鄰域之白頭翁與烏頭翁母鳥擇偶偏好是否足以構成生殖隔離之研究

Abstract

雜交帶的動態反映了相鄰兩族群的基因交流情形，因此長久以來被視為「演化之窗」。然而許多研究指出，此基因交流因雌性擇偶偏好而受阻。若雌性擇偶結果為同型交配，則會造成強化作用，使雜交的兩族群產生交配前生殖隔離；當兩個族群漸趨穩定分離，雜交帶範圍也會因而無法擴張。 在臺灣一百三十多種陸棲鳥類中，只有烏頭翁與白頭翁的地理分布呈現東西分隔的情形。雖然此兩相鄰的鵯屬族群可輕易地以其分布範圍與個體的頭羽顏色辨別，但是兩種鳥不論在身型大小、繁殖季節、食性、歌曲與行為都難以區分。最近以烏、白頭翁遺傳標記進行的研究更顯示兩族群為由同一祖先分支的姊妹種；即便存在雜交帶，野外觀察發現兩族群地理分布的東西相隔情形仍屬穩定。為了解兩族群是否已產生了交配前生殖隔離，我們從非雜交帶之野外捕捉烏頭翁與白頭翁，並設計了三隔室鳥籠分別檢驗兩種雌鳥對兩種不同雄鳥的擇偶偏好。實驗結果為73%的白頭翁雌鳥偏好白頭翁雄鳥，83%的烏頭翁雌鳥偏好烏頭翁雄鳥，皆屬同型交配。此結果顯示兩個相鄰的鵯屬族群已產生交配前生殖隔離，而頭部羽色的差異可能為其擇偶之主要依據。然而從捕獲雜交後代機率較高的宜蘭地區所取得的白頭翁雌鳥並未對同種雄鳥展現明顯偏好，其機制尚待未來研究。

Hybrid zone has been regarded as “the window on evolutionary process.” The dynamics of hybrid zone are closely related with the gene flow between the two adjacent populations, which have been reported in many cases to be hindered by female mate choice. Positive assortative mating can lead to reinforcement, and thus prevents the hybrid zone from expanding. There are two parapatric Pycnonotus species in Taiwan, P. sinensis formosae and P. taivanus. Although the two bulbuls can be distinguished by their distributions and distinct patterns of head plumage, they are similar in size, breeding season, dietary habits, song and behavior. In addition, recent studies by genetic markers have shown that they are actually sister taxa, diverging from one ancestor. Despite the existence of hybrid zones, the distributions of these two species have remained stable. To understand whether there is differential mating preference, we examined the female mate choice of both P. s. formosae and P. taivanus between conspecific and heterospecific males in a designed aviary. Trial birds were collected from outside hybrid zones. Results from trials revealed positive assortative mating in the two species, suggesting premating reproductive isolation between the two parapatric bulbul species and head plumage color pattern likely a primary cue for mate choice.