Abstract
Background: The evolutionary trend of short tandem repeats (STRs) at the crossroads of speciation remains largely elusive and attributed to random evolution for the most part. To explore this trend, we selected nine species, which shared sequential chronological ancestors, including rat, mouse, olive baboon, gelada, macaque, gorilla, chimpanzee, bonobo, and human, and collected three sets of data on the abundance of all classes of dinucleotide STRs (≥6-repeats) for three regions of every chromosome, each region spanning 10 Mb of DNA. Results: In all three datasets, we found directional shrinkage of the dinucleotide STR compartment as follows: rodents>Old World monkeys>great apes (P=0.000). The decremented gradient observed for the dinucleotide STRs was not detected for a number of other classes of STRs, such as mono and trinucleotide STRs. Conclusion: We report the first instance of massive and directional gradient of STRs, which may link with the evolution of Old World monkeys and great apes.