Members of the ascomycete fungi genus Fusarium range from plant pathogens to soil dwellers. The publication of a genomic comparison of several Fusarium species sheds light on the differences between these species and shows that horizontal transfer of entire chromosomes between species has important consequences for the lifestyle and host range of these pathogens.

Ma and colleagues sequenced the genomes of Fusarium verticillioides and Fusarium oxysporum f. sp. lycopersici; a third genome, that of Fusarium graminearum , had been sequenced previously. The host ranges of F. graminearum and F. verticillioides are narrow compared with that of F. oxysporum, which infects a wide range of plants, although each strain has a specific and narrow host range. Using comparative genomics, the authors identified a core genome shared by the three sequenced organisms. Interestingly, the genome of F. oxysporum f. sp. lycopersici is notably larger than that of the other two species and contains many strain-specific genes. Each genome has a different number of chromosomes: 4 in F. graminearum, 11 in F. verticillioides and 14 in F. oxysporum f. sp. lycopersici. Further analysis showed that F. oxysporum f. sp. lycopersici is more closely related to F. verticillioides than to F. graminearum.

F. oxysporum f. sp. lycopersici-specific genes are likely to have been acquired through horizontal gene transfer

To understand the interactions of F. oxysporum f. sp. lycopersici with its host, the tomato plant, the authors then focused on the genomic regions detected only in this strain. Analysis of codon usage indicated that the F. oxysporum f. sp. lycopersici-specific genes are likely to have been acquired through horizontal gene transfer, and phylogenetic analysis of these genes across seven ascomycete genomes indicated that this is likely to have involved horizontal transfer of whole chromosomes from other Fusarium species, although the identity of these species is unknown. To determine whether the horizontal transfer of chromosomes can influence the host range of the fungus, the authors focused on chromosome 14 of F. oxysporum f. sp. lycopersici, which encodes multiple proteins involved in host-specific interactions. The authors mixed F. oxysporum f. sp. lycopersici in which chromosome 14 encoded an antibiotic marker with other strains of F. oxysporum that are pathogenic for different plants and that contained a different antibiotic cassette. Selecting for fungi with both resistances, the authors found that chromosome 14 was transferred from F. oxysporum f. sp. lycopersici to the other strains at a rate of 0.1 to 10 per million spores. Recombination was not detected for other regions of the genome, so the chromosome must have been horizontally transferred. Acquisition of the F. oxysporum f. sp. lycopersici chromosome 14 increased the ability of some recipient strains to colonize tomato plants, indicating that the transfer of chromosomes can alter the host range.

Not all strains that acquired chromosome 14 could grow on tomato plants, and those that could were not as pathogenic as F. oxysporum f. sp. lycopersici. Therefore, loci on other chromosomes also have a role in full adaptation to a particular host. Nonetheless, this is a powerful demonstration that these pathogenic fungi can exchange entire chromosomes and that this can lead to the changes in their host range.