Recent range expansion and agricultural landscape heterogeneity have only minimal effect on the spatial genetic structure of the plant pathogenic fungus Mycosphaerella fijiensis

Abstract

Understanding how geographical and environmental features affect genetic variation at both the population and individual levels is crucial in biology, especially in the case of pathogens. However, distinguishing between these factors and the effects of historical range expansion on spatial genetic structure remains challenging. In the present study, we investigated the case of Mycosphaerella fijiensis-a plant pathogenic fungus that has recently colonized an agricultural landscape characterized by the presence of potential barriers to gene flow, including several commercial plantations in which disease control practises such as the use of fungicides are applied frequently, and low host density areas. We first genotyped 300 isolates sampled at a global scale on untreated plants in two dimensions over a 50 Â 80-km area. Using two different clustering algorithms, no genetic structure was detected in the studied area, suggesting expansion of large populations and/or no influence of potential barriers. Second, we investigated the potential effect of disease control practises on M. fijiensis diversity by comparing populations sampled in commercial vs food-crop plantations. At this local scale, we detected significantly higher allelic richness inside commercial plantations compared with the surrounding food-crop plantation populations. Analysis of molecular variance indicated that 99% of the total genetic variance occurred within populations. We discuss the suggestion that high population size and/or high migration rate between populations might be responsible for the absence of any effect of disease control practises on genetic diversity and differentiation.

Genetic diversity and spatial structure of the Rufous‐throated Antbird (Gymnopithys rufigula), an Amazonian obligate army‐ant follower

“…To test for recent changes in the effective population size, we used the software Bottleneck v.1.2.02 (Peery et al., 2012). In recently bottlenecked populations, a heterozygosity excess relative to the number of alleles present in the population is expected, while a deficit is expected in the case of a population expansion (Cornuet & Luikart, 1996; Rieux, De Lapeyre De Bellaire, Zapater, Ravigne, & Carlier, 2013). We performed the analysis with 9,999 iterations under the assumptions of the stepwise‐mutation model (SMM) and the two‐phase mutation model (TPM; variance = 20, proportion of SMM = 95%).…”

A novel bioassay to monitor fungicide sensitivity in Mycosphaerella fijiensis

“…The neutral genetic diversity analysis could only be performed on a smaller conidial sample, but the findings revealed twofold more genotype mixtures within individual lesions than detected in the germination tests. This result was expected because the resistance frequency in the farm where the lesions were collected was roughly 50%, and hence, because loci are independent owing to recombination, the probability of not distinguishing with an MBC germination test two isolates having different genotypes is about 50%. The fact that minority and majority genotypes were spatially located in different parts of the lesion suggests that several separate infections might have occurred at adjacent sites on the leaf, but these infections could not be differentiated when the lesion was cut off.…”

“…Total DNA was extracted from each isolate, as described in Section 2.4.1. Among all microsatellite markers described in M. fijiensis , eight markers were screened for which high genetic diversity ( He > 0.4) had been reported in Cameroonian populations . In these conditions, the probability P that two randomly selected individuals in the population will have the same combination of alleles at these eight loci is very low .…”

Genetic variability of Pseudocercospora fijiensis, the black Sigatoka pathogen of banana (Musa spp.) in Mexico

“…This is also compatible with the hypothesis of Muller et al . (), which proposed that the variability exists on macro‐ as well as microgeographical scales, as well as the hypothesis of ‘stratified dispersal combination' (Parisod & Bonvin, ; Rieux et al ., ).…”