B chromosomes direct cell division for survival, and new research identifies key genes involved in this process in rye, including DCR28. Unlike standard A chromosomes, additional B chromosomes are not necessary for the normal growth and development of organisms. As of 2024, B chromosomes have been identified in approximately 3,000 species across all eukaryotic phyla. Although a low number of B chromosomes usually has no apparent selective effect, a higher number can lead to phenotypic abnormalities and reduced fertility.

To prevent extinction, many B chromosomes influence cell division in their favor and increase their copy number. This phenomenon is called chromosomal drive. Thus, the “selfish” B chromosomes are activated only when their existence is threatened, not for the benefit of the plant.

Problems studying the movement of the B chromosome

Control mechanisms in B chromosome systems have been studied in many species and in different contexts using different technologies, from classical genetics to cytogenetics.

But although B chromosomes are an ideal test case for studying the deep mechanisms of chromosomal drive, B chromosome studies have been slow to take advantage of the explosion of information caused by the DNA sequencing explosion: B chromosomes have high structural complexity, reproducibility, and so on. and multiplicity, which makes them resistant to pseudomolecular assembly of chromosomes, especially recent advances in long-read sequencing.

Therefore, understanding at the gene level of the specific mechanisms controlling chromosome movement is quite limited, and specific candidate genes involved in this phenomenon have not yet been identified.

Groundbreaking development in rye B chromosome research

In order to identify the factor(s) that control the driving force of the rye B chromosome, an international research group led by the Institute Leibniz initially narrowed down the size of the area that controls the driving force. Next, using long DNA sequences, the researchers assembled the rye B chromosome into a single ~430 Mb pseudomolecule and performed a detailed transcriptome analysis.

“Using the newly created B chromosome pseudomolecule, we identified five candidate genes whose roles as moderators of chromosome movement were confirmed by additional studies,” explains Jianyong Chen, first author of the study. “The DCR28 gene stands out, which is probably responsible for the regulation of this process,” emphasizes Professor Andreas Huben, head of the IPK “Structure and Function of Chromosomes” research group. Additionally, the B chromosome has been shown to be derived from parts of all seven standard rye A chromosomes.