Upgrade to virulence: Conditionally dispensable chromosomes of fungal plant pathogens and their role in host infection.

Abstract

Fungal pathogens are a significant threat to agriculture and world food security. The fungal plant pathogens Fusarium oxysporum, Nectria haematococca and Alternaria alternate can infect a very broad range of hosts and are significant threats to agriculture. Pathogenic strains of these fungi all carry so-called conditionally dispensable (CD) chromosomes. These small chromosomes are dispensable for saprophytic growth, but essential for growth on a specific niche, i.e. a host plant. They are functionally similar to bacterial virulence plasmids, which allow bacteria to undergo rapid host jumps when these plasmids are acquired via horizontal gene transfer (HGT). HGT between eukaryotes was thought to be limited to a few ancient events, but sequence analyses of CD chromosomes have strongly suggested that CD chromosomes are transferred horizontally between fungal populations. This thesis will give an overview of how CD chromosomes have enabled filamentous fungi to infect new hosts and to what extent horizontal transfer of CD chromosomes has enabled this. CD chromosomes contain only a few genes, many of which encode virulence related genes. The presence of a CD chromosome in the genome of fungi is linked to virulence on specific hosts. This is evident in A. alternata and F. oxysporum as these species can be subdivided into several pathotypes or formae speciales, which can infect a very broad range of hosts. Evidence strongly proposes that CD chromosomes are responsible for determining host range of these pathotypes. A mechanism for horizontal transfer of CD chromosomes in fungi has not yet been elucidated, but experimental evidence strongly suggests that chromosomes can be exchanged during conidial anastomosis. Horizontal chromosome transfer of CD chromosomes may enable the formation of super pathogens with a very broad host range. Studying the makeup, origins and propagation of CD chromosomes therefore offers a unique insight into how new fungal plant pathogens emerge and what kind of genes are essential for this process. Furthermore, by determining how chromosomes are transferred, it may be possible to stop diseases spreading.