- 學位論文
球背象鼻蟲姊妹種的生殖隔離
Reproductive isolation of two sister species of Pachyrhynchus weevils (Coleoptera: Curculionidae)
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摘要
生殖隔離是物種形成的關鍵因子，包含合子前隔離（行為隔離、物理隔離或生態隔離）和合子後隔離（子代發育遲緩或子代不孕）。球背象鼻蟲(Pachyrhynchus)在台灣、菲律賓等島嶼上形成高度多樣化的分類群，然而物種如何形成及形成的機制仍然未知。本研究探討兩個近緣物種（Pachyrhynchus moniliferus 和 P. orbifer azureus）的生殖隔離機制，比較合子前隔離（雄性佔據雌性的時間、次數，同性之間的競爭、雌性抗拒與交配成功）與合子後隔離（產卵量、孵化率、各齡期存活率、幼蟲與成蟲形態、發育時間與性別比）的差異，探討個體是否有交配上的偏好，並了解純種後代是否比雜交後代具有更高的適存度。在合子前隔離實驗中，雄性 P. orbifer azureus和雌性 P. moniliferus 偏好與同種交配，而雌性 P. orbifer azureus和雄性 P. moniliferus 與異種交配的機率比與同種高。同種交配時，異種的競爭強度較低，但雜交時，同種的競爭強度顯著較高。而合子後隔離實驗顯示，產卵量、孵化率、幼蟲不同齡期存活率、體長與發育時間在雜交與純種間並無顯著差異，但羽化到性成熟階段的雜交個體存活率顯著低於純種。所有雜交子代的色彩與斑紋皆為介於兩種親代的中間型。交配競爭的實驗中，無論雄蟲的先後交配順序，產生的子代均為雜交外型。本研究顯示 P. orbifer azureus和 P. moniliferus 的性擇偏好是不對稱的，可能因兩物種性別比的差異導致不同性別的生殖策略不同，而合子後隔離可能為兩姐妹物種生殖隔離的關鍵。雖然雜交子代的成蟲存活率低，但少數的存活個體或許亦可提供球背象鼻蟲雜交種化的機會。
並列摘要
Reproductive isolation is a key factor in speciation, including prezygotic isolation (behavioral, physical, or ecological isolation) and postzygotic isolation (offspring stunting or infertile). Pachyrhynchus weevils form highly diverse taxa on insular islands, such as Taiwan and the Philippines; however, what mechanisms shape species diversity are still unknown. In this study, prezygotic isolation (time and frequency of male occupation, male-male/female-female competition, female resistance, and mating success) and postzygotic isolation (egg quantity, hatching rate, survival rate of each instar, larval and adult size, development time and sex ratio) between two closely related species, Pachyrhynchus moniliferus and P. orbifer azureus, were compared. Mating preference between different species, and the fitness between purebred and hybrid offsprings were evaluated. In prezygotic isolation experiment, male P. orbifer azureus and female P. moniliferus preferred to mate with conspecifics, while female P. orbifer and male P. moniliferus were more likely to mate with heterospecies than with conspecifics. In intraspecific matings, competition intensity was low, but in interspecific matings, the competition intensity within the same species was significantly higher. Postzygotic isolation experiment showed that the survival rate of hybrid individuals from emergence to sexual maturity was significantly lower than that of purebreds. All the hybrid offspring display intermediate color and pattern between two parent species. There was no significant difference in egg quantity, hatching rate, survival rate of each instar, body length and development time between hybrids and purebreds. In mating competition experiment, all the offspring displayed a hybrid phenotype regardless of the mating order of males from different species. This study shows that the sexual selection preferences of P. orbifer azureus and P. moniliferus are asymmetrical, which may be due to the sex ratio difference between the two species, leading to different reproductive strategies of different sexes. Postzygotic isolation may be the key mechanism of reproductive isolation between the two sister species. This study indicates that despite lower fitness in hybrid individuals, the limited number of surviving individuals may contribute to the species diversity.
參考文獻
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