Department of Biology, Indiana University, Bloomington, IN 47405.
Proc Natl Acad Sci U S A. 2013 Sep 24;110(39):15740-5. doi: 10.1073/pnas.1313388110. Epub 2013 Aug 19.
Despite much theoretical work, the molecular-genetic causes and evolutionary consequences of asexuality remain largely undetermined. Asexual animal species are rare, evolutionarily short-lived, and thought to suffer mutational meltdown as a result of lack of recombination. Whole-genome analysis of 11 sexual and 11 asexual genotypes of Daphnia pulex indicates that current asexual lineages are in fact very young, exhibit no signs of purifying selection against accumulating mutations, and have extremely high rates of gene conversion and deletion. The reconstruction of chromosomal haplotypes in regions containing SNP markers associated with asexuality (chromosomes VIII and IX) indicates that introgression from a sister species, Daphnia pulicaria, underlies the origin of the asexual phenotype. Silent-site divergence of the shared chromosomal haplotypes of asexuals indicates that the spread of asexuality is as recent as 1,250 y, although the origin of the meiosis-suppressing element or elements could be substantially older. In addition, using previous estimates of the gene conversion rate from Daphnia mutation accumulation lines, we are able to age each asexual lineage. Although asexual lineages originate from wide crosses that introduce elevated individual heterozygosities on clone foundation, they also appear to be constrained by the inbreeding-like effect of loss of heterozygosity that accrues as gene conversion and hemizygous deletion expose preexisting recessive deleterious alleles of asexuals, limiting their evolutionary longevity. Our study implies that the buildup of newly introduced deleterious mutations (i.e., Muller's ratchet) may not be the dominant force imperiling nonrecombining populations of D. pulex, as previously proposed.
尽管进行了大量的理论研究，但无性繁殖的分子遗传原因和进化后果在很大程度上仍未确定。无性繁殖的动物物种很少见，在进化上寿命较短，并且由于缺乏重组而被认为会发生突变崩溃。对 11 种有性和 11 种无性 Daphnia pulex 基因型的全基因组分析表明，目前的无性谱系实际上非常年轻，没有表现出对积累突变的净化选择的迹象，并且具有极高的基因转换和缺失率。在含有与无性繁殖相关的 SNP 标记的区域（染色体 VIII 和 IX）中重建染色体单倍型表明，来自姊妹种 Daphnia pulicaria 的基因渗入是无性表型起源的基础。无性单倍型共享染色体单倍型的沉默位点分歧表明，无性繁殖的传播时间与 1250 年前一样近，尽管减数分裂抑制因子的起源可能要早得多。此外，我们使用先前从 Daphnia 突变积累系中估计的基因转换率来对每个无性谱系进行定龄。尽管无性谱系起源于广泛的杂交，在克隆基础上引入了升高的个体杂合性，但它们似乎也受到因基因转换和半合缺失而导致的杂合性丢失的近交样效应的限制，这会暴露出无性个体的先前存在的隐性有害等位基因，限制其进化寿命。我们的研究表明，新引入的有害突变的积累（即 Muller's ratchet）可能不是危及 D. pulex 非重组群体的主导力量，如先前提出的那样。
