Evolution of subterminal satellite (StSat) repeats in hominids

Subterminal satellite (StSat) repeats, consisting of 32-bp-long AT-rich units (GATATTTCCATGTT(T/C)ATACAGATAGCGGTGTA), were first found in chimpanzee and gorilla (African great apes) as one of the major components of heterochromatic regions located proximal to telomeres of chromosomes. StSat repeats have not been found in orangutan (Asian great ape) or human. This patchy distribution among species suggested that the StSat repeats were present in the common ancestor of African great apes and subsequently lost in the lineage leading to human. An alternative explanation is that the StSat repeats in chimpanzee and gorilla have different origins and the repeats did not occur in human. The purpose of the present study was quantitative evaluation of the above alternative possibilities by analyzing the nucleotide variation contained in the repeats. We collected large numbers of sequences of repeat units from genome sequence databases of chimpanzee and gorilla, and also bonobo (an African great ape phylogenetically closer to chimpanzee). We then compared the base composition of the repeat units among the 3 species, and found statistically significant similarities in the base composition. These results support the view that the StSat repeats had already formed multiple arrays in the common ancestor of African great apes. It is thus suggested that humans lost StSat repeats which had once grown to multiple arrays.