Abstract

Humanity has been taking advantage of plant breeding and chemical fungicides to avoid the damage by diseases. In spite of such efforts, plant diseases are presently a big threat to food production, and the annual worldwide loss by plant diseases is estimated $220 billion (Agrios 2005). Studies on the origin of plant pathogens would help to understand pathogenicity mechanisms and to establish an effective method to control the diseases. Although a common accepted theory is that a plant pathogen originates from a non-pathogen, few studies have demonstrated the theory in scientific way so far. Here, we introduce our studies attempting to demonstrate the theory using Solanum lycopersicum (cultivated tomato) and the tomato wilt fungus Fusarium oxysporum f. sp. lycopersici (FOL). S. lycopersicum is estimated to originate in Southern America, such as Chile, Ecuador, Peru, and Bolivia, where several Solanum species still inhabit at this moment. It has been reported that a part of these species (possibly S. pimpinellifolium) have spread to Mexico, and tomato was domesticated as S. lycopersicum var. cerasiforme prehistorically. In Mexico, there are diverse traditional tomato varieties so-called "jitomate criollo", having irregular multiloculed type fruits, and they are supposed to be the transitional forms between var. cerasiforme and the modern cultivars (Jenkins 1948). F. oxysporum is a well-known plant pathogenic fungus causing diseases in many plants. Although this fungus does not have a broad host range, each isolate of the fungus has narrow host range and this host specificity is defined as Ôforma specialis, f. sp.Õ based on each specific pathogenicity. For example, the causal pathogen of tomato wilt disease, FOL, invades from the roots, expands in xylem, and causes symptoms such as wilt, yellows, and plant death in tomato, but not in other plants. On the other hand, F. oxysporum f. sp. fragariae causes strawberry wilt diseases, but not in other plants including tomato. Moreover, an enormous population of non-pathogenic F. oxysporum (NPF) distributes in diverse environments mainly in soil. So we hypothesized that NPFs, which have ability to colonize tomato tissues acquired pathogenicity during tomato domestication and breeding, resulted in the emergence of FOL. To test this hypothesis, we have collected F. oxysporum, which was colonizing wild, transitional, and cultivated tomatoes, and compared the phylogenetic relationship with FOL. For the purpose, we isolated F. oxysporum from tissues and rhizosphere soil of the following healthy samples; 1) wild species of tomato (S. chilense and S. peruvianum in Chile, and S. pimpinellifolium in Ecuador), 2) transitional tomatoes (var. cerasiforme and jitomate criollo in Mexico), and 3) modern tomato cultivars worldwide. We have obtained 273 isolates of F. oxysporum, and pathogenicity assay showed that all of the isolates were NPF. A phylogeny of the NPF isolates obtained in this study and FOL isolates collected worldwide was constructed based on rDNA intergenic spacer (IGS) region. The phylogeny revealed that only 11 NPF isolates were closely related to FOL. All of the 11 NPF were the isolates obtained from jitomate criollo, var. cerasiforme, or modern cultivars, but not from the wild species in Andes. Furthermore, we have targeted three host-determining genes (SIX1, SIX3, SIX4) found in FOL (Rep et al. 2004; Houterman et al. 2008; 2009). SIX genes are unique in FOL, and are not found on the genomes of other organisms. These genes are located on a small chromosome enriched with mobile elements, according to the FOL Genome Database (Broad Institute, MIT, USA). Thus, we analyzed if any of the NPFs isolates have SIX genes, but none of the NPF including the 11 isolates phylogenetically close to FOL had the genes. Based on the results, we consider a scenario that 11 NPFs having ability to colonize tomato, acquired SIX genes or the chromosome carrying the genes during /after tomato domestication phase in Mexico, resulted in the emergence of FOL.