GENE CONVERSION AS A SECONDARY MECHANISM OF SHORT INTERSPERSED ELEMENT (SINE) EVOLUTION

The Alu repetitive family of short interspersed elements (SINEs) in pr imates can be subdivided into distinct subfamilies by specific diagnos tic nucleotide changes. The older subfamilies are generally very abund ant, while the younger subfamilies have fewer copies. Some of the youn gest Alu elements are absent in the orthologous loci of nonhuman prima tes, indicative of recent retroposition events, the primary mode of SI NE evolution. PCR analysis of one young Alu subfamily (Sb2) member fou nd int eh low-density lipoprotein receptor gene apparently revealed th e presence of this element in the green monkey, orangutan, gorilla, an d chimpanzee genomes, as well as the human genome. However, sequence a nalysis of these genomes revealed a highly mutated, older, primate-spe cific Alu element was present at this position in the nonhuman primate s. Comparison of the flanking DNA sequences upstream of this Alu inser tion corresponded to evolution expected for standard primate phylogeny , but comparison of the Alu repeat sequences revealed that the human e lement departed from this phylogeny. The change in the human sequence apparently occurred by a gene conversion event only within the Alu ele ment itself, converting it from one of the oldest to one of the younge st Alu subfamilies. Although gene conversions of Alu elements are clea rly very rare, this finding shows that such events can occur and contr ibute to specific cases of SINE subfamily evolution.