Tomatoes resistant to the fungus Cladosporium fulvum possess the so-called Cf resistance genes. Kruijt investigated the evolution of these genes in wild tomato species. The researcher managed to demonstrate that a number of these Cf resistance genes were already present in an ancestral tomato species. In all probability, the fungus C. fulvum was already a pathogen of this ancestral tomato species, and therefore the resistance genes Cf-4 and Cf-9 have been retained in the various modern wild tomato species.
The researcher also discovered that wild plants on which the Peruvian or berry tomatoes grow contain not one but three resistance genes, all of which recognise the same fungal factor. These three genes are the result of a series of changes that have led to complete pieces of DNA being duplicated.
Kruijt also demonstrated that DNA exchange between the various Cf genes has led to a new Cf resistance gene.
Cladosporium fulvum causes a fungal disease in tomato plants. Wild tomato species contain many resistance genes against this fungus. In particular, the Cf-9 gene from the wild berry tomato Lycopersicon pimpinellifolium is frequently used in commercial tomato varieties. Each of the Cf resistance genes recognises a different product from the fungus and this recognition in the tomato leaf causes a number of cells around the fungus to die. This 'scorched earth tactic' ensures that the fungus cannot infect the rest of the plant.
The research was funded by the Netherlands Organisation for Scientific Research.