Abstract
BackgroundRecent work has shown that Fusarium species and genotypes most commonly associated with human infections, particularly of the cornea (mycotic keratitis), are the same as those most commonly isolated from plumbing systems. The species most dominant in plumbing biofilms is Fusarium keratoplasticum, a cosmopolitan fungus known almost exclusively from animal infections and biofilms. To better understand its diversity and population dynamics, we developed and utilized a nine-locus sequence-based typing system to make inferences about clonality, recombination, population structure, species boundaries and hybridization.ResultsHigh levels of genetic diversity and evidence for recombination and clonality were detected among 75 clinical and 156 environmental isolates of Fusarium keratoplasticum. The multilocus sequence typing system (MLST) resolved 111 unique nine-locus sequence types (STs). The single locus bifactorial determinants of mating compatibility (mating types MAT1-1 and MAT1-2), were found in a ratio of 70:30. All but one of the 49 isolates of the most common ST (FSSC 2d-2) came from human infections, mostly of the cornea, and from biofilms associated with contact lenses and plumbing surfaces. Significant levels of phylogenetic incongruence were found among loci. Putative clonal relationships among genotypes were estimated, showing a mixture of large clonal complexes and unrelated singletons. Discordance between the nuclear ribosomal rRNA and other gene genealogies is consistent with introgression of ribosomal RNA alleles of phylogenetic species FSSC 9 into F. keratoplasticum. No significant population subdivision based on clinical versus non-clinical sources was found.ConclusionsIncongruent phylogenetic trees and the presence of both mating types within otherwise identical STs were observed, providing evidence for sexuality in F. keratoplasticum. Cryptic speciation suggested in a published three-locus MLST system was not supported with the addition of new loci, but evidence of introgression of ribosomal RNA genes from another strongly supported phylogenetic species (FSSC 9), also known from plumbing systems and human infections, was detected in two isolates. Overall, F. keratoplasticum is a diverse and geographically unstructured species with a mixed clonal and recombinant life history.
Highlights
Recent work has shown that Fusarium species and genotypes most commonly associated with human infections, of the cornea, are the same as those most commonly isolated from plumbing systems
Along with Aspergillus, it is a frequent cause of trauma-associated mycotic keratitis in the tropics [3] and it is a predominant cause of contact-lens associated mycotic keratitis worldwide [4,5,6,7,8,9]
sequence types (STs) diversity was 0.94 among the nine-locus STs, and phylogenetic diversity (PD) of the nine-locus dataset calculated in SPLITSTREE4 was 0.45 (Table 3)
Summary
Recent work has shown that Fusarium species and genotypes most commonly associated with human infections, of the cornea (mycotic keratitis), are the same as those most commonly isolated from plumbing systems. To better understand its diversity and population dynamics, we developed and utilized a nine-locus sequence-based typing system to make inferences about clonality, recombination, population structure, species boundaries and hybridization. Several MLST systems have been developed for studying the genetic diversity and population biology of fungal human pathogens including, among others, Aspergillus [14], Blastomyces [15], Candida [16], Coccidioides [17], Cryptococcus [18], Histoplasma [19], and Pneumocystis [20]. Several studies utilizing MLST and genealogical concordance phylogenetic species recognition in Fusarium have revealed strongly supported species boundaries and have elucidated the spectrum of fusaria associated with human pathogenicity [4,9,12,21,22,23,24,25,26,27]. Typing systems are valuable for epidemiological inference and for providing a framework for understanding the distribution of important phenotypes within species such as antimicrobial drug resistance [24]
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