Formation and centromere inactivation of fusion chromosomes in two allotetraploid species from the Saccharum complex

Chromosome fusion can cause a change in the basic chromosome number of a species, thus imposing a major impact on speciation. However, there are few studies on the structure and evolution of fusion chromosomes in plants. Erianthus rockii and Narenga porphyrocoma are allotetraploid species within the Saccharum complex. Both species possess five distinct fusion chromosomes, each resulting from the fusion of two ancestral chromosomes. We developed a high-resolution oligonucleotide (oligo)-based painting technique and constructed a detailed cytogenetic map for each of the 10 fusion chromosomes, enabling visualization of the breakpoints and fusion events of their ancestral chromosomes. We observed several common features associated with the 10 fusion chromosomes. The breakpoints of these fusion chromosomes were consistently located in the proximal regions near the centromeres of the ancestral chromosomes. Each fusion event was accompanied by the inactivation of one of the two ancestral centromeres, along with the loss of the associated centromeric repeats in the inactivated centromere. We conclude that the 10 fusion chromosomes likely originated through different mechanisms, including nested chromosome fusion, end-to-end fusion, and centromere misdivision followed by rejoining of the resulting fragments.