Stage-specificity of spontaneous mutation at a tandem repeat DNA locus in the mouse germline

Mouse expanded simple tandem repeat (ESTR) loci are the most unstable loci in the mouse genome. Despite the fact that over the last decade these loci have been extensively used for studying germline mutation induction in mice, to date little is known about the mechanisms underlying spontaneous and induced ESTR mutation. Here we used flow cytometry and single-molecule PCR to compare the frequency of ESTR mutation in four flow-sorted fractions of the mouse male germ cells - spermatogonia, spermatocytes I, round and elongated spermatids. The frequency and the spectrum of ESTR mutation did not significantly differ between different stages of mouse spermatogenesis. Considering these data and the results of other publications, we propose that spontaneous ESTR mutation is mostly attributed to replication slippage in spermatogonia and these loci may be regarded as a class of expanded microsatellites.