Figure 2.

Mechanisms of karyotype evolution. (I) Centromere repositioning can occur when a native centromere actively recruiting CENP-A (red) becomes inactivated (dark blue) through DNA- (loss of centromeric sequence) or epigenetic- (loss of CENP-A incorporation) mediated events, or due to a chromosomal fission event wherein one chromosome segment loses the native centromere. In both cases, centromere repositioning occurs when CENP-A is recruited and fixed to an ectopic location which now serves as the functioning centromere. (II) satellite expansions (blue) can mediate karyotypic evolution via mis-segregation and mitotic defects caused by defects to chromatin structure. Large satellite expansions can produce chromatin defects, causing chromatids to fail to segregate (bottom right). (III) Transposable elements and other repeats (orange and brown) sharing sequence identity across chromosomes can result in non-allelic homologous recombination (NAHR), wherein unequal crossing over results in chromosome modifications across regions that are not alleles (white chromosome and blue chromosome are not haplotypic). Below, a zoomed panel (blue box) shows a possible NAHR event occurring between two transposable elements (orange and brown arrows), leading to duplication and deletion events on different chromosomes. (IV) Other chromosomal mechanisms of rearrangement can contribute to karyotypic variation, including chromosome inversions, deletions, and duplications. The fate of segments from two non-allelic chromosomes (white and blue) are indicated. Red dots demarcate the CENP-A-delimited centromere and orange lines represent a locus.