The accessory genome as a cradle for adaptive evolution in pathogens

By comparison, the core genome carries essential genes that encode basal functions such as cellular metabolism and reproduction. Because of its essential functions, the core genome is thought to be shielded from high recombination and mutation rates and insertion of novel genes. Similar to the processes observed for the mating type loci in Cryptococcus neoformans and Neurospora tetrasperma [18], [19], as well as heteromorphic animal sex chromosomes [20], an accessory chromosome is expected to gradually decay in a population unless homologous pairings help purge deleterious mutations. Understanding the evolutionary dynamics of accessory chromosomes in fungi will require consideration of both the beneficial effects of virulence genes located on these chromosomes and their transmission characteristics through both meiosis and horizontal chromosome transfer.