In the absence of sex and recombination, genomes are expected to accumulate deleterious mutations via an irreversible process known as Muller’s ratchet, especially in the case of polyploidy. In contrast, no genome-wide mutation accumulation was detected in a transcriptome of facultative apomictic, hexaploid plants of the Ranunculus auricomus complex. We hypothesize that mutations cannot accumulate in flowering plants with facultative sexuality because sexual and asexual development concurrently occurs within the same generation. We assume a strong effect of purging selection on reduced gametophytes in the sexual developmental pathway because previously masked recessive deleterious mutations would be exposed to selection. We test this hypothesis by modeling mutation elimination using apomictic hexaploid plants of the R. auricomus complex. To estimate mean recombination rates, the mean number of recombinants per generation was calculated by genotyping three F1 progeny arrays with six microsatellite markers and character incompatibility analyses. We estimated the strength of purging selection in gametophytes by calculating abortion rates of sexual versus apomictic development at the female gametophyte, seed and offspring stage. Accordingly, we applied three selection coefficients by considering effects of purging selection against mutations on (1) male and female gametophytes in the sexual pathway (additive, s = 1.000), (2) female gametophytes only (s = 0.520), and (3) on adult plants only (sporophytes, s = 0.212). We implemented recombination rates into a mathematical model considering the three different selection coefficients, and a genomic mutation rate calculated from genome size of our plants and plant-specific mutation rates. We revealed a mean of 6.05% recombinants per generation. This recombination rate eliminates mutations after 138, 204 or 246 generations, depending on the respective selection coefficients (s = 1.000, 0.520, and 0.212). Our results confirm that the empirically observed frequencies of facultative recombination suffice to prevent accumulation of deleterious mutations via Muller’s ratchet even in a polyploid genome. The efficiency of selection is in flowering plants strongly increased by acting on the haplontic (reduced) gametophyte stage.

中文翻译：

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一点点的性别即使在无融合生殖的多倍体植物中也能防止突变积累

在没有性别和重组的情况下，尤其是在多倍性的情况下，基因组有望通过称为穆勒棘轮的不可逆过程积累有害突变。相反，在毛an毛的兼性无融合生殖，六倍体植物的转录组中未检测到全基因组突变积累。我们假设突变不能在具有兼性的开花植物中积累，因为在同一世代中同时发生性和无性发育。我们假设清除选择对性发育途径中减少的配子体有很强的作用，因为以前掩盖的隐性有害突变会暴露于选择中。我们通过模拟使用R. auricomus复合物的无融合生殖六倍体植物消除突变来验证该假设。为了估计平均重组率，通过对具有六个微卫星标记的三个F1后代阵列进行基因分型并进行字符不相容性分析，计算出每代重组子的平均数量。我们通过计算雌配子体，种子和后代阶段性与无融合生殖发育的流产率，估算了配子体清除选择的强度。因此，我们考虑了清除针对突变的选择对以下三个因素的应用系数：（1）性途径中的雄性和雌性配子体（加成，s = 1.000），（2）仅雌性配子体（s = 0.520），和（3 ）仅在成年植物上（孢子体，s = 0.212）。考虑到三个不同的选择系数，我们将重组率应用于数学模型，以及根据我们植物的基因组大小和植物特异性突变率计算出的基因组突变率。我们揭示了平均每代6.05％的重组体。该重组率消除了138、204或246代后的突变，具体取决于各自的选择系数（s = 1.000、0.520和0.212）。我们的结果证实，凭经验观察到的兼性重组频率足以防止即使在多倍体基因组中，通过穆勒棘轮也能防止有害突变的积累。选择的效率是通过作用于触毛（减少的）配子体阶段而大大增强的开花植物。取决于各自的选择系数（s = 1.000、0.520和0.212）。我们的结果证实，凭经验观察到的兼性重组频率足以防止即使在多倍体基因组中，通过穆勒棘轮也能防止有害突变的积累。选择的效率是通过作用于触毛（减少的）配子体阶段而大大提高的开花植物。取决于各自的选择系数（s = 1.000、0.520和0.212）。我们的结果证实，凭经验观察到的兼性重组频率足以防止即使在多倍体基因组中，通过穆勒棘轮也能防止有害突变的积累。选择的效率是通过作用于触毛（减少的）配子体阶段而大大增强的开花植物。