Resolving the <i>Globisporangium ultimum</i> (<i>Pythium ultimum</i>) species complex

The <i>Globisporangium ultimum</i> (formerly <i>Pythium ultimum</i>) species complex was previously composed of two morphological varieties: var. <i>ultimum</i> and var. <i>sporangiiferum</i>. Prior attempts to resolve this morphology-based species complex using molecular techniques have been inconclusive or conflicting. The increased availability of sequenced genomes and isolates identified as <i>G. ultimum</i> var. <i>ultimum</i> and var. <i>sporangiiferum</i> has allowed us to examine these relationships at a higher resolution and with a broader scope than previously possible. Using comparative genomics, we identified highly variable gene regions and designed primers for four new protein-coding genes for phylogenetics. These were then used alongside three known markers to generate a nuclear multigene genealogy of the species complex. From a collection of 163 isolates belonging to the target taxa, a subset of 29 was chosen to be included in this study (verified with nuclear rDNA internal transcribed spacer 1 [ITS1] and mitochondrial cytochrome <i>c</i> oxidase subunit 1 [<i>cox1</i>] sequences). Seventeen isolates of var. <i>ultimum</i> were selected to be representative of variations in genotype, morphology, and geographic collection location. The 12 isolates of var. <i>sporangiiferum</i> included all available specimens identified either morphologically (in previous studies) or through sequence similarity with ITS1 and <i>cox1</i>. Based on the fulfillment of reciprocal monophyly and observed genealogical concordance under the genealogical concordance phylogenetic species recognition, we determined that the <i>Globisporangium ultimum</i> species complex is composed of four genetically distinct species: <i>Globisporangium ultimum, Globisporangium sporangiiferum, Globisporangium solveigiae</i>, and <i>Globisporangium bothae</i>.