Relationships among Brazilian and worldwide isolates of Fusarium oxysporum f. sp. lactucae race 1 inferred from ribosomal intergenic spacer (IGS-rDNA) region and EF-1α gene sequences

Fusarium wilt, caused by Fusarium oxysporum f. sp. lactucae (FOLac), is amongst the main diseases affecting lettuce in subtropical regions. Although nationwide surveys indicated the exclusive presence of FOLac race 1 in Brazil, no detailed studies are available providing molecular evidences if these isolates were introduced into the country via contaminated seeds or if they are endemic populations. The translation elongation factor 1α (EF-1α) gene and rDNA intergenic spacer (IGS-rDNA) region represent the most comprehensive databases for comparative analyses of Fusarium isolates. Our aim was to assess the genetic relationships of 23 Brazilian FOLac race 1 isolates with a collection of FOLac isolates of worldwide origin, using the information from these genomic regions. A consistent single-cluster pattern was observed for FOLac race 1 isolates from Brazil, California-USA, Arizona-USA, Japan, Italy, as well as the novel FOLac race 4 isolates from the Netherlands based upon the EF-1α (604 nucleotides) and the IGS-rDNA (1859 nucleotides) sequences. Our analysis (based upon six single nucleotide polymorphisms identified only in the IGS-rDNA sequence) allowed the identification of intra-race 1 variation with the discrimination of four haplotypes. Isolates from Brazil, Italy, and a subset from the USA were classified into a single haplotype. The low diversity levels and the presence of only a single haplotype across the entire country are strong indications that Brazilian FOLac race 1 isolates are result of recent introduction event(s). This fast and widespread distribution of FOLac race 1 in Brazil has occurred more likely via importation and planting of contaminated seeds.