Lynch M, Blanchard J L
Department of Biology, University of Oregon, Eugene 97403, USA.
Genetica. 1998;102-103(1-6):29-39.
It is well established on theoretical grounds that the accumulation of mildly deleterious mutations in nonrecombining genomes is a major extinction risk in obligately asexual populations. Sexual populations can also incur mutational deterioration in genomic regions that experience little or no recombination, i.e., autosomal regions near centromeres, Y chromosomes, and organelle genomes. Our results suggest, for a wide array of genes (transfer RNAs, ribosomal RNAs, and proteins) in a diverse collection of species (animals, plants, and fungi), an almost universal increase in the fixation probabilities of mildly deleterious mutations arising in mitochondrial and chloroplast genomes relative to those arising in the recombining nuclear genome. This enhanced width of the selective sieve in organelle genomes does not appear to be a consequence of relaxed selection, but can be explained by the decline in the efficiency of selection that results from the reduction of effective population size induced by uniparental inheritance. Because of the very low mutation rates of organelle genomes (on the order of 10(-4) per genome per year), the reduction in fitness resulting from mutation accumulation in such genomes is a very long-term process, not likely to imperil many species on time scales of less than a million years, but perhaps playing some role in phylogenetic lineage sorting on time scales of 10 to 100 million years.
基于理论依据，在严格无性繁殖的种群中，非重组基因组中轻度有害突变的积累是主要的灭绝风险，这一点已得到充分证实。有性繁殖种群在经历很少或没有重组的基因组区域，即着丝粒附近的常染色体区域、Y染色体和细胞器基因组中，也可能发生突变退化。我们的研究结果表明，对于动物、植物和真菌等多种物种中的一系列基因（转运RNA、核糖体RNA和蛋白质）而言，线粒体和叶绿体基因组中产生的轻度有害突变相对于重组核基因组中产生的轻度有害突变，其固定概率几乎普遍增加。细胞器基因组中这种选择性筛子宽度的增加似乎并非是选择放松的结果，而是可以通过单亲遗传导致的有效种群大小减少所引起的选择效率下降来解释。由于细胞器基因组的突变率非常低（每年每个基因组约为10^(-4)），由此类基因组中突变积累导致的适应性下降是一个非常长期的过程，在不到一百万年的时间尺度上不太可能危及许多物种，但可能在一亿到十亿年的时间尺度上对系统发育谱系分选起到一定作用。
