Accessory Chromosome Contributes to Virulence of Banana Infecting <i>Fusarium oxysporum</i> Tropical Race 4

Filamentous fungi have evolved compartmentalised genomes comprising conserved core regions and dynamic accessory regions, which are thought to drive adaptation to changing environments, including interactions with host organisms. Tropical Race 4 (TR4) is a lineage of banana-infecting Fusarium spp. and causes a devastating Fusarium wilt epidemic in the industrial banana cultivar Cavendish. A recent study showed that TR4 contains a single accessory chromosome (chromosome 12), which in some strains has undergone extensive internal duplications, tripling its size compared to other closely related strains. However, the contribution of this accessory chromosome to virulence is currently unknown. Here we show that the induced loss of accessory chromosome 12 in the TR4 reference strain II5 leads to reduced virulence on banana plants. Moreover, loss of chromosome 12 co-occurs with structural rearrangements of conserved core chromosomes. Together, our results provide new insights into the chromosome dynamics of the banana-infecting Fusarium TR4 lineage and highlight the importance of its unique accessory chromosome in virulence.