Retrotransposons regulate host genes in mouse oocytes and preimplantation embryos.

A comprehensive analysis of transposable element (TE) expression in mammalian full-grown oocytes reveals that LTR class III retrotransposons make an unexpectedly high contribution to the maternal mRNA pool, which persists in cleavage stage embryos. The most abundant transcripts in the mouse oocyte are from the mouse transcript (MT) retrotransposon family, and expression of this and other TE families is developmentally regulated. Furthermore, TEs act as alternative promoters and first exons for a subset of host genes, regulating their expression in full-grown oocytes and cleavage stage embryos. To our knowledge, this is the first example of TEs initiating synchronous, developmentally regulated expression of multiple genes in mammals. We propose that differential TE expression triggers sequential reprogramming of the embryonic genome during the oocyte to embryo transition and in preimplantation embryos.