Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, ON, Canada Department of Biology, University of Toronto at Mississauga, Mississauga, ON, Canada
Max Planck Institute of Molecular Plant Physiology, Potsdam-Golm, Germany.
Mol Biol Evol. 2015 Apr;32(4):896-905. doi: 10.1093/molbev/msu345. Epub 2014 Dec 21.
Sexual reproduction is nearly universal among eukaryotes. Theory predicts that the rarity of asexual eukaryotic species is in part caused by accumulation of deleterious mutations and heightened extinction risk associated with suppressed recombination and segregation in asexual species. We tested this prediction with a large data set of 62 transcriptomes from 29 species in the plant genus Oenothera, spanning ten independent transitions between sexual and a functionally asexual genetic system called permanent translocation heterozygosity. Illumina short-read sequencing and de novo transcript assembly yielded an average of 16.4 Mb of sequence per individual. Here, we show that functionally asexual species accumulate more deleterious mutations than sexual species using both population genomic and phylogenetic analysis. At an individual level, asexual species exhibited 1.8 × higher heterozygosity than sexual species. Within species, we detected a higher proportion of nonsynonymous polymorphism relative to synonymous variation within asexual compared with sexual species, indicating reduced efficacy of purifying selection. Asexual species also exhibited a greater proportion of transcripts with premature stop codons. The increased proportion of nonsynonymous mutations was also positively correlated with divergence time between sexual and asexual species, consistent with Muller's ratchet. Between species, we detected repeated increases in the ratio of nonsynonymous to synonymous divergence in asexual species compared with sexually reproducing sister taxa, indicating increased accumulation of deleterious mutations. These results confirm that an important advantage of sex is that it facilitates selection against deleterious alleles, which might help to explain the dearth of extant asexual species.
有性繁殖在真核生物中几乎普遍存在。理论预测，无性真核生物物种的稀有性部分是由于有害突变的积累以及与无性物种中重组和分离受到抑制相关的灭绝风险增加所致。我们使用来自植物属 Oenothera 的 29 个物种的 62 个转录组的大型数据集对此进行了测试，这些转录组跨越了十个独立的从有性到称为永久性易位杂合性的功能性无性遗传系统的转变。Illumina 短读测序和从头转录组装平均为每个个体产生了 16.4Mb 的序列。在这里，我们通过群体基因组学和系统发育分析表明，功能性无性物种积累了比有性物种更多的有害突变。在个体水平上，无性物种的杂合度比有性物种高 1.8 倍。在物种内，我们检测到与有性物种相比，无性物种中的非同义多态性相对于同义变异的比例更高，表明纯化选择的效率降低。无性物种还表现出更多具有过早终止密码子的转录本。非同义突变的比例增加也与有性和无性物种之间的分歧时间呈正相关，与 Muller's ratchet 一致。在物种间，我们检测到无性物种中非同义与同义分歧的比值相对于有性生殖姐妹类群重复增加，表明有害突变的积累增加。这些结果证实，性的一个重要优势是它有助于选择有害等位基因，这可能有助于解释现存无性物种的匮乏。
