Charlesworth B, Lapid A, Canada D
Department of Ecology and Evolution, University of Chicago, IL 60637-1573.
Genet Res. 1992 Oct;60(2):115-30. doi: 10.1017/s0016672300030809.
Data were collected on the distribution of nine families of transposable elements among a sample of autosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization of biotinylated probes to polytene chromosomes. There is no general tendency for elements to accumulate at the tips of chromosomes. Elements tend to be present in excess of random expectation in the euchromatin proximal to the centromeres of the major autosomes, and on chromosome four. There is considerable heterogeneity between different families in the extent of this excess. The overall abundance of element families is inversely related to the extent to which they accumulate proximally. The level of proximal accumulation for the major autosomes is similar to that on the fourth chromosome, but less than that for the X chromosome. There is an overall deficiency of elements in the mid-section of the X compared with the mid-sections of the major autosomes, with considerable heterogeneity between families. The magnitude of this deficiency is positively related to the extent to which elements accumulate proximally. No such deficiency is seen if the proximal regions of the X and autosomes are compared. There is a small and non-significant excess of elements in third chromosomes carrying inversions. There is some between-year heterogeneity in element abundance. The implications of these findings are discussed, and it is concluded that they generally support the hypothesis that transposable element abundance is regulated primarily by the deleterious fitness consequences of meiotic ectopic exchange between elements. If this is the case, such exchange must be very infrequent in the proximal euchromatin, and the elements detected in population surveys of this kind must be inserted into sites where they have negligible mutational effects on fitness.
通过将生物素化探针原位杂交到多线染色体上，收集了关于从黑腹果蝇自然种群中分离出的常染色体样本中九个转座因子家族分布的数据。转座因子没有在染色体末端积累的普遍趋势。在主要常染色体着丝粒近端的常染色质以及第四条染色体上，转座因子的存在往往超过随机预期。不同家族在这种过量程度上存在相当大的异质性。转座因子家族的总体丰度与它们在近端积累的程度呈负相关。主要常染色体的近端积累水平与第四条染色体相似，但低于X染色体。与主要常染色体的中段相比，X染色体中段的转座因子总体上有所缺乏，家族之间存在相当大的异质性。这种缺乏的程度与转座因子在近端积累的程度呈正相关。如果比较X染色体和常染色体的近端区域，则没有这种缺乏现象。携带倒位的第三条染色体上有少量且不显著的转座因子过量。转座因子丰度在年份之间存在一些异质性。讨论了这些发现的意义，并得出结论，它们总体上支持这样的假设，即转座因子的丰度主要由元件之间减数分裂异位交换的有害适应性后果所调节。如果是这样，这种交换在近端常染色质中肯定非常罕见，并且在这类种群调查中检测到的转座因子必须插入到对适应性产生可忽略不计的突变影响的位点。
