Do A-stretches inhibit the emergence of new variants in STR loci?

Penta E and D18S51 loci are known to be highly polymorphic in humans. In this study, Japanese macaque PCR products amplified with human Penta E and D18S51 primers were examined. Electrophoresis and sequence analyses revealed that 13 Japanese macaque products amplified with human Penta E primer were homogeneous. The Japanese macaque sequence was extremely similar with human Penta E alleles except for the insertion of contiguous adenosine repeats, called 'A-stretch', at 5'-side of AAAGA repeats. Among the 11 Japanese macaque PCR products amplified with human D18S51 primers, only two variants were observed. The sequences of these Japanese macaque products were similar to those of human D18S51 alleles. However, the Japanese macaque sequences also contained the insertion of A-stretch at 5'-side of AGAA repeats. Less polymorphism in the Japanese macaque sequences, in contrast with highly polymorphic human Penta E and D18S51 loci, suggested that A-stretches might inhibit the emergence of new variants at the STR loci.