Microsatellite repeats are not randomly distributed within Norway spruce (Picea abies K.) expressed sequences

A Norway spruce (Picea abies K.) cDNA library obtained from vegetative bud tissue was screened for the presence of (AG)n and (AC)n microsatellite repeats. Ten (AG)n and six (AC)n microsatellites were found, with an average length of 25.5 repeat units. Most of the microsatellites are simple perfect repeats. The microsatellite distribution within the clones is clearly non-random, with different classes of repeats lying in different positions relative to the coding region and in a highly conserved orientation. An estimate of the frequency of dinucleotide microsatellites in expressed regions was obtained, showing that SSRs (simple sequence repeats) are found in genes about 20 times less frequently than in random genomic clones, with (AG)n repeats more frequent than (AC)n repeats. Potential applications of these sequences as expressed region-based molecular markers are shown by developing six SSR markers for the detection of natural variation in Norway spruce populations and testing two of them for the identification of illegitimate progenies from a mapping population. Key words: Picea abies, microsatellites, SSRs, ESTs, population genetics, trees.