Inferring outcrossing in the homothallic fungus Sclerotinia sclerotiorum using linkage disequilibrium decay

The occurrence and frequency of outcrossing in homothallic fungal species in nature is an unresolved question. Here we report detection of frequent outcrossing in the homothallic fungus Sclerotinia sclerotiorum. In using multilocus linkage disequilibrium (LD) to infer recombination among microsatellite alleles, high mutation rates confound the estimates of recombination. To distinguish high mutation rates from recombination to infer outcrossing, 8 population samples comprising 268 S. sclerotiorum isolates from widely distributed agricultural fields were genotyped for 12 microsatellite markers, resulting in multiple polymorphic markers on three chromosomes. Each isolate was homokaryotic for the 12 loci. Pairwise LD was estimated using three methods: Fisher’s exact test, index of association (IA) and Hedrick’s D′. For most of the populations, pairwise LD decayed with increasing physical distance between loci in two of the three chromosomes. Therefore, the observed recombination of alleles...自然界中同宗配合真菌物种的异交发生情况与发生频率仍是一个尚未解决的科学问题。本研究报道了在同宗配合真菌核盘菌（Sclerotinia sclerotiorum）中频繁发生异交的检测结果。以往利用多位点连锁不平衡（linkage disequilibrium, LD）推断微卫星等位基因间重组时，高突变率会干扰重组频率的估算。为区分高突变与重组事件以推断异交发生情况，我们对采自广泛分布的农业生境的8个种群样本、共计268株核盘菌分离株，使用12个微卫星标记进行基因分型，最终在3条染色体上均获得了多个多态性标记。所有分离株的12个基因座均为同核体状态。采用三种方法估算成对连锁不平衡：费希尔精确检验（Fisher’s exact test）、关联指数（index of association, IA）以及赫德里克D′值（Hedrick’s D′）。对于绝大多数种群而言，在3条染色体中的2条上，成对连锁不平衡程度随基因座间物理距离的增加而逐渐衰减。因此，本研究观测到的等位基因重组现象……