title: Estimating genomic diversity and population differentiation - an empirical comparison of microsatellite and SNP variation in Arabidopsis halleri.
abstract: Microsatellite markers are widely used for estimating genetic diversity within and differentiation among populations. However, it has rarely been tested whether such estimates are useful proxies for genome-wide patterns of variation and differentiation. Here, we compared microsatellite variation with genome-wide single nucleotide polymorphisms (SNPs) to assess and quantify potential marker-specific biases and derive recommendations for future studies. Overall, we genotyped 180 Arabidopsis halleri individuals from nine populations using 20 microsatellite markers. Twelve of these markers were originally developed for Arabidopsis thaliana (cross-species markers) and eight for A. halleri (species-specific markers). We further characterized 2 million SNPs across the genome with a pooled whole-genome re-sequencing approach (Pool-Seq).
  
  
  
  Our analyses revealed that estimates of genetic diversity and differentiation derived from cross-species and species-specific microsatellites differed substantially and that expected microsatellite heterozygosity (SSR-He) was not significantly correlated with genome-wide SNP diversity estimates (SNP-Heand θWatterson) in A. halleri. Instead, microsatellite allelic richness (Ar) was a better proxy for genome-wide SNP diversity. Estimates of genetic differentiation among populations (FST) based on both marker types were correlated, but microsatellite-based estimates were significantly larger than those from SNPs. Possible causes include the limited number of microsatellite markers used, marker ascertainment bias, as well as the high variance in microsatellite-derived estimates. In contrast, genome-wide SNP data provided unbiased estimates of genetic diversity independent of whether genome- or only exome-wide SNPs were used. Further, we inferred that a few thousand random SNPs are sufficient to reliably estimate genome-wide diversity and to distinguish among populations differing in genetic variation.
  
  
  
  We recommend that future analyses of genetic diversity within and differentiation among populations use randomly selected high-throughput sequencing-based SNP data to draw conclusions on genome-wide diversity patterns. In species comparable to A. halleri, a few thousand SNPs are sufficient to achieve this goal.
authors.affiliation: ETH Zürich, Institute of Integrative Biology, Universitätstrasse 16, 8092, Zürich, Switzerland. martin.fischer@env.ethz.ch.
authors.forename: Martin C
authors.initials: MC
authors.lastname: Fischer
authors.affiliation: WSL Swiss Federal Research Institute, Zürcherstrasse 111, 8903, Birmensdorf, Switzerland.
authors.forename: Christian
authors.initials: C
authors.lastname: Rellstab
authors.affiliation: ETH Zürich, Institute of Integrative Biology, Universitätstrasse 16, 8092, Zürich, Switzerland.
authors.forename: Marianne
authors.initials: M
authors.lastname: Leuzinger
authors.affiliation: ETH Zürich, Institute of Integrative Biology, Universitätstrasse 16, 8092, Zürich, Switzerland.
authors.forename: Marie
authors.initials: M
authors.lastname: Roumet
authors.affiliation: WSL Swiss Federal Research Institute, Zürcherstrasse 111, 8903, Birmensdorf, Switzerland.
authors.forename: Felix
authors.initials: F
authors.lastname: Gugerli
authors.affiliation: Institute of Evolutionary Biology and Environmental Studies and Institute of Plant Biology, University of Zurich, Winterthurerstrasse 190, 8057, Zürich, Switzerland.
authors.forename: Kentaro K
authors.initials: KK
authors.lastname: Shimizu
authors.affiliation: ETH Zürich, Institute of Integrative Biology, Universitätstrasse 16, 8092, Zürich, Switzerland.|WSL Swiss Federal Research Institute, Zürcherstrasse 111, 8903, Birmensdorf, Switzerland.
authors.forename: Rolf
authors.initials: R
authors.lastname: Holderegger
authors.affiliation: ETH Zürich, Institute of Integrative Biology, Universitätstrasse 16, 8092, Zürich, Switzerland.
authors.forename: Alex
authors.initials: A
authors.lastname: Widmer
mesh_terms: D017360:Arabidopsis / Q000235:genetics*; D018745:Genome, Plant / Q000235:genetics; D023281:Genomics; D018895:Microsatellite Repeats / Q000235:genetics*; D020641:Polymorphism, Single Nucleotide
keywords: Arabidopsis halleri; Conservation units; Expected heterozygosity; Genetic diversity; Microsatellites; Pool-Seq; Population genomics; SNPs; SSR; Whole-genome re-sequencing