LEVELS OF REGIONAL PHENOTYPIC ADAPTATION (QST) INDICATE THAT NEUTRALITY HAS SHAPED THE POPULATION STRUCTURE OF THE SOYBEAN-INFECTING PATHOGEN Rhizoctonia solani AG-1 IA

ABSTRACT Populations of the soybean leaf blight pathogen (Rhizoctonia solani AG-1 IA) are highly genetically differentiated along a latitudinal gradient in the major soybean growing regions of Brazil. However, the evolutionary processes leading to regional adaptation are still unknown. The objective of this study was to evaluate the relative importance of neutral genetic variation and natural selection on the divergence and regional adaptation of populations of the soybean-infecting pathogen R. solani AG-1 IA. Therefore, we compared the phenotypic differentiation in quantitative traits (QST) and the neutral genetic differentiation (FST, based on microsatellites data) among three pairs of populations. As measures of phenotypic responses of the fungus (quantitative traits), we estimated the tolerance to temperature stress and the tolerance to a broad-spectrum fungicide (copper oxychloride) under optimal (25 °C) and high temperature conditions (33.5 °C). In general there was an increase in genetic variance with a positive effect on the heritability for tolerance to copper fungicide under temperature stress. The genetic differences among populations were the main determinants of thermal adaptation in R. solani AG-1 IA (h2 > 0.70). The analysis of neutral genetic structure (FST) indicated subdivision between the three pairs of populations. Although population pairwise comparisons between FST and QST values did not follow a single pattern, the majority of QST values did not differ significantly from FST, indicating that, for the quantitative characters studied, neutrality (or neutral evolution) had a major role in the regional adaptation of R. solani AG-1 IA populations.摘要：在巴西主要大豆种植区，大豆叶枯病病原菌（Rhizoctonia solani AG-1 IA）的种群沿纬度梯度呈现高度遗传分化。然而，介导其种群形成区域适应性的演化机制仍不明晰。本研究旨在解析中性遗传变异与自然选择在该侵染大豆的病原菌立枯丝核菌AG-1 IA种群分化与区域适应性中的相对贡献。为此，我们针对三对种群，比较了其数量性状表型分化（QST）与基于微卫星（microsatellites）数据计算的中性遗传分化（FST）。本研究以真菌的表型响应指标（即数量性状）为测定对象，分别在适宜温度（25℃）与高温胁迫条件（33.5℃）下，评估了其对温度胁迫的耐受性以及对广谱杀菌剂（氧氯化铜，copper oxychloride）的耐受性。总体而言，在温度胁迫下，与铜杀菌剂耐受性相关的性状遗传方差有所提升，且对其遗传力产生正向影响。立枯丝核菌AG-1 IA种群间的遗传差异是其热适应性的主要决定因素（h²>0.70）。中性遗传结构（FST）分析结果显示，三对种群间存在遗传亚群分化。尽管FST与QST值的种群两两比较未呈现统一模式，但绝大多数QST值与FST值无显著差异，这表明针对本研究中的数量性状而言，中性进化（或中性演化）在立枯丝核菌AG-1 IA种群的区域适应性中发挥了主导作用。