Discrete lineages within Alternaria alternata species group: identification using new highly variable loci and support from morphological characters

The Alternaria alternata species group is ubiquitous in the environment acting as saprotrophs, human allergens, and plant pathogens. Many morphological species have been described within the group and it is unclear whether these represent re-descriptions of the same species or discrete evolutionary taxa. Evolutionary relationships within the A. alternata species group were established using a phylogenetic approach based on functional genes. Sequencing of five highly variable loci identified three major lineages within the A. alternata species group. These loci included two loci previously shown to be variable within the Alternaria genus (endo-PG, Alt a1) as well as three new phylogenetic loci (TMA22, PGS1, and REV3) identified as highly variable based on publically available genome sequence data for Dothideomycete species. Results indicated that the three lineages have recently diverged and as such were considered as subspecies within a single species A. alternata. Lineages were identified as A. alternata ssp. arborescens, A. alternata ssp. tenuissima, and A. alternata ssp. gaisen in accordance with the placement of reference isolates. The phylogenetic results were supported by morphological analysis, which differentiated strains in A. alternata ssp. arborescens and A. alternata ssp. tenuissima and also aligned with previous morphological species descriptions for A. arborescens and A. tenuissima. However, phylogenetic analysis placed the morphologically described species A. alternata and A. mali within the A. alternata ssp. tenuissima and did not support them as discrete taxa. As A. alternata are of phytosanitary importance, the molecular loci used in this study offer new opportunities for molecular identification of isolates by national plant protection organizations.