UBIQUITOUS MAMMALIAN-WIDE INTERSPERSED REPEATS (MIRS) ARE MOLECULAR FOSSILS FROM THE MESOZOIC ERA

Short interspersed elements (SlNEs) are ubiquitous in mammalian genome s. Remarkable variety of these repeats among placental orders indicate s that most of them amplified in each lineage independently, following mammalian radiation. Here, we present an ancient family of repeats, w hose sequence divergence and common occurrence among placental mammals , marsupials and monotremes indicate their amplification during the Me sozoic era. They are called MIRs for abundant Mammalian-wide Intersper sed Repeats. With approximately 120,000 copies still detectable in the human genome (0.2-0.3% DNA), MIRs represent a 'fossilized' record of a major genetic event preceding the radiation of placental orders.