探討花蜜之咖啡因在生態扮演之角色 – 以西方蜂之生理學角度切入

Abstract

植物產生之次級代謝物具有防禦的功能，在植物的葉子或種子中大量累積，以阻止草食性動物的侵害。有趣的是，次級代謝物也被產生在植物的花蜜中，能被協助植物授粉的昆蟲所攝取，使得次級代謝物在生態中扮演的角色出現矛盾。諸多研究認為植物利用次級代謝物來吸引授粉昆蟲，片面增加植物本身的授粉率，而降低了回饋的蜜源品質。然而，相對較少有研究探討次級代謝物在授粉昆蟲的作用機制，使得「次級代謝物剝削說」忽略了授粉昆蟲所能得到的益處。本研究以西方蜜蜂作為模式物種，並以植物次級代謝物——咖啡因作為研究材料，以西方蜜蜂生理學角度切入，探討咖啡因在西方蜜蜂與植物的交互作用中扮演的角色，最終以生態的角度解釋研究結果。我們調查咖啡因對蜜蜂大腦中腺苷(adenosine,Ado)路徑的影響，研究了下游二次信使(secondmessenger)環狀腺苷酸(cyclicAMP,cAMP)以調查配體-受體(ligand-receptor)相互作用。在咖啡因處理後，進一步研究生理和行為結果，如能量代謝、基因表達、神經活動和記憶功能。還檢驗了蜜蜂對食源的偏好，以更好地理解蜜蜂是否總是會選擇含咖啡因的花蜜。cAMP測量結果顯示，咖啡因在蜜蜂大腦中與其配體競爭對手Ado發揮協同作用(synergisticeffects)。啟動cAMP/PKA/CREB路徑，有助於ATP生產、基因表達和大腦中的鈣離子活動，包括增強的長期記憶。於抗病毒的實驗中，我們亦證實餵食咖啡因能增強蜜蜂的免疫相關基因表現，並降低病毒在蜜蜂體內的複製量。蜜蜂對糖水的偏好性測試結果顯示，含咖啡因的花蜜並不總是蜜蜂的首選，表明咖啡因作為植物的片面利用工具效果有限。我們的研究從生理和行為角度提供了關於蜜蜂有效利用咖啡因的觀點。以咖啡因與蜜蜂的例子，次級代謝物的確可以在周圍蜜源養分含量相當的情況下，為植物帶來更高的吸引力，然而授粉昆蟲並不會一昧的被植物的次級代謝物吸引，相反的，他們偏好挑選養分含量較高的蜜源。取食到咖啡因的蜜蜂也並不會單純被植物剝削，而是利用了咖啡因提升自身的生理、行為與抗病的表現，而其作用即是透過強化cAMP/PKA/CREB路徑。我們總結，花蜜中的咖啡因應為加強植物-傳粉者共生關係的互利共生工具，植物與授粉昆蟲皆能從咖啡因獲得相應的益處。未來，更多的次級代謝物與授粉昆蟲能個別納入研究與討論，為花蜜中的次級代謝物提出更完整的生態地位。

Plants produce secondary metabolites as a defending strategy. Interestingly, plants also produce those phytochemicals in nectar, which is generally a reward for pollinators. Previous studies suggested this phenomenon might be ‘pollinator manipulation,’ that plants used secondary metabolites to influence the foraging decision of pollinators regardless the nutrition quality. However, few studies focused on pollinators’ perspectives of the positive effects brought by these secondary metabolites in nectar and their physiological mechanisms. In the present study, pollinating insect, honey bees, and common phytochemical in nectar, caffeine, were used as our model. The caffeine mechanisms were focused on the interactions with adenosine receptors of honey bee. Both results in vitro and in vivo experiments suggested that caffeine synergistically enhanced the down-stream signaling of adenosine pathway. The enhancements on ATP production, gene expression, and memory ability were also examined after caffeine treatments. The other possible caffeine mechanisms already known in previous studies such as ryanodine receptor agonism were also explored, and the results showed negative. The benefits of consuming caffeine were confirmed in honey bee’s immune responses and antiviral effects. The results of foraging preference tests indicated that sugar content was a more important factor to influence the decision of foragers for searching food. Finally, our results supported that caffeine in nectar might strengthen the plant-pollinator mutualism, rather than unilateral exploitation, since both plants and pollinators could potentially benefit from the secondary metabolites in nectar.