ALT Sequence identity heat maps from forward-in-time simulations of centromere evolution. On the left, the starting point is the Col-0 centromere of chromosome 1, about 2.5 Mb long. Three independent simulations are shown as rows, each at 30,000, 60,000, 90,000, 120,000 and 150,000 generations. Red squares along the diagonal mark homogenized blocks of highly similar repeats. Over time, blocks expand, shrink, disappear and newly emerge, with some reaching megabase sizes similar to those in natural centromeres. Centromere length also varies between simulations, from about 1 to 5 Mb.