Division of Biotechnology, Dalian Institute of Chemical Physics, CAS, Dalian, People's Republic of China.
Mol Biol Rep. 2010 Mar;37(3):1551-7. doi: 10.1007/s11033-009-9557-8. Epub 2009 May 17.
What caused spliceosomal introns gain remains an unsolved problem. To this, defining what spliceosomal introns arise from is critical. Here, the introns density of the genomes is calculated for four species, indicating:(1) sex chromosomes in mammals have lower intron densities, (2) despite that, the proportion of UTRs (untranslated regions) with introns in sex chromosomes is higher than other ones, and (3) AT content of introns is more similar to that of intergenic regions when these regions comprise the majority of a chromosome, and more similar to that of exons, when exons are the majority of the chromosome. On the other hand, introns have been clearly demonstrated to invade genetic sequences in recent times while sex chromosomes evolved from a pair of autosomes within the last 300 millions years. One main difference between sex chromosomes and autosomes in mammalian is that sex chromosomes recombination stopped. Thus, recombination might be the main determinant for eukaryotes gaining spliceosomal introns. To further prove that and avoid giving weak signal, the whole genomes from eight eukaryotic species are analyzed and present strong signal for above the trend (3) in three species (t-test, P = 0.55 for C. elegans, P = 0.72 for D. melanogaster and P = 0.83 for A. thaliana). These results suggest that the genome-wide coincidence as above (3) can only be caused by the large-scale random unequal crossover in eukaryote meiosis, which might have fueled spliceosomal introns but hardly occurred in prokaryotes.
剪接体内含子获得的原因仍然是一个悬而未决的问题。为此，定义剪接体内含子的起源至关重要。这里，计算了四个物种的基因组内含子密度，结果表明：（1）哺乳动物的性染色体中内含子密度较低，（2）尽管如此，性染色体上具有内含子的 UTR（非翻译区）的比例高于其他染色体，（3）当这些区域构成染色体的大部分时，内含子的 AT 含量与基因间区更为相似，而当外显子构成染色体的大部分时，内含子的 AT 含量与外显子更为相似。另一方面，内含子在最近的时期已经被明确证明会侵入遗传序列，而性染色体在过去的 3 亿年内从一对常染色体进化而来。哺乳动物中性染色体和常染色体的一个主要区别是，性染色体的重组停止了。因此，重组可能是真核生物获得剪接体内含子的主要决定因素。为了进一步证明这一点，并避免信号较弱，对来自八个真核物种的全基因组进行了分析，结果在三个物种中均显示出上述趋势（3）的强烈信号（t 检验，C. elegans 的 P = 0.55，D. melanogaster 的 P = 0.72，A. thaliana 的 P = 0.83）。这些结果表明，只有在真核生物减数分裂中大规模随机的非均等交叉才能导致上述（3）的全基因组一致性，这可能为剪接体内含子提供了动力，但在原核生物中很少发生。
