Many fungal pathogens are categorized as ‘opportunistic’. This is because, unlike many bacteria and viruses, these fungi are perfectly happy growing on dead or decaying material and do not require a host to complete their lifecycle. For many plant pathogens, the switch from environmental growth to infective parasite is associated with accessory chromosomes. These chromosomes are not found in non-virulent strains and are generally specific to a given host, i.e. one accessory chromosome may allow a strain to infect tomato, while another allows a different strain to infect melon. The origin of these chromosomes is a mystery, and likely involves horizontal gene transfer. How these chromosomes are formed, and subsequently maintain or spread through populations is a major outstanding question and is of central importance in the fight against crop pathogens. The current research projects of the group involve understanding what genes may contribute to the evolution of accessory genomes, including transposable elements and meiotic drivers.