中，並且在它們宿主中發揮著不可或缺的作用。這些腸道微生物能為宿主帶來益處，並且可能促進宿主與共生菌的交互作用的形成以及在宿主食性適應的過程中與宿主發生共同演化。果蠅科昆蟲具有高度食性多樣性，並且其與微生物關係相當緊密。先前的研究對果蠅和微生物之間的關係已有相當程度的了解，然而在自然環境的條件下腸道微生物菌群與果蠅生態適應的關聯仍存在著許多未解的問題。本論文係以專食性與廣食性的不同果蠅物種為研究對象，比較腸道菌群在各食性不同的果蠅間的差異性，來探討腸道菌群與果蠅食性適應的關聯性。 為了解微生物菌群在果蠅生態適應中的重要性，本論文第一章節回顧了近十年對於野生果蠅群腸道菌相的研究。為了解在自然環境中影響腸道菌相組成的決定因子，本論文第二章節針對具有專一食性的姑婆芋小蠅以及其天南星科寄主植物的微生物菌進行16S 核糖體RNA (16S ribosomal RNA) 的定序並分析。結果顯示小蠅的腸道菌相組成差異主要是受到採集的年份，其次是小蠅物種與寄主植物的物種所影響。此外，在比較小蠅與寄主植物的菌群後發現兩者間的菌群有很大的差異，這表示小蠅的腸道菌群的建立並非是在取食的過程中從單純地從食物取得，小蠅的體內環境亦會對所獲取的細菌進行篩選。為進一步了解腸道菌群在果蠅科昆蟲食性適應的過程所扮演的角色，本論文第三章節對所有針對野生果蠅腸道菌群研究的數據進行收集。總共收錄了四類食性不同的十個果蠅物種，包括三類分別以天南星科植物為食、以菇類為食與以仙人掌為食的專食性果蠅，以及廣食性的果蠅。經重新分析並比較腸道菌群在各食性不同的果蠅間的差異性後發現，專食性與廣食性果蠅的腸道菌群有組成及功能上的分化。這些分化反映了專食性與廣食性果蠅對食物利用的需求上差異性，包括對糖及高分子碳水化合物的代謝、氨基酸的代謝、維生素B12的獲取以及解毒上的差異。顯示腸道菌群在果蠅的食性適應中可能扮演功能上互補的角色。最後為進一步證實食物轉變會對腸道菌群產生影響，本論文第四章節比較不同果蠅物種與品系在取食具有毒性諾麗果後，腸道菌相改變的異同。結果發現在測試的果蠅中，具有諾麗果適應的塞席爾果蠅 (D. sechellia) 以及具有諾麗果毒性抗性的黃果蠅 (D. melanogaster) 中的Oregon-R 品系，在取食諾麗果後腸道菌相組成會有一致的改變，意味著腸道菌在果蠅食物利用中具有功能性，以及腸道菌在面對宿主食物轉變時，具有迅速調節的可塑性。 綜合以上，本論文提出了腸道菌群與果蠅食物利用的關聯性，顯示了腸道菌在動物食性演化過程中可能扮演著比過去認為還要重要的角色。因此建議往後在進行動物食性演化的研究時，腸道菌也必須作為驅動及促進動物食性演化助力。

Bacteria commonly colonize the guts of almost all animals, playing indispensable roles in their animal hosts. The benefits of gut bacteria may facilitate host-symbiont interactions and coevolution in the dietary specialization of animal hosts. Drosophilid flies utilize a variety of diets and have a strong bond with microbes. Although the relationship between flies and microbes has been documented, there are still many unsolved questions about the association between flies and microbiota under natural conditions. To clarify the importance of microbiota of drosophilid flies in ecological adaptation, I first reviewed the studies of gut microbiota in the wild population of Drosophila species over the decade (chapter one). To reveal the ecological determinants of the gut microbiota composition in drosophilids, I focused on the Colocasiomyia flies, which only feed and breed in the inflorescence of Araceae plants. I sequenced and analyzed 16S rRNA amplicons of the microbiota from Colocasiomyia flies and their Araceae host plants. The results showed that the microbiota of Colocasiomyia flies was mainly shaped by the environmental factor, year, as well as fly species and host plant. Besides, the flies harbored a distinct gut microbiota from the microbiota in the host plants, suggesting that the microbiota of flies was not simply derived from the diets but was filtered by the intrinsic conditions of the flies (chapter two). To further understand the microbiota roles during the fly dietary specialization, I incorporated and re-analyzed published microbiota data from the wild population of drosophilids, including Araceae-feeding, mycophagous, and cactophilic specialists and generalists. The results showed that the gut microbiota were compositional and functional differentiated. The differences were mainly associated with the higher utilization of structural complex carbohydrates, protein utilization, vitamin B12 acquisition, and demand for detoxification in specialists than in generalists. It revealed functionally complementary roles of microbiota in the fly hosts (chapter three). Finally, to confirm the dietary effects on the microbiota composition, I investigated changes in microbiota after diet manipulation among different species and strains of Drosophila flies. I found a consistent shift in microbiota composition in D. sechellia and D. melanogaster Oregon-R, which are noni resistant or tolerant, after being manipulated to feed on noni fruit (chapter four). Altogether, this thesis demonstrated the association between gut microbiota and dietary utilization of drosophilid flies, revealing that the role of gut microbiota may be more important in dietary specialization than previous thought. Gut microbiota should be considered a force to drive and facilitate the dietary specialization of the animal host in the future.