The localization of centromere protein A is conserved among tissues

CENP-A/CENP-B uncoupling in the evolutionary reshuffling of centromeres in equids

BACKGROUND

While CENP-A is the epigenetic determinant of the centromeric function, the role of CENP-B, a centromeric protein binding a specific DNA sequence, the CENP-B-box, remains elusive. In the few mammalian species analyzed so far, the CENP-B box is contained in the major satellite repeat that is present at all centromeres, with the exception of the Y chromosome. We previously demonstrated that, in the genus Equus, numerous centromeres lack any satellite repeat.

RESULTS

In four Equus species, CENP-B is expressed but does not bind the majority of satellite-based centromeres, or the satellite-free ones, while it is localized at several ancestral, now-inactive, centromeres. Centromeres lacking CENP-B are functional and recruit normal amounts of CENP-A and CENP-C. The absence of CENP-B is related to the lack of CENP-B boxes rather than to peculiar features of the protein itself. CENP-B boxes are present in a previously undescribed repeat which is not the major satellite bound by CENP-A. Comparative sequence analysis suggests that this satellite was centromeric in the equid ancestor, lost centromeric function during evolution, and gave rise to a shorter CENP-A bound repeat not containing the CENP-B box but enriched in dyad symmetries.

CONCLUSIONS

We propose that the uncoupling between CENP-B and CENP-A may have played a role in the extensive evolutionary reshuffling of equid centromeres. This study provides new insights into the complexity of centromere organization in a largely biodiverse world where the majority of mammalian species still have to be studied.

The structure, function, and evolution of plant centromeres

Centromeres are essential regions of eukaryotic chromosomes responsible for the formation of kinetochore complexes, which connect to spindle microtubules during cell division. Notably, although centromeres maintain a conserved function in chromosome segregation, the underlying DNA sequences are diverse both within and between species and are predominantly repetitive in nature. The repeat content of centromeres includes high-copy tandem repeats (satellites), and/or specific families of transposons. The functional region of the centromere is defined by loading of a specific histone 3 variant (CENH3), which nucleates the kinetochore and shows dynamic regulation. In many plants, the centromeres are composed of satellite repeat arrays that are densely DNA methylated and invaded by centrophilic retrotransposons. In some cases, the retrotransposons become the sites of CENH3 loading. We review the structure of plant centromeres, including monocentric, holocentric, and metapolycentric architectures, which vary in the number and distribution of kinetochore attachment sites along chromosomes. We discuss how variation in CENH3 loading can drive genome elimination during early cell divisions of plant embryogenesis. We review how epigenetic state may influence centromere identity and discuss evolutionary models that seek to explain the paradoxically rapid change of centromere sequences observed across species, including the potential roles of recombination. We outline putative modes of selection that could act within the centromeres, as well as the role of repeats in driving cycles of centromere evolution. Although our primary focus is on plant genomes, we draw comparisons with animal and fungal centromeres to derive a eukaryote-wide perspective of centromere structure and function.