Delimitation of Ophiostoma quercus and its synonyms using multiple gene phylogenies

Abstract

Ophiostoma quercus is a morphologically variable species that causes sapstain on mostly hardwood hosts worldwide. Several species have been suggested as synonyms of O. quercus in the past, including O. fagi, O. roboris, O. valachicum, O. kubanicum, and O. catonianum. A recent collection of isolates resembling O. quercus from Azerbaijan provided the opportunity to reconsider the accuracy of these synonymies based on morphology and DNA sequence data. Four gene regions, the ribosomal internal transcribed spacer regions including the 5.8 s gene, part of the β-tubulin gene, translation elongation factor-1α, and histone gene, were used to determine the phylogenetic relationships between the various species and isolates of different origin. In all four resulting phylogenetic trees, isolates of O. quercus, O. fagi, and O. roboris formed a single, well-supported cluster, but with some internal variation. All the other species in the analyses, including O. piceae and O. catonianum, grouped distinctly with good node support. These results thus support the synonymy of O. fagi and O. roboris with O. quercus, and confirm that O. piceae and O. catonianum are distinct taxa. Ophiostoma valachicum and O. kubanicum could not be considered due to the absence of cultures, but based on published descriptions, we argue that O. valachicum should be regarded as a valid species in need of neotypification. Ophiostoma kubanicum was never validly described and should be excluded from the list of synonyms of O. quercus.

Both mating types in the heterothallic fungus Ophiostoma quercus contain MAT1-1 and MAT1-2 genes

“…The ITS sequences of the 10 O. quercus isolates used in this study were similar to those reported previously for isolates of this species (Grobbelaar et al 2009), confirming their identity. The heterothallic behaviour of these isolates were also confirmed (Table 1) and agreed with that previously determined (Brasier and Kirk 1993;De Beer et al 2003;Kamgan et al 2008).…”

“…After analysis of chromatograms with Chromas Lite v. 2.01 (Technelysium Pty. Ltd.; http://www.technelysium.com.au), the sequences of all isolates were compared to the ITS sequences of authentic O. quercus isolates (De Beer et al 2003;Grobbelaar et al 2009;Harrington et al 2001). …”

“…For example, the Dutch elm disease pathogens and O. quercus are heterothallic (Brasier and Kirk 1993;Harrington et al 2001;Solla et al 2008) where sexual reproduction requires the interaction of two individuals of opposite mating type (Coppin et al 1997). In contrast, individuals of homothallic species (e.g., O. arduennense and O. minus) are typically self-fertile and capable of completing the sexual cycle in the absence of a second individual (Carlier et al 2006;Gorton and Webber 2000;Grobbelaar et al 2009). …”

“…Conventional mating tests are laborious and time-consuming as they involve mating all available isolates in every possible combination and subsequent assignment of mating specificities. This traditional approach has been used widely for heterothallic species of Ophiostoma (Brasier and Kirk 1993;De Beer et al 2003;Grobbelaar et al 2009;Harrington et al 2001;Zhou et al 2004). …”

“…quercus, a number of mating tester strains is available from previous mating studies (Brasier and Kirk 1993;De Beer et al 2003;Grobbelaar et al 2009;Kamgan et al 2008). The aim of this study was to characterise the MAT genes in some of these O. quercus isolates.…”