[Article] 控制翅紋的基因重新定義毒蝶的擬態歷史

摘要 [原文網址]

The mimetic butterflies Heliconius erato and Heliconius melpomene have undergone parallel radiations to form a near-identical patchwork of over 20 different wing-pattern races across the Neotropics. Previous molecular phylogenetic work on these radiations has suggested that similar but geographically disjunct color patterns arose multiple times independently in each species. The neutral markers used in these studies, however, can move freely across color pattern boundaries, and therefore might not represent the history of the adaptive traits as accurately as markers linked to color pattern genes. To assess the evolutionary histories across different loci, we compared relationships among races within H. erato and within H. melpomene using a series of unlinked genes, genes linked to color pattern loci, and optix, a gene recently shown to control red color-pattern variation. We found that although unlinked genes partition populations by geographic region,optix had a different history, structuring lineages by red color patterns and supporting a single origin of red-rayed patterns within each species. Genes closely linked (80–250 kb) to optix exhibited only weak associations with color pattern. This study empirically demonstrates the necessity of examining phenotype-determining genomic regions to understand the history of adaptive change in rapidly radiating lineages. With these refined relationships, we resolve a long-standing debate about the origins of the races within each species, supporting the hypothesis that the red-rayed Amazonian pattern evolved recently and expanded, causing disjunctions of more ancestral patterns.

在系統發生學中，如果我們想要知道一個特徵的演化起源，也就是大家最想看的「怎麼演化」，通常會有個樹，上面有很多顏色很漂亮，然後你追著那個你想看的顏色，就會知道這個特徵發生了幾次，然後你在看圖說故事（譬如多年前有人探討竹節蟲翅的演化來源一樣）。重點就在於通常我們重建一棵樹的時候，理論告訴我們要選中性或偏中性的分子標記，不能選那些受到強烈天擇作用的標記，這樣才能純粹的反應物種間的關係，然後再用你想看的特徵去追溯，才會知道演化的來源。

而這如果用在擬態的演化這件事上會怎樣？通常你會看到不同擬態樣子的物種混在同一個支系裡，然後你用這個樹去追溯擬態的特徵，九成九會告訴你「所有的擬態特徵都是每個物種獨立演化多次來的」，所以哪個特徵先出現會變得模糊不清，原因就在於這些中性的分子標記是可以在各個物種間傳來傳去的，不一定會跟著強烈天擇的翅紋走，因此基於這些標記重建的樹無法反應擬態特徵的演化歷史。

所以為了知道這個特徵的演化來源與方向，這篇文章就用了控制斑紋的基因重建親緣關係，結果發現不同物種間但同樣斑紋的種類親緣關係較近，也就是在分子的層級上，擁有相同擬態翅紋的物種是趨同的，這就可以用來解釋很多事情，像是提供穆氏擬態的演化假說證據（註）等等。

當然要做到這個層級的前提有很多，像是要先知道各個控制斑紋的基因是哪個，先花大錢把序列全部定出來，實驗動物本身要能夠雜交容易飼養繁殖等等，相較於其他地區的擬態群來說，毒蝶提供很多這類的優勢，自然是個很好的模式動物。

註：穆氏擬態的演化假說是不同物種間的斑紋會越來越像，以求達最適化。