Phylogenetic relationship between Australian Fusarium oxysporum isolates and resolving the species complex using the multispecies coalescent model.

Background: The Fusarium oxysporum species complex (FOSC) is a ubiquitous group of fungal species readily isolated from agroecosystem and natural ecosystem soils which includes important plant and human pathogens. Genetic relatedness within the complex has been studied by sequencing either the genes or the barcoding gene regions within those genes. Phylogenetic analyses have demonstrated a great deal of diversity which is reflected in the differing number of clades identified: three, five and eight. Genetic limitation within the species in the complex has been studied through Genealogical Concordance Phylogenetic Species Recognition (GCPSR) analyses with varying number of phylogenetic 'species' identified ranging from two to 21. Such differing views have continued to confuse users of these taxonomies.
Results: The phylogenetic relationships between Australian F. oxysporum isolates from both natural and agricultural ecosystems were determined using three datasets: whole genome, nuclear genes, and mitochondrial genome sequences. The phylogenies were concordant except for three isolates. There were three concordant clades from all the phylogenies suggesting similar evolutionary history for mitochondrial genome and nuclear genes for the isolates in these three clades. Applying a multispecies coalescent (MSC) model on the eight single copy nuclear protein coding genes from the nuclear gene dataset concluded that the three concordant clades correspond to three phylogenetic species within the FOSC. There was 100% posterior probability support for the formation of three species within the FOSC. This is the first report of using the MSC model to estimate species within the F. oxysporum species complex. The findings from this study were compared with previously published phylogenetics and species delimitation studies.
Conclusion: Phylogenetic analyses using three different gene datasets from Australian F. oxysporum isolates have all supported the formation of three major clades which delineated into three species. Species 2 (Clade 3) may be called F. oxysporum as it contains the neotype for F. oxysporum.