- 期刊
Karyotype Polymorphism in Hybrid Populations of Drosophila nasuta and D. albomicans
紅果蠅和輝顏果蠅雜交族群之染色體核型多態性
摘要
同胞種（sibling species）紅果蠅（Drosophila albomicans, 2n=6）與輝顏果蠅（D. nasuta, 2n=8）具有不同的染色體核型（karyotype），卻有相同的外型。前者的分布從日本、臺灣到泰國，後者從印度到非洲東岸。牠們在實驗室中可以很容易的雜交而且後代可繼續繁殖，但在野外卻看不到任何混雜的核型。我們試圖探討兩個問題：一、核型的多態性（polymorphism）是否可長久維持？二、是否可能在雜交後代中，一條染色體的頻率變化影響到來自同種的非同源（non homologous）染色體的頻率變化？我們以世代不重疊的累代培養方法，建立了22個雜交品系。每個品系最初是由一對印度的輝顏果蠅雄蟲和琉球的紅果蠅處女雌蟲在實驗室中雜交產生。將第45代（兩年半之後）幼蟲及成蟲分別作核型分析及酯酶電泳，可區辨其染色體分別來自親本的那一方。由核型的分析可顯示第三、第四及性染色體X來自紅果蠅（癒合型的3-X及較大的4）或是來自輝顏果蠅（原始的分離型式3與X），而酯酶電泳可供辨認第二對染色體。雜交後代的第四對和第二對染色體分別有92%及38%來自紅果蠅，這兩對染色體的組成比例與族群起始值（50%）作卡方分析，均有極顯著之差異。由頻率分布顯示第四對染色體受方向性選汰（directional selection）趨向紅果蠅的染色體，而第二對則受平衡性選汰（balancing selection）其平均值偏向輝顏果蠅的染色體。另外，3-X的比例佔72.2%（雌、雄分別為72.8%及70.3%），此觀測值與66.7%的族群起始值之間，利用卡方檢測發現並無顯著性差異。這些數據顯示核型的多態性可以長久維持在雜交族群中，同時各個染色體有自己的特性，非同源染色體各受自己的選汰作用，因此我們可以設計實驗來研究紅果蠅性染色體的進化而不必顧慮其非同源染色體頻率變化的干擾，這個部分的研究正在進行中。
並列摘要
Drosophila albomicans (2n = 6) and D. nasuta (2n = 8) are 2 sibling species with indistinguishable morphology; but distinct karyotypes. The former is distributed from Japan through Taiwan to Thailand, and the latter from India to the east coast of Africa. Although they are completely crossable in the laboratory, no polymorphic karyotype has ever been found in anyone population of these two species in nature. In this report, 2 questions are addressed. First, does karyotype polymorphism exist in hybrid populations of these 2 species after long-term cultivation? Second, do the chromosomes originating from the same species interact with each other during their transmission from generation to generation in hybrid offspring? To answer these questions, 22 interspecific hybrid strains were established. Each strain was initiated by crossing one D. nasuta male and one D. albomicans female. Their hybrid offsprings were maintained with the non-overlapping generation method for 2.5 y (i.e., 45 generations) and then subjected to karyotype analysis. The origins of the 3rd, 4th, and sex chromosomes in the hybrid offspring were identified by larval karyotyping, while an esterase electrophoretic technique was applied for distinguishing the origin of the 2nd chromosome. Among these hybrid offsprings, 92% of the 4th chromosomes and 38% of the 2nd chromosomes are derived from D. albomicans. The frequencies of the 4th and the 2nd chromosomes are significantly different from their initial values (50%), therefore, we may conclude that chromosome 4 is leaning to D. albomicans, and chromosome 2 to D. nasuta in hybrid strains. On the contrary, 72.2% of the 3rd and X chromosomes (70.3% in male and 72.8% in female) are derived from D. albomicans, which is close to the initial value (66.7%). Our data clearly reveal that karyotype polymorphism exists in hybrid offspring after long-term cultivation, and the evolution of the 3rd and X chromosomes is apparently not associated with other chromosomes.