Selection processes in simple sequence repeats suggest a correlation with their genomic location: insights from a fungal model system

Background: Adaptive processes shape the evolution of genomes and the diverse functions of different genomic regions are likely to have an impact on the trajectory and outcome of this evolution. The main underlying hypothesis of this study is that the evolution of Simple Sequence Repeats (SSRs) is correlated with the evolution of the genomic region in which they are located, resulting in differences of motif size, number of repeats, and levels of polymorphisms. These differences should be clearly detectable when analyzing the frequency and type of SSRs within the genome of a species, when studying populations within a species, and when comparing closely related sister taxa. By coupling a genome-wide SSR survey in the genome of the plant pathogenic fungus Heterobasidion irregulare with an analysis of intra-and interspecific variability of 39 SSR markers in five populations of the two sibling species H. irregulare and H. annosum, we investigated mechanisms of evolution of SSRs.Results: Results showed a clear dominance of trirepeats and a selection against other repeat number, i.e. di- and tetranucleotides, both in regions inside Open Reading Frames (ORFs) and upstream 5' untranslated region (5'UTR). Locus per locus AMOVA showed SSRs both inside ORFs and upstream 5'UTR were more conserved within species compared to SSRs in other genomic regions, suggesting their evolution is constrained by the functions of the regions they are in. Principal coordinates analysis (PCoA) indicated that even if SSRs inside ORFs were less polymorphic than those in intergenic regions, they were more powerful in differentiating species. These findings indicate SSRs evolution undergoes a directional selection pressure comparable to that of the ORFs they interrupt and to that of regions involved in regulatory functions.Conclusions: Our work linked the variation and the type of SSRs with regions upstream 5'UTR, putatively harbouring regulatory elements, and shows that the evolution of SSRs might be affected by their location in the genome. Additionally, this study provides a first glimpse on a possible molecular basis for fast adaptation to the environment mediated by SSRs

List of number of microsatellite repeats for fungus specific locus in stands of Cetraria aculeata, Usnea antarctica and U. aurantiacoatra

Eight consistently amplifying markers were used for Cetraria aculeata. Samples of Usnea aurantiacoatra and U. antarctica were genotyped using 21 and 22 microsatellites markers, respectively. Detailed information on primers and PCR amplification can be found in Lutsak et al. (2016) and Lagostina et al. (2017). PCR amplicons were electrophoresed using an Applied Biosystems 3730 sequencer, with the LIZ 500 (C. aculeata)or LIZ 600 (Usnea sp.) size standards (Applied Biosystems, Waltham, Mass., USA). Allele sizes were manually scored using the Geneious 10 microsatellites tool (Kearse et al. 2012). Those SSR markers were used to study the effects of dispersal strategy and phylogeographic history on the population genetic structure of Antarctic lichens

Microsatellite markers and ITS sequences to investigate Usnea antarctica and U. aurantiacoatra species pair

Lichens are symbiotic associations consisting of a fungal (mycobiont) and one or more photosynthetic (photobionts) partners and are the dominant component, and most important primary producers, of Antarctic terrestrial ecosystems. The most common lichens in the maritime Antarctic are Usnea antarctica and U. aurantiacoatra, a so-called "species pair" in which U. antarctica shows asexual reproduction and propagation via soredia and U. aurantiacoatra forms ascospores in apothecia. Previous molecular analyses were not able to unambiguously distinguish the two morphotypes as species. Therefore, the goal of this study was to find out whether fast-evolving SSR (Single Sequence Repeats) markers are able to separate morphotypes more clearly and help to clarify their taxonomy. We investigate 190 individuals from five mixed stands of both morphotypes collected in King George Island and Elephant Island (South Shetland Islands, Antarctica). Based on 23 microsatellite markers designed from sequenced genomes, discriminant analysis of principal components (DAPC), Bayesian clustering analysis and coalescent-based estimation of gene flow show clear evidence for the existence of two different species distinguishable by reproductive mode. We did not detect any statistical association between genetic clusters and three previously reported chemical races of each species