Genetic expansion of randomly mating founder populations of 1 Alternaria brassicicola infecting Cakile maritima in Australia

1 Founder populations of fungal plant pathogens are expected to have low levels of 2 genetic diversity, coupled with further genetic drift due to e.g. limited host availability which 3 should result in additional population bottlenecks. This study uses microsatellite markers in 4 the interaction between Cakile maritima and the fungal pathogen Alternaria brassicicola to 5 explore genetic expectations associated with such situations. The host, C. maritima was 6 introduced to Australia approximately 100 years ago but it is unknown whether the pathogen 7 was already present in Australia as it has a wide occurrence, or whether it has been 8 introduced to Australia on brassicaceous hosts. Eleven A. brassicicola populations were 9 studied and all show moderate levels of gene and genotypic diversity. Chi-square tests on 10 frequencies of mating-type alleles, a large number of genotypes and linkage equilibrium 11 among SSR loci, all suggest A. brassicicola reproduces sexually. Significant genetic 12 differentiation was found among populations, but there was no evidence for isolation by 13 distance effects. Bayesian analyses identified eight clusters where the inferred clusters did not 14 represent geographical populations but instead consisted of individuals admixed from all 15 populations. Further analysis indicated that fungal populations are more likely to have 16 experienced a recent population expansion than a population bottleneck. It is suggested that 17 A. brassicicola has been introduced multiple times to Australia, potentially increasing the 18 diversity and size of any A. brassicola populations already present in Australia; combined 19 with its ability to reproduce sexually, such processes appear to have increased the 20 evolutionary potential of this pathogen through recent population expansions. 21 on O cber 2, 2017 by gest ht://aem .sm .rg/ D ow nladed fom