Data from: Drift load in populations of small size and low density

According to theory, drift load in randomly mating populations is determined by past population size, because enhanced genetic drift in small populations causes accumulation and fixation of recessive deleterious mutations of small effect. In contrast, segregating load due to mutations of low frequency should decline in smaller populations, at least when mutations are highly recessive and strongly deleterious. Strong local selection generally reduces both types of load. We tested these predictions in 13 isolated, outcrossing populations of Arabidopsis lyrata that varied in population size and plant density. Long-term size was estimated by expected heterozygosity at 20 microsatellite loci. Segregating load was assessed by comparing performance of offspring from selfings versus within-population crosses. Drift load was the heterosis effect created by interpopulation outbreeding. Results showed that segregating load was unrelated to long-term size. However, drift load was significantly higher in populations of small effective size and low density. Drift load was mostly expressed late in development, but started as early as germination and accumulated thereafter. The study largely confirms predictions of theory and illustrates that mutation accumulation can be a threat to natural populations.根据演化理论，随机交配种群中的漂变负载（drift load）由过往种群规模决定，因为小型种群中增强的遗传漂变会导致效应微弱的隐性有害突变积累并固定。与之相对，由低频突变引发的分离负载（segregating load）在小型种群中应会降低——至少在突变高度隐性且强有害的情况下如此。强烈的局部选择通常会降低这两类负载。我们针对13个隔离的、异交的琴叶拟南芥（Arabidopsis lyrata）种群开展实验，这些种群的规模与植株密度均存在差异。长期种群规模通过20个微卫星位点（microsatellite loci）的期望杂合度进行估算。分离负载通过比较自交后代与种群内杂交后代的适合度表现进行评估。漂变负载则由种群间远交产生的杂种优势效应表征。研究结果显示，分离负载与长期种群规模并无关联，但在有效种群规模较小、植株密度较低的种群中，漂变负载显著更高。漂变负载大多在发育后期显现，但早在萌发阶段就已开始出现，并随发育进程持续积累。本研究在很大程度上验证了相关理论预测，并表明突变积累可能对自然种群构成威胁。