Fig. 3.

—Diagram of mechanisms for gaining genetic variability during LTR retrotransposons life-cycle. The life-cycle of LTR retrotransposon clans may include two hypothetical phases for gaining genetic variability: a fast phase “generating” variability from the overall replication infidelity during transposition bursts (top, orange background), and a long-lasting quiescent phase that involves the “acquisition” of polymorphisms resulting from ageing (bottom, blue background). In this model, the rate at which a clan bursts is assumed to be higher—on long-term average—than the rate at which the random acquisition of deleterious mutations eventually disturbs its life-cycle. If this condition is not met, a clan may ultimately be driven to extinction due to lethal ageing. Other contingent factors may also be implicated in clan extinction, such as truncation or deletion of its members mediated by host chromosomal recombination or rearrangements. Note that the classical life-cycle from figure 1 and the recombinogenic steps from figure 2 would be contained within the upper fast “generating” variability phase.