The genomes of many plants, animals, and fungi frequently comprise dispensable B chromosomes that rely upon various chromosomal drive mechanisms to counteract the tendency of non-essential genetic elements to be purged over time. The B chromosome of rye - a model system for nearly a century - undergoes targeted nondisjunction during first pollen mitosis, favouring segregation into the generative nucleus, thus increasing their numbers over generations. However, the genetic mechanisms underlying this process are poorly understood. Here, using a newly-assembled, ~430 Mb-long rye B chromosome pseudomolecule, we identify five candidate genes whose role as trans-acting moderators of the chromosomal drive is supported by karyotyping, chromosome drive analysis and comparative RNA-seq. Among them, we identify DCR28, coding a microtubule-associated protein related to cell division, and detect this gene also in the B chromosome of Aegilops speltoides. The DCR28 gene family is neo-functionalised and serially-duplicated with 15 B chromosome-located copies that are uniquely highly expressed in the first pollen mitosis of rye.

Biology Centre Czech Academy of Sciences Ceske Budejovice Czech Republic

Department of Molecular Signal Processing Leibniz Institute of Plant Biochemistry Halle Germany

Institute of Agricultural and Nutritional Sciences Martin Luther University Halle Wittenberg Halle Germany

Institute of Biology Department of Plant Cell Biology Philipps University Marburg Marburg Germany

Institute of Experimental Botany of the Czech Academy of Sciences Centre of Plant Structural and Functional Genomics Olomouc Czech Republic

KWS SAAT SE and Co KGaA Einbeck Germany

Leibniz Institute of Plant Genetics and Crop Plant Research Gatersleben Seeland Germany

School of Agriculture Forestry and Ecosystem Science The University of Melbourne Parkville VIC Australia

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