果蠅族群中染色體逆位多態性和隱性不良因子累積之相關性

Abstract

在自然族群中，染色體逆位多態性被認為和物種的適應有高度關係。目前認為族群維持染色體逆位多態性的機制，是透過其攜帶具選汰優勢的一組基因，因逆位異型合子抑制遺傳重組，而被同時保留在族群裡造成共適應的現象。然而，對於染色體逆位異型合子抑制重組造成的不良因子累積並增加突變負荷很少受到討論。不良因子的累積會隨著逆位同結合型在族群中被消除，進而增加逆位異型合子在族群裡的頻率。黃果蠅的熱帶非洲族群，同時具有高度的染色體逆位多態性和高頻的隱性不良因子累積，此現象提供了一個相當適合的研究材料。藉由定位隱性不良因子在染色體上累積的位置是否會真如預期在重組被高度抑制的位置，例如染色體逆位的端點，來探討染色體逆位和不良因子之間的關係，我們利用重組定位和基因缺失組定位出隱性不良因子累積的位置，結果在分別來自八個不同單雌系的八條三號染色體上定位出十四個不同的隱性致死因子，並且所有的致死因子和染色體逆位端點相距非常近，只有不到三個厘摩根。同時，隱性致死因子分佈在染色體上的位置和族群內的逆位多態性相當一致。本研究顯示隱性不良因子的累積受到染色體逆位多態性所抑制的重組率影響，在染色體上呈現不隨機的分佈。同時，也暗示染色體逆位和隱性不良因子之間強烈的相關性。

Chromosomal inversion polymorphisms have been demonstrated to play adaptive roles in natural populations by capturing local co-adapted alleles within recombination-suppressed regions of inversion heterozygotes. On the other hand, a less studied but important role of inversion polymorphisms is that recombination suppression by heterozygous chromosomal rearrangement may accumulate recessive deleterious mutations and thus cause a great amount of mutation load. Deleterious mutations will be eliminated when homozygotes and in turn increase heterozygosity of various inversions. The Afrotropical population of Drosophila melanogaster with high chromosomal inversion heterozygosity and high ratio of recessive lethals provides an ideal material to test any association between them by examining the accumulation pattern of recessive lethals. Recessive lethals are predicted to locate near the inversion breakpoints where the recombination is greatly suppressed by inversion heterokaryotypes. By using recombination and deficiency mappings, 14 recessive lethal alleles from eight lethal-bearing third chromosomes (each from eight distinct isofemale lines, respectively) were identified. All of recessive lethals were mapped into the regions close to (less than 3 centi-Morgan) the inversion breakpoints which were polymorphic in the African population. This result clearly shows that recessive lethal alleles are accumulated by recombination suppression and distributed non-randomly along inverted chromosomes. The data also provide the strong association between chromosomal inversions and the accumulation of recessive deleterious mutations.