Functional monocentricity with holocentric characteristics and chromosome-specific centromeres in a stick insect.

Centromeres are essential for chromosome segregation in eukaryotes, yet their specification is unexpectedly diverse among species and can involve major transitions such as those from localized to chromosome-wide centromeres between monocentric and holocentric species. How this diversity evolves remains elusive. We discovered within-cell variation in the recruitment of the major centromere protein CenH3, reminiscent of variation typically observed among species. While CenH3-containing nucleosomes are distributed in a monocentric fashion on autosomes and bind tandem repeat sequences specific to individual or groups of chromosomes, they show a longitudinal distribution and broad intergenic binding on the X chromosome, which partially recapitulates phenotypes known from holocentric species. Despite this variable CenH3 distribution among chromosomes, all chromosomes are functionally monocentric, marking the first instance of a monocentric species with chromosome-wide CenH3 deposition. Together, our findings illustrate a potential transitional state between mono- and holocentricity or toward CenH3-independent centromere determination and help to understand the rapid centromere sequence divergence between species.