球背象鼻蟲與擬硬象天牛之譜系發育、生物地理與擬態

Abstract

物種如何演化出多樣的斑紋，與其他物種間的交互關係又是如何形塑外形的多樣性是研究生物多樣性重要的議題。主要分布在菲律賓群島等海洋島嶼的球背象鼻蟲（Pachyrhynchus）具有多樣且色彩繽紛的斑紋，且和共域的擬硬象天牛（Doliops）有極為相似的斑紋。雖然球背象鼻蟲的斑紋被認為和警戒訊號或隱蔽等功能有關，但是斑紋的演化過程仍不清楚，因此本研究第一部分藉由親緣基因體學的方式重建球背象鼻蟲的親緣關係以及物種分化的分歧時間，並釐清斑紋的演化過程，檢測球背象鼻蟲的地理分佈與斑紋的關係。親緣關係結果顯示球背象鼻蟲具有八個主要的分支，並且大多數的物種分化發生在上新世。球背象鼻蟲可能主要由呂宋島起源，而特定的斑紋與地理有顯著的關聯性，如方格斑紋與呂宋北方島嶼的物種、圓斑與民都洛島的物種。斑紋之間的轉換可能是很頻繁的，較常見的圓斑被推論為斑紋的祖先特徵，而網狀斑紋的演化則較保守。由於球背象鼻蟲堅硬的外骨骼以及亮麗的外觀可作為降低捕食的防禦機制，因此擬硬象天牛翅鞘上的色彩斑紋被認為與球背象鼻蟲的貝氏擬態有關。然而擬硬象天牛與球背象鼻蟲的演化關係仍不清楚，加上擬態對之間的界定主要依據人為主觀的判斷，因此本研究第二部分透過基於親緣關係樹的距離以及演化事件的方法評估兩者共演化的程度，並且估算擬態對之間的分歧時間。此外，使用光譜儀測量斑紋及身體的顏色反射光譜，以六種潛在掠食者的視覺模型計算可偵測差異值（JNDs）以量化球背象鼻蟲以及擬硬象天牛的顏色相似程度。研究結果顯示擬硬象天牛與球背象鼻蟲的演化關係具有關聯性，然而大多數擬硬象天牛的分歧時間早於被擬態的球背象鼻蟲。視覺系統模型的結果發現擬硬象天牛與球背象鼻蟲的彩色顏色難以被掠食者區分，因此擬硬象天牛可能透過擬態球背象鼻蟲的顏色而得到保護。

How species evolve diverse color pattern and how species interaction shape the diverse appearance are the important issues to biodiversity research. Pachyrhynchus weevils are insular species that predominantly distributed across the Philippine archipelago, and display extremely diverse and colorful pattern. Sympatric Doliops longhorn beetles also show the similar colorful pattern with Pachyrhynchus weevils. Although those color patterns of Pachyrhynchus weevils are known to function as an aposematic signal or camouflage, the evolutionary history of color patterns on Pachyrhynchus weevils remain unclear. In chapter 1, the phylogenomic methods were used to reconstruct the phylogenetic relationship and to estimate divergent time of diversification in Pachyrhynchus weevils, and the evolutionary process of patterns was inferred. I also investigated the association between geographic distribution and patterns. The result of phylogenetic inference showed that there were eight major clades in Pachyrhynchus weevils, and most of diversification events occurred in the Pliocene. Pachyrhynchus weevils might origin from Luzon, and the significant associations between checker and northern islands, and between spots and Mindoro were found. Patterns have transited within Pachyrhynchus weevils frequently. Spots pattern was inferred as the ancestral character, and the evolution of reticulation pattern was relatively conserved. The hard elytra and colorful appearance in Pachyrhynchus weevils may function as antipredator mechanisms, and Doliops longhorn beetles are hypothesized to mimic Pachyrhynchus weevils with similar color pattern. However, coevolutionary relationships between Doliops longhorn beetles and Pachyrhynchus weevils were still unclear, and the mimetic pairs were defined subjectively. In chapter 2, I examined the degree of coevolution by using phylogenetic distance-based approach and event-based approach, and assessed divergent time within mimetic pairs. Besides, reflectance spectrum of color patches on pattern and body was measured to quantify color similarity between Doliops longhorn beetles and Pachyrhynchus weevils by Just Noticeable Differences (JNDs) based on the six visual model systems of potential predators. The results showed that the co-phylogeny occurred in Doliops longhorn beetles and Pachyrhynchus weevils, and most of Doliops longhorn beetles diverged earlier than Pachyrhynchus weevils. The visual model analyses indicated that chromatic colors in mimetic pairs were difficult to be distinguished by the predators, Doliops longhorn beetles might mimic the color of Pachyrhynchus weevils to get the protection.