High mutation rate and mutational bias at (TAA)(n) microsatellite loci in chickpea (Cicer arietinum L.)

Microsatellites, very short tandemly repeated DNA sequences, are being exte nsively used in evolutionary genetics and molecular breeding of crop plants , because of their high degree of allelic variability, which is presumably caused by a high rate of mutation that changes microsatellite array length. In humans and various animals, mutation rates vary greatly and fall within the range of 10(-3) to 10(-6). In plants, the mutation rate at microsatell ite loci seems to be higher than in animals, but no experimental estimates are available yet. Here, we report high spontaneous mutation rates (mu) and mutational bias at fifteen perfect (TAA)(n) microsatellite loci in inbred populations of chickpea. We show a significantly higher mutation rate, aver aged across all loci, in the long-lived variety Ghab 2 (mu = 1.0 x 10(-2) ; detected in 16,050 allele-generations) compared to the variety Syrian Loca l (mu = 3.9 x 10(-3); detected in 15,600 allele-generations), which has a s hort life-span, with the majority of mutants (96.9%) in both varieties diff ering by 1 repeat unit. Compared to animals, higher mutation rates in chick pea are likely to be due to the presence of long (TAA)n microsatellite repe at arrays and the larger number of DNA replications that meristematic initi als of the plants undergo before reaching the reproductive phase. Thus, the long-lived variety undergoes more DNA replications, resulting in an accumu lation of more mutations than in the variety with the shorter life-span.