Temporal trends of genetic diversity in European barley cultivars (Hordeum vulgare L.)

The changes of genetic diversity over time were monitored in 504 European barley cultivars released during the 20th century by genotyping with 35 genomic microsatellites. For analysis, the following four temporal groups were distinguished: 1900-1929 (TG1 with 19 cultivars), 1930-1949 (TG2 with 40 cultivars), 1950-1979 (237 cultivars as TG3), and 1980-2000 (TG4 consisting of 208 cultivars). After rarefaction of allelic diversity data to the comparable sample size of 18 varieties, of the 159 alleles found in the first group (TG1) 134 were retained in the last group (TG4) resulting in a loss of only 15.7% of alleles. On the other hand 51 novel alleles were discovered in the group representing the last investigated time period (TG4) in comparison with the TG1. Novel alleles appeared evenly distributed over the genome, almost at all investigated genomic loci, with up to five such novel alleles per locus. Alleles specific for a temporal group were discovered for all investigated time periods, however analysis of molecular variance (AMOVA) did not reveal any significant population structure attributable to temporal decadal grouping. Only 2.77% of the total observed variance was due to differences between the four temporal groups and 1.42% between individual decades of the same temporal group, while 95.81% of the variance was due to variation within temporal groups. The distinction between two-rowed and six-rowed genetic types accounted for 19.5% of the total observed variance by AMOVA, whereas the comparison between 'winter' and 'spring' types accounted for 17% of the total observed variation. The analysis of linkage disequilibrium did not reveal statistically significant differences between the temporal groups. The results indicated that the impact of breeding effort and variety delivery systems did not result in any significant quantitative losses of genetic diversity in the representative set of barley cultivars over the four time periods.