Banana Genome Contributes to Understand Grass Chromosome Number Reduction and Genome Repatterning Mechanism

Following repeated occurrence of polyploidies, angiosperm chromosome numbers have usually been restored to a narrow range, possibly contributing to the recovery of diploid heredity. A comparative analysis of banana and grass genomes shows that the common ancestor of grasses had 7 basic chromosomes before a grass-common tetraploidization, after which the number of chromosomes doubled and then reduced to 12, which have been preserved in rice but continued to reduce in other grasses. Then, we showed that chromosome number reduction (CNR) was best explained by intra- and/or inter-chromosomal crossovers to form new chromosomes that utilize the existing telomeres of ‘invaded’ and centromeres of ‘invading’ chromosomes, the alternative centromeres and telomeres being lost. The ‘invading’ and ‘invaded’ chromosomes are frequently homoeologs, originating from duplication of a common ancestral chromosome and with greater-than-average DNA-level correspondence to one another. Finally, we proposed a telomere-centric model to describe genome repatterning after polyploidization, which may be a general mechanism of restoring small linear chromosome numbers in higher eukaryotes.