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.

Adaptability of Rhizoctonia solani AG-1 IA for mancozeb sensitivity under temperature stress

“…As there was a restriction in mycelial growth at 33.5 °C for the pathogen populations sampled from Maranhão and Tocantins, this was considered a stress temperature. In contrast, the mean of mycelial growth for the population from Mato Grosso was not affected at 33.5 °C, indicating adaptation to more extreme fluctuations in temperatures (Ferro et al, 2019). This potential for adaptation to higher temperature was reported for populations of R. solani AG-1 IA associated with rice and signal grass pastures in Colombia (Ramos et al, 2019).…”

“…Considering the fungicide effect as a stress factor, mancozeb reduced the mycelial growth rate of R. solani AG-1 IA and inhibited sclerotia production at 25 °C or 35 °C. Similarly, copper oxychloride, another broad-spectrum fungicide, inhibited sclerotia production (Ferro et al, 2019). The inhibition of sclerotia production under fungicide stress occurred due to the resulting lower fungal mycelia growth, as large amounts of mycelium are required for sclerotia production (Youssef et al, 2012).…”

“…Rhizoctonia solani AG-1 IA isolates were obtained in 2006 from soybean infected plants sampled from cropping areas in Maranhão, Mato Grosso, and Tocantins States, Brazil Ferro et al, 2019). For this study, twelve genetically distinct isolates were randomly selected from each of these three populations.…”

“…The fungi were incubated at 25 °C, considered the optimal growth temperature for R. solani AG-1 IA (Costa et al, 2007) and at 33.5 °C, considered stress temperature (Ferro et al, 2019). After 48 h incubation, the mean radial mycelial growth was measured along the line of the longest diameter and its perpendicular line.…”

“…For this reason, it is assumed that fungal growth in the presence of dithiocarbamate is a polygenic base character. A recent study indicated that the populations of R. solani AG-1 IA from soybean under high-temperature stress conditions (33.5 °C) increased genetic variance and heritability for sensitivity to copper oxychloride, another broadspectrum fungicide (Ferro et al, 2019).…”