Copy and paste: the impact of a new non-L1 retroposon on the gonosomal heterochromatin of Microtus agrestis.

Mobile elements are most abundant in the mammalian genome, comprising at least 40-50% of the DNA. They are differentiated into two most prominent families: the LINE elements, which are preferentially located in the G-bands, and SINES, which are clustered in the R-bands. We report here a novel mammalian non-L1-retroposon, which invaded the genome of Microtus agrestis in a very short time from an evolutionary viewpoint. No relevant sequence homology could be demonstrated to known sequences in the NCBI database. However, cross-hybridizing sequences exist in the genomes of all other Microtus species analyzed, but not in Mus musculus, indicating the recent evolutionary origin of this element. This retroposon is enriched in the entire heterochromatin of the X and Y chromosomes, but is also interspersed in autosomal locations in euchromatic portions of the genome. We show that the retroposon is heavily transcribed from the heterochromatin during female meiosis prerequisite for the subsequent retrotransposition. The estimated rate of retrotransposition is at least 1-2 x 10(-2) per generation, which is hundred-fold higher than that of the majority of invertebrate retroposons and also higher than the transposition rate of a murine L1 element, which was calculated to be 3 x 10(-3) per generation.