Figure 3.

Schematic of the evolutionary mechanisms that impact centromere repeats. (a) Two models for the derivation of species-specific satellites are shown: (left) A satellite array evolves from a library of satellites, culminating in a dominant satellite; (right) TE insertion(s) followed by mutations, such as deletions, lead to the evolution of new satellites. In both cases, a homogenized array evolves through molecular drive mechanisms, such as intra-array concerted evolution. Stabilization of the arrays into HOR arrays defines the active centromere core, where CENP-A nucleosomes (red) are assembled. Other events, such as inter-array conversion, can lead to the spread of new HORs or changes in HOR copy number (bottom). (b) Two homologous chromosomes share the same satellite repeat (red), but one homolog experiences an expansion of that repeat through de novo mutations. During female meiosis, the larger centromere attracts more microtubules, resulting in the loss of the homolog with the weaker centromere into the polar body during meiosis I. The larger centromere is preferentially driven to the viable egg following unequal distribution of chromatids during meiosis II.