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Abstract
Small-spored Alternaria species are a taxonomically challenging group of fungi with few morphological or molecular characters that allow unambiguous discrimination among taxa. The protein-coding genes most commonly employed in fungal systematics are invariant among these taxa, so noncoding, anonymous regions of the genome were developed to assess evolutionary relationships among these organisms. Nineteen sequence-characterized amplified regions (SCAR) were screened for phylogenetic utility by comparing sequences among reference isolates of small-spored Alternaria species. Five of nineteen loci were consistently amplifiable and had sufficient phylogenetic signal. Phylogenetic analyses were performed with 150 small-spored Alternaria isolates using sequence data from an endopolygalacturonase gene and two anonymous loci. Associations among phylogenetic lineage, morphological classification, geography and host were evaluated for use as practical taxonomic characters. Samples included isolates from citrus in Florida, pistachio in California, desert plants in Arizona, walnuts in France/Italy and apples in South Africa. No associations were found between host or geographic associations and phylogenetic lineage, indicating that these characters were not useful for cladistic classification of small-spored Alternaria. Similarly strict congruence between morphology and phylogenetic lineage was not found among isolates grouped morphologically with A. alternata or A. tenuissima. In contrast 34 isolates grouped morphologically with A. arborescens fell into discrete clades for all datasets. Although 5–9 well supported clades were evident among isolates, it is currently unclear if these clades should be considered phylogenetic species or emerging evolutionary line-ages within the phylogenetically defined alternata species-group.
The authors thank Linda M. Kohn, University of Toronto, for helpful suggestions that improved the manuscript. We are grateful for the assistance of Pedro W. Crous, Centraalbureau voor Schimmelcultures, who provided the Alternaria apple core-rot isolates used in this study. We also acknowledge Derek Pouchnik, Laboratory for Biotechnology and Bioanalysis at Washington State University, for providing assistance with sequencing. Research was supported by National Science Foundation award DEB-0416314.