Model for the maturation of a centromere during evolution. Different pathways can be envisaged leading to a fully mature satellite-based repositioned centromere (D) from an ancestral centromere with satellite repeats (A) through satellite-free intermediates (B,C,E,F). The first route (A–D) follows the previously proposed model (Piras et al. 2010): a neocentromere arises in a satellite-free region; satellite repeats may then colonize this repositioned centromere at a later stage, giving rise to a “mature” centromere; meanwhile the ancestral satellite DNA is lost. Alternative routes (A, B, E, D or A, B, C, F, D) imply that, at an already functional satellite-free centromere, amplification occurs as an intermediate step toward complete maturation of the neocentromere. In this model, neocentromere maturation and loss of satellite DNA from the old centromere site are independent events that can occur at different stages during evolution. Donkey chromosomes exemplifying each step are listed, taking into account the position of satellite DNA as previously described (Piras et al. 2010). Horse Chromosome 11 is also reported since its evolutionary stage (C) was previously analyzed (Wade et al. 2009). We cannot exclude that sequence amplification may precede neocentromere formation (G?) but we have no data supporting this possibility.
