High levels of genetic diversity and cryptic recombination is widespread in introduced Diplodia pinea populations

Abstract

Introduced populations of organisms typically have reduced diversity compared to those that are native. It is, therefore, unusual that introduced populations of the fungal tree pathogen Diplodia pinea have been shown to have high levels of genetic diversity, even surpassing diversity in some native regions. This is thought to be due to multiple introductions over time or the existence of a cryptic and yet undiscovered sexual cycle. In this study, we consider whether populations of D. pinea in Southern Hemisphere countries have similar patterns of diversity, share some level of genetic identity and how they might be influenced by sexual recombination. A total of 173 isolates from Argentina, Australia, Ethiopia and South Africa were characterized using 12 microsatellite markers. The results show that all these populations have high gene and genotype diversities, with the Australian population having the lowest diversity. Very few private alleles were found, suggesting that isolates from different countries might share a source of introduction. However, based on allele distribution and frequency, each of the populations appeared to be evolving independently. The results showed that in all but the Australian population, alleles are randomly associated, suggesting that widespread sexual recombination has influenced population structure.

MAT gene idiomorphs suggest a heterothallic sexual cycle in a predominantly asexual and important pine pathogen

“…Previous reports of spermatia (Wingfield and Knox-Davies, 1980;Palmer et al, 1987), as well as the random association of loci and high genetic diversity in D. pinea suggested the presence of an active sexual state in D. pinea (Bihon et al 2012a(Bihon et al , 2012b. The observation of an intact MAT locus and the even distribution of mating genes (Coppin et al, 1997;Hull et al, 2000;Lengeler et al, 2002;Poggeler, 2002) strengthen these previous indications of the presence of a cryptic sexual cycle in D. pinea.…”

“…These were the same populations of isolates that have been used in previous population genetic studies (Bihon et al 2012a(Bihon et al , 2012b. PCR was conducted using the two idiomorph primer sets described above and amplification verified using gel-electrophoresis.…”

“…Furthermore, the alleles at unlinked loci in sexually 3 reproducing populations are expected to be recombining randomly as indicated by linkage disequilibrium, unlike the situation in exclusively asexual fungi where alleles are not reassorted (Smith et al, 1993). Population genetic studies using microsatellite markers have consistently shown these patterns expected for sexually reproducing species in D. pinea, namely high genotypic diversity in populations and linkage disequilibrium between alleles (Burgess et al, 2004;Bihon et al, 2012aBihon et al, , 2012b. Together with the observations of spermatia discussed above, these results demand a deeper investigation into the reproductive biology of D. pinea.…”

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

“…I A tests have been used to infer recombination (Taylor et al, 1999a;Bihon et al, 2012;Klaassen et al, 2012) despite high mutation rates of microsatellite loci (Li et al, 2002;Lynch et al, 2008). However, if the marker system has a high mutation rate, it might confound the inferences of recombination.…”

Invasive Everywhere? Phylogeographic Analysis of the Globally Distributed Tree Pathogen Lasiodiplodia theobromae

“…It has been shown for other organisms, for example lizards, that the invasive populations could be more diverse than native populations if introduced multiple times and from various isolated native populations [69]. This has also been observed in fungi such as D. sapinea in parts of its invasive range (e.g., in South Africa; [22,23]). …”