Gene-for-gene interactions in rust fungi

Did You Know?

• Rust fungi belong to the phylum Basidiomycota.
• Not all rusts are obligate pathogens; some have been cultured without requiring a living host.
• Rust fungi can have up to five different spore stages, though not all stages occur in every species.

Genetic components

The gene-for-gene theory involves two types of genes: resistance (R) genes from the host plant and avirulence (Avr) genes from the pathogen. R genes are vertically transferred in the host, while Avr genes reside in the rust. Flor observed that both susceptibility/resistance and virulence/avirulence depend on a single gene. Avirulence genotypes are either dominant (AVR) or recessive (avr); resistance genotypes are dominant (RR) or recessive (rr). A pathogen with a dominant AVR genotype will not infect a host that carries a dominant RR genotype, but will infect a host with a recessive rr genotype. A pathogen with a recessive avr genotype can infect hosts regardless of the host’s R genotype. Physiological races of rust fungi are determined by different alleles at the avirulence loci. Flor classified races by comparing 18 flax rust lines, each containing a single rust-conditioning gene, to known North American races based on diagnostic resistances or susceptibilities. He believed this method would promote rust resistance and help understand the role of hybridization and mutation in the emergence of new races.

Evolutionary impact

Gene-for-gene models indicate that dispersal of pathogen–host interactions functions in metapopulations, where the same species is spatially distributed in habitat patches that independently determine their own dynamics. A two-simulation model with five resistance and five virulence genes distributed in populations with gene flow, migration, mutation, and co-evolution determined that gene-for-gene interactions exert a fitness cost in both pathogen and host, affecting their life histories. Co-evolution involves mutual genetic changes in rusts and their hosts over time. It has been suggested that further research into the exact genes involved in mutual polymorphism should examine the types of nucleotides in DNA sequences and mutations, and that pathogen and host genotypes should be compared side by side. Co-evolution involving gene-for-gene interactions could have biomedical importance in terms of new ways to fight diseases. In agriculture, humans alter seed genetics and rule out the evolutionary arms race, whereas natural populations experience different inheritance patterns. Gene-for-gene co-evolution plays a role in local agriculture because crops must maintain resistance to pathogens.

Ecological and economic implications

A study of the rust fungus Triphragmium ulmariae and its host Filipendula ulmaria in metapopulations on 70 islands examined how climate change affects pathogen prevalence, severity, and incidence. Evidence showed an increase in extinction of T. ulmariae in relation to warming temperatures, with lower prevalence of the pathogen in F. ulmaria. Mild winter temperatures led to a great increase in pathogen prevalence, causing epidemics, while high temperatures increased extinction rates of rust pathogens. Pathogens allow host plants to be genetically diverse in resistance, providing plants the opportunity to diversify their resistance genes. It has been suggested that a lack of pathogens might disrupt natural ecosystems and change community structure and interactions. Understanding gene-for-gene interactions in agriculture is important because resistance genes play a role in crop production. Co-evolution by gene-for-gene interactions strengthens the plant immune system, and knowledge of these genetics could help create technologies to alter crop immune systems, such as introducing PAMP-recognition for certain pathogens in the ETI immune system.