Asynchronous evolution of centromeric sequences across chromosomes in Pyricularia oryzae.

Abstract

Centromeres are essential for chromosome segregation, yet they are among the most rapidly evolving regions of the genome. The mechanisms driving this rapid evolution of centromeric sequences are still not well understood. In this study, we identified the centromeric sequences of the wheat-infecting fungus Pyricularia oryzae (strain Br48) using CENP-A chromatin immunoprecipitation followed by high-throughput sequencing. The Br48 centromeres range from 71 kb to 101 kb in length and are highly AT-rich (72.1-75.5%) and repeat-rich (63.4-85.0%). These regions are also enriched for H3K9me3 and 5-methylcytosine but depleted of H3K4me2 and H3K27me3. During the analysis of repetitive sequences in the Br48 centromere, we identified a stretch of approximately 530 bp that is tightly associated with centromeres in P. oryzae. We named this element the CenIR (centromere-associated IR element), as it often forms inverted repeat structures with two elements adjacent in reverse orientation. A comparison of putative centromere sequences across phylogenetically distinct P. oryzae strains suggests that changes in centromeric sequences are non-uniform across chromosomes and do not always align with the fungal phylogenetic relationships. Repeat-induced point mutation (RIP)-like C:G to T:A transitions likely accelerate base substitutions in the centromeres of Pyricularia fungi.