How do B chromosomes evolve? – Are they a product of chromoanagenesis and a stepwise sequence accumulation process?

• Applicant: Privatdozent Dr. Andreas Houben
• Subject Area: Plant Genetics and Genomics ; General Genetics and Functional Genome Biology
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 559365585

In a wide range of fungi, plants and animals, dispensable B chromosomes (Bs) exist in addition to the standard (A) chromosomes. Although the first B was identified more than 100 years ago, they still represent an uncharted genome component in most B chromosome-carrying species. We aim to unravel the origin of this enigmatic chromosome using the related plant grass species rye, Aegilops speltoides and Aegilops mutica, as well as the eudicot Brachycome dichromosomatica. Our preliminary analysis revealed that the Bs of rye and Ae. speltoides represent a mosaic of A chromosomal and organelle-derived sequences. This raises fundamental questions: Are chromoanagenesis and subsequent stepwise sequence accumulation involved in the genesis of Bs? Did Bs in cereals evolve multiple times, or do they share the same origin? When and how did B chromosomes accumulate nucleus-, chloroplast- and mitochondrion-derived sequences? Does the (peri)centromere composition differ between A and B chromosomes? To address these questions, we will sequence and assemble the B chromosomes of these species and perform a detailed evolutionary analysis. Our research on B chromosomes will provide insights into the plasticity of genomes as well as on the mechanisms underlying the maintenance and modification of the B chromosome structure.

• DFG Programme: Research Grants