紅果蠅和輝顏果蠅的不對稱生殖隔離

Abstract

一個生物學上相當重要的議題，就是在演化的過程中原始的族群是如何分化成現在的不同物種？紅果蠅 (Drosophila albomicans, 2n = 6) 與輝顏果蠅 (D. nasuta, 2n = 8)，在形態上是無法區分的同胞種 (sibling species)，卻有不同的核型 (karyotpe)。在實驗室中可以交配產生可孕後代並且可培養數百代，但在自然界中卻從未看過混雜的核型。因此推測可能在這兩個物種之間存在著某種隔離機制。而本實驗將重點放於行為所造成的交尾前生殖隔離，利用了各種族群箱交尾選擇實驗，以多重選擇 (multiple choice)、雌性選擇 (female choice)、無選擇 (no choice) 三種不同類型的交尾實驗為主要的方法來探討可能的機制。實驗結果顯示了體型大不是造成交尾成功的主要因素。繼而比較無選擇和雌性選擇交尾實驗，其差異代表雄性之間有交互影響。無選擇交尾實驗排除了雄性之間的互相影響，顯示紅果蠅雌蟲偏好同種，而輝顏果蠅的雌蟲沒有顯著的交尾偏好性，讓我們有機會去探討兩個關於不對稱生殖隔離的假說。又因為符合 Watanabe & Kawanishi 所提出的假說，進一步推論物種分化之初有基因交流的可能性。最後，根據紅果蠅雄蟲有較強的 lek 行為和它特殊的染色體構造，提出假說：紅果蠅雄蟲的性染色體上具有較多的雄性特殊表現基因。並以雜合消去法 (suppressive subtractive hybridization) 建立一批轉殖菌株提供未來驗證此假說的材料。

One of the most important issues in biology is: “How evolutionary process creates divergent groups known as species?” Drosophila albomicans (2n = 6) and D. nasuta (2n = 8) are sibling species with indistinguishable morphology but distinct karyotypes. Although these two species can easily produce viable and fertile hybrids in laboratory, no hybrids have ever been found in nature. Thus, we suggest that there may exist reproductive isolation between them. In this study, we focused on behavior isolation of premating mechanisms, and performed three kinds of mating choice experiments (i.e., multiple, female, and no choice). Size was ruled out as an important factor for mating success. The difference between no choice experiment and female choice revealed male-male interaction. If male-male interaction was excluded, what left is significant female choice of D. albomicans for conspecific males, but D. nasuta females mate randomly. This asymmetric female choice which fits Watanabe & Kawanishi’s hypothesis infers divergence of the ancestral population with gene flow. Due to strong lek behavior (i.e., stronger lunge) and the unique karyotype in D. albomicans male, we proposed that the neo-Y chromosome of D. albomicans may harbor more male specific genes to cause the difference. Clones of the male specific genes in these two species have been generated by suppressive subtractive hybridization. Further study is needed to verify this hypothesis.