Origin of the Alu family: a family of Alu-like monomers gave birth to the left and the right arms of the Alu elements.

The Alu dimeric elements are a common feature of the primate genomes, where they constitute a family of related sequences (1). The identification of a free left Alu monomer (FLAM) family plus a free right Alu monomer (FRAM) family suggests that the dimeric structure results from the fusion of a FLAM sequence with a FRAM sequence (2). Here, we describe a very old Alu-like monomeric family, referred to as FAM for fossil Alu monomer. This family arose from a 7SL RNA sequence and gave birth to the FLAM and FRAM families. From the results obtained, the evolution of the Alu family can be subdivided into two phases. The first phase, which involves only monomeric elements, is characterized by deep remodelling of the progenitor sequences and ends with the appearance of the first Alu dimeric element through the fusion of a FLAM and a FRAM element. The second phase, still in progress, starts with the first Alu dimeric element. This phase is characterized by the stabilization of the progenitor sequences.