Reassessment of Species Boundaries in the Fusarium oxysporum Species Complex: Insights from Korean Isolates Using Multilocus Phylogenetics, GCPSR, and Coalescent-Based Approaches

The Fusarium oxysporum species complex (FOSC) comprises globally distributed fungi exhibiting diverse ecological lifestyles, including plant pathogenic, endophytic, saprotrophic, and opportunistic associations. However, species boundaries within the complex remain controversial due to limited genetic divergence, morphological similarity, and incongruence among gene genealogies. In this study, species diversity and evolutionary relationships within the FOSC were reassessed using multilocus phylogenetic analyses, the Genealogical Concordance Phylogenetic Species Recognition (GCPSR) concept, and multispecies coalescent-based species delimitation approaches (STACEY and BPP) based on five loci (tef1, rpb1, rpb2, CaM, and tub2). The concatenated multilocus phylogeny formed the primary basis for recognizing 32 phylogenetic species among 49 previously described taxa, representing the species boundaries adopted in this study. Additional GCPSR and multispecies coalescent analyses provided further insight into the relationships among these phylogenetic species and supported the proposal of an 18-lineage scenario as a set of putative evolutionary lineages. However, these putative evolutionary lineages require further taxonomic evaluation before formal taxonomic changes can be proposed. Among the 92 Korean isolates examined, 11 species were identified, including nine species newly recorded in Korea. Of these, F. cugenangense was the most frequently recovered species and exhibited the broadest host/substrate associations. This study improves our understanding of species boundaries and evolutionary relationships within the FOSC while expanding current knowledge of species diversity and host/substrate associations in Korea.