When microsatellites were divided into three classes,
consisting of AT-rich (greater than 50% A or T
in the motif), AT/GC-balanced (limited to di- and
tetra-nucleotide motifs that fit this criterion), or GC-rich
motifs, the longest repeat motifs corresponded to AT-rich
microsatellites where the average number of repeat units
was 19 and the average length of the repeated sequence
was 48 nt. The AT/GC-balanced class averaged 17 repeats,
while the GC-rich tri-nucleotides showed the least
potential for expansion, with an average repeat tract
length of 8.5 units, consisting of 26 nt (Fig. 2). The
longest tract of perfect repeats corresponded to the
di-nucleotide (AT)128 (RM8135). These statistics are of
interest because length of repeat has a demonstrated association
with rates of polymorphism in rice.9,23