Some chromosomes, particularly B chromosomes, have the ability to increase their own inheritance rate for their own benefits. These additional chromosomes are present in numerous species of plants, animals, and fungi. They utilize various tactics to avoid being removed over time since most living organisms tend to eliminate unnecessary genetic material. However, the specific genetic mechanisms that enable B chromosomes to avoid elimination remain largely unclear. A team of researchers from the IPK Leibniz Institute has pinpointed genes on the rye B chromosome that are likely involved in directing this process. This breakthrough was published today in the journal Nature Communications.
Unlike standard A chromosomes, supernumerary B chromosomes are not essential for the normal growth and development of organisms. As of 2024, B chromosomes have been identified in nearly 3,000 species across various eukaryotic groups. While most B chromosomes do not show any obvious negative effects at low counts, higher numbers can lead to unusual physical traits and decreased fertility. To prevent their own removal, many B chromosomes manipulate cell division in their favor, promoting an increase in their number. This behavior is termed chromosome drive. These “selfish” B chromosomes primarily become active when their survival is threatened, rather than for the benefit of the host plant.
Research on chromosome drive mechanisms within B chromosome systems has been conducted across various species and settings, utilizing techniques from traditional genetics to cytogenetics. Despite their potential to provide valuable insights into the mechanisms of chromosome drive, progress in B chromosome research has been slow, particularly in taking advantage of the surge of data from recent DNA sequencing advancements. B chromosomes are structurally complex, repetitive, and often numerous, which complicates efforts to assemble them at the pseudomolecule level, especially before advancements in long-read sequencing. Consequently, our understanding of the genes that control chromosome drive is still quite limited, with few specific gene candidates identified up to now.
To discover the factors controlling drive in the rye B chromosome, the international research team from the IPK Leibniz Institute first focused on narrowing down the drive-control region’s size. They then employed long DNA reads to create a comprehensive assembly of the rye B chromosome into a single ~430 Mb-long pseudomolecule followed by an extensive transcriptome analysis. “Using the newly assembled B chromosome pseudomolecule, we found five candidate genes, whose roles in moderating chromosome drive are confirmed by additional studies,” explains Jianyong Chen, the study’s lead author. “Among them, the DCR28 gene, believed to be crucial in regulating this process, was particularly notable,” highlights Prof. Andreas Houben, who leads the “Chromosome Structure and Function” research group at IPK. Moreover, it was discovered that the B chromosome originated from fragments of all seven rye standard A chromosomes.
These discoveries may also be valuable for understanding genetic disorders linked to uneven chromosome distribution.
