In order to assess the feasibility of using microsatellites as markers in plant genetics, a survey of published DNA sequence data for presence, abundance and ubiquity in higher plants of all types of dinucleotide and trinucleotide repeats with a minimum number of 10 and 7 units, respectively, was conducted. This search revealed that such microsatellites are frequent and widely distributed; they were uncovered in 34 species, with a frequency of one every 50 kb. AT repeats were by far the most frequently observed class of dinucleotide microsatellites, whereas ACffG repeats, which are common in animals, were observed only once. TAT repeats prevailed among trinucleotides. Polymerase chain reaction amplification of (AT), and (TAT), micro-satellites in soybean (Glycine max (L.) Merr.) revealed that they are highly polymorphic, as a consequence of length variation, somatically stable and inherited in a co-dominant Mendelian manner. The abundance and amount of information derived from such markers, together with the ease by which they can be identified, make them ideal markers for plant genetic linkage and physical mapping, population studies and varietal identification.

SummaryIn order to assess the feasibility of using microsatellites as markers in plant genetics, a survey of published DNA sequence data for presence, abundance and ubiquity in higher plants of all types of dinucleotide and trinucleotide repeats with a minimum number of 10 and 7 units, respectively, was conducted. This search revealed that such microsatellites are frequent and widely distributed; they were uncovered in 34 species, with a frequency of one every 50 kb. AT repeats were by far the most frequently observed class of dinucleotide microsatellites, whereas ACffG repeats, which are common in animals, were observed only once. TAT repeats prevailed among trinucleotides. Polymerase chain reaction amplification of (AT), and (TAT), microsatellites in soybean (Glycine max (L.) Merr.) revealed that they are highly polymorphic, as a consequence of length variation, somatically stable and inherited in a co-dominant Mendelian manner. The abundance and amount of information derived from such markers, together with the ease by which they can be identified, make them ideal markers for plant genetic linkage and physical mapping, population studies and varietal identification.

Norway spruce (Picea abies) genomic libraries were screened for presence of dinucleotide AC/GT and AG/CT microsatellites (or simple sequence repeats). On average, one (AG)n microsatellite every 194 kb and one (AC)n microsatellite every 406 kb were found. Forty-six positive clones were sequenced and primers flanking 24 AG microsatellites and 12 AC microsatellites diesigned. Only seven (20%) of them produced the expected single-locus polymorphic pattern when used to amplify Norway spruce DNAs. The other primer pairs gave either multiple bands or bad amplification, or a single monomorphic fragment. Such a small proportion of successful primer pairs was attributed to the high level of complexity of the Norway spruce genome. Dot blot analysis of the clones showed that many of them contained repetitive DNA and that those giving the single-locus polymorphic patterns usually corresponded to single-copy sequences. A family of repetitive DNA that contained AG repeats was identified and was present in about 40,000 copies per haploid genome. Simple Mendelian inheritance was observed for all the polymorphisms tested. The average number of alleles was 13, ranging from 6 to 22, and the expected heterozygosity was 0.79 when seven microsatellites were used to genotype a panel of 18 trees representing different populations. Compared with isozymes, microsatellites are about five times more informative and could provide an extremely valuable source of markers for genome mapping and genetic diversity studies.

Genetic linkage maps have been produced for a wide range of organisms during the last decade, thanks to the increasing availability of molecular markers. The use of microsatellites (or Simple Sequence Repeats, SSRs) as genetic markers has led to the construction of "second-generation" genetic maps for humans, mouse and other organisms of major importance. We constructed a second-generation single-tree genetic linkage map of Norway spruce (Picea abies K.) using a panel of 72 haploid megagametophytes with a total of 447 segregating bands [366 Amplified Fragment Length Polymorphisms (AFLPs), 20 Selective Amplification of Microsatellite Polymorphic Loci (SAMPLs) and 61 SSRs, each single band being treated initially as a dominant marker]. Four hundred and thirteen markers were mapped in 29 linkage groups (including triplets and doublets) covering a genetic length of 2198.3 cM, which represents 77.4% of the estimated genome length of Picea abies (approximately 2839 cM). The map is still far from coalescing into the expected 12 chromosomal linkage groups of Norway spruce (2n = 2x = 24). A possible explanation for this comes from the observed non-random distribution of markers in the framework map. Thirty-eight SSR marker loci could be mapped onto 19 linkage groups. This set of highly informative Sequence Tagged Sites (STSs) can be used in many aspects of genetic analysis of forest trees, such as marker-assisted selection, QTL mapping, positional cloning, gene flow analysis, mating system analysis and genetic diversity studies.

Microsatellites or simple sequence repeats are stretches of short tandemly repeated DNA sequence motifs, dispersed throughout the genomes of most eukaryotes. Simple sequence repeat polymorphisms (SSRPs) have recently been reported in plants. Here we present the genetic map position of seven different soybean (Glycine max (L.) Merr. and Glycine soja Sieb. and Zucc.) SSRPs contained in sequenced genes, four of which represent newly mapped positions for these genes. The other three SSRPs coincided with independently established RFLP map positions for the corresponding genes. When a set of 61 soybean accessions was screened at four of these loci by using agarose gels, the average number of alleles per locus was 7.75, the effective number of alleles (ne) was 2.57, and the level of allele differentiation (delta(t)) was 0.62. Allelic variation decreased sharply with increasing levels of domestication, with the level of differentiation going from 84% in the wild soybean to 43% in the elite germplasm. Variation levels observed on a subset made of 19 of the 61 lines were always higher for SSRPs than for RFLP markers, with the average number of alleles per locus going from 4.25 to 2.15. In comparison with RFLP markers, SSRPs are more informative and are easier to analyse but require more effort to develop.