Allelic variation at (TAA)(n) microsatellite loci in a world collection ofchickpea (Cicer arletinum L.) germplasm

A set of 12 randomly selected (TAA), microsatellite loci of the cultivated chickpea (Cicer arietinum L.) were screened in a worldwide sample comprisin g 72 landraces, four improved cultivars and two wild species of the primary gene pool (C. reticulatum and C. echinosperum) to determine the level and pattern of polymorphism in these populations. A single fragment was amplifi ed from all the accessions with each of 12 sequence-tagged microsatellite s ite markers,except; for one locus where no fragment was obtained from eithe r of the two wild species. There was a high degree of intraspecific polymor phism at these microsatellite loci, although isozymes, conventional RFLPs a nd RAPDs show very little or no polymorphism. Overall, the repeat number at a locus (excluding null alleles) ranged from 7 to 42. The average number o f alleles per locus was 14.1 and the average genetic diversity was 0.86. Ba sed on the estimates obtained, 11 out of the 12 frequency distributions of alleles at the loci tested can be considered to be non-normal. A significan t positive correlation between the average number of repeats (size of the l ocus) and the amount of variation was observed, indicating that replication slippage may be the molecular mechanism involved in generation of variabil ity at the loci. A comparison between the infinite allele and stepwise muta tion models revealed that for 11 out of the 12 loci the number of alleles o bserved fell in between the values predicted by the two models. Phylogeneti c analysis of microsatellite polymorphism in C. arietinum showed no relatio nship between accession and geographic origin, which is compatible with the recent expansion of this crop throughout the world.