共域分布但利用不同寄主植物的蝶類隱藏種群為異域起源

文獻來源: McBride CS, Velzen RV & Larsen TB. 2009. Allopatric origin of cryptic butterfly species that were discovered feeding on distinct host plants in sympatry. Molecular Ecology 18(17): 3639-3651. [Abstract]

近年熱帶昆蟲的研究持續發現隱藏種－意指由於形態近似，過去被歸類為同一種，但其實具生殖隔離的不同物種。然而此類研究大多局限於單一地點，因此對於這些隱藏種的地理分布與種內變異所知極為有限，這也造成在推測何種力量促成此多樣性時的困難。例如在許多廣食性的植食或寄生昆蟲被發現實為多種專食性物種的研究中，寄主轉移被認為與種化有關，但是寄主轉移發生於何時、何地？是否受到地理隔離的幫助？此研究使用最近於西非喀麥隆中部被發現，利用不同寄主植物，且在過去被認為皆屬Cymothoe egesta的兩個隱藏種蛺蝶來回答這些問題。作者群首先利用粒線體DNA的COI片段，試圖區分採自兩種不同寄主植物上的個體，再擴及所有採自泛熱帶非洲區的個體。結果出乎作者的意料之外，這兩種蛺蝶的分布格局幾乎是異域的，僅在初次被發現兩者共域的西非中部有小範圍的交集，再加上遺傳變異的分析結果，顯示分化過程應為異域種化，發生的時間可能在更新世。作者最後探討在理解新近於熱帶非洲發現的隱藏種種化時，此研究結果的意義何在，並認為在地理分布模式與寄主利用的課題上需要更多研究投入。

Abstract

Surveys of tropical insects are increasingly uncovering cryptic species – morphologically similar yet reproductively isolated taxa once thought to comprise a single interbreeding entity. The vast majority of such species are described from a single location. This leaves us with little information on geographic range and intraspecific variation and limits our ability to infer the forces responsible for generating such diversity. For example, in herbivorous and parasitic insects, multiple specialists are often discovered within what were thought to be single more generalized species. Host shifts are likely to have contributed to speciation in these cases. But when and where did those shifts occur, and were they facilitated by geographic isolation? We attempted to answer these questions for two cryptic species within the butterfly Cymothoe egesta that were recently discovered on different host plants in central Cameroon. We first used mtDNA markers to separate individuals collected on the two hosts within Cameroon and then extended our analysis to incorporate individuals collected across the entire pan-Afrotropical range of the original taxon. To our surprise, we found that the species are almost entirely allopatric, dividing the original range and overlapping only in the narrow zone of West-Central Africa where they were first discovered in sympatry. This finding, combined with analyses of genetic variation within each butterfly species, strongly suggests that speciation occurred in allopatry, probably during the Pleistocene. We discuss the implications of our results for understanding speciation among other cryptic species recently discovered in the tropics and argue that more work is needed on geographic patterns and host usage in such taxa.