2 Origin, Function, and Transmission of Accessory Chromosomes

Fungal genomes are highly diverse and remarkably variable—even within species. Presence/absence polymorphisms of entire chromosomes are frequently found between individuals of a population. The affected chromosomes are considered accessory and therefore not essential for growth. How these accessory chromosomes are maintained in a population is unknown, but it may either be due to a fitness benefit or a transmission advantage during meiosis or mitosis. Although many fungal accessory chromosomes were shown to confer a fitness benefit, e.g., by encoding virulence determinants essential for virulence on specific hosts, new findings demonstrate that also transmission advantages play a role in their maintenance in fungal genomes. Here, we report the current knowledge of the origin and function of fungal accessory chromosomes with a particular focus on their modes of transmission during mitosis and meiosis. We discuss transmission advantages as possible mechanisms for the widespread occurrence of these chromosomes.