ASCERTAINMENT BIAS CANNOT ENTIRELY ACCOUNT FOR HUMAN MICROSATELLITES BEING LONGER THAN THEIR CHIMPANZEE HOMOLOGS

A large majority of human microsatellite markers are longer than their homologues in chimpanzees, suggesting that more expansion mutations h ave occurred in the lineage leading to humans. However, such a length difference has also been explained as arising from the selection of un usually long microsatellites as genetic markers. In order to resolve t his controversy and to establish the true source of the observed lengt h differences, we have now conducted the necessary reciprocal study. W e have compared the lengths of size-selected markers cloned from chimp anzees between this species and humans. We find that of 19 markers whi ch were informative and polymorphic in both species, 13 are longer in humans. This result is incompatible with ascertainment bias being the sole explanation for the inter-specific length differences. We estimat e that dinucleotide repeat microsatellites are an average of 3.2 repea t units longer in RESULTS humans than in chimpanzees, implying a mutat ional bias in favour of microsatellite expansions and a higher average genome-wide microsatellite mutation rate in the human lineage.