Genome complexity: centromere control in plants (Introduction)

by David Turell , Friday, November 12, 2021, 19:51 (896 days ago) @ David Turell

Held in tight bounds:

https://www.science.org/doi/10.1126/science.abi7489

"Centromeres are key for anchoring chromosomes to the mitotic spindle, but they have been difficult to sequence because they can contain many repeating DNA elements. These repeats, however, carry regularly spaced, distinctive sequence markers because of sequence heterogeneity between the mostly, but not completely, identical DNA sequence repeats. Such differences aid sequence assembly. Naish et al. used ultra-long-read DNA sequencing to establish a reference assembly that resolves all five centromeres in the small mustard plant Arabidopsis. Their view into the subtly homogenized world of centromeres reveals retrotransposons that interrupt centromere organization and repressive DNA methylation that excludes centromeres from meiotic crossover repair. Thus, Arabidopsis centromeres evolve under the opposing forces of sequence homogenization and retrotransposon disruption. (my bold)

"The centromeres of eukaryotic chromosomes assemble the multiprotein kinetochore complex and thereby position attachment to the spindle microtubules, allowing chromosome segregation during cell division. The key function of the centromere is to load nucleosomes containing the CENTROMERE SPECIFIC HISTONE H3 (CENH3) histone variant [also known as centromere protein A (CENPA)], which directs kinetochore formation. Despite their conserved function during chromosome segregation, centromeres show radically diverse organization between species at the sequence level, ranging from single nucleosomes to megabase-scale satellite repeat arrays, which is termed the centromere paradox. Centromeric satellite repeats are variable in sequence composition and length when compared between species and show a high capacity for evolutionary change, both at the levels of primary sequence and array position along the chromosome. However, the genetic and epigenetic features that contribute to centromere function and evolution are incompletely understood, in part because of the challenges of centromere sequence assembly and functional genomics of highly repetitive sequences. New long-read DNA sequencing technologies can now resolve these complex repeat arrays, revealing insights into centromere architecture and chromatin organization.

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"Our Col-CEN assembly and functional genomics analysis have implications for understanding centromere sequence evolution in eukaryotes. We propose that a recombination-based homogenization process, occurring between allelic or nonallelic locations on the same chromosome, maintains the CEN180 library close to the consensus optimal for CENH3 recruitment. The advantage conferred to ATHILA retrotransposons by integration within the centromeres is presently unclear. They may be engaged in centromere drive, supporting the hypothesis that centromere satellite homogenization acts as a mechanism to purge driving elements. Each Arabidopsis centromere appears to represent different stages in cycles of satellite homogenization and ATHILA-driven diversification. These opposing forces provide both a capacity for homeostasis and a capacity for change during centromere evolution. In the future, assembly of centromeres from multiple Arabidopsis accessions and closely related species may further clarify how centromeres form and the evolutionary dynamics of CEN180 and ATHILA repeats."

Comment: Note my bolds. Such tight controls are automatic, and in life provide persisting homeostasis, which is why so much energy is required by life. Staying alive is a constant biochemical battle.