On transposons and totipotency.

Our perception of the role of the previously considered 'selfish' or 'junk' DNA has been dramatically altered in the past 20 years or so. A large proportion of this non-coding part of mammalian genomes is repetitive in nature, classified as either satellites or transposons. While repetitive elements can be termed selfish in terms of their amplification, such events have surely been co-opted by the host, suggesting by itself a likely altruistic function for the organism at the subject of such natural selection. Indeed numerous examples of transposons regulating the functional output of the host genome have been documented. Transposons provide a powerful framework for large-scale relatively rapid concerted regulatory activities with the ability to drive evolution. Mammalian totipotency has emerged as one key stage of development in which transposon-mediated regulation of gene expression has taken centre stage in the past few years. During this period, large-scale (epigenetic) reprogramming must be accomplished in order to activate the host genome. In mice and men, one particular element murine endogenous retrovirus with leucine tRNA primer (MERVL) (and its counterpart human ERVL (HERVL)) appears to have acquired roles as a key driving force in this process. Here, I will discuss and interpret the current knowledge and its implications regarding the role of transposons, particularly of long interspersed nuclear elements (LINE-1s) and endogenous retroviruses (ERVs), in the regulation of totipotency. This article is part of a discussion meeting issue 'Crossroads between transposons and gene regulation'.