01 Nov Ustilago

Ustilago

Background

Ustilago is the largest genus of the Ustilaginaceae in the order of smut fungi, Ustilaginales, with about 200 currently accepted species (Vánky 2013). Ustilago and related genera contain many important plant pathogens that destroy the inflorescence or culms of grasses (Poaceae) (Vánky 2011). Some agriculturally important pathogens of grain and edible crops are U. tritici on wheat (Triticum), U. hordei on barley (Hordeum) and U. maydis on corn (Zea mays). Species of Ustilago have been used as model organisms for the study of plant disease pathways and mating types (Andrews et 2000; Bakkeren et al. 2008; Kellner et al. 2011), as well as for studies in the co-evolution of pathogens with their hosts (Begerow et al. 2004). The genomes of U. maydis and U. hordei were released in 2003 and 2012, respectively (Kamper et al. 2006; Laurie et al. 2012). Ustilago was until recently a catch-all genus for smut fungi on a diversity of host families, including the Carophyllaceae, Cyperaceae, Poaceae, Polygonaceae, Restionaceae, and Tilliaceae (McTaggart et al. 2012b). Closely related genera were not easily distinguished from Ustilago by morphology and formed a complex (Stoll et al. 2003, 2005). Subsequent systematic studies reserved Ustilago s. lat.  for species that infected Poaceae, with Ustilago s. str. restricted to the tribe Pooideae (McTaggart et al. 2012a; Stoll et al. 2005). Soral morphology and host range were later found to be synapomorphic character states for the smut genera Anthracocystis, Langdonia, Sporisorium, Stollia, and Triodiomyces, which were differentiated from Ustilago (McTaggart et al. 2012c). Melanopsichium is closely related to Ustilago and appears to have jumped hosts from Poaceae to Polygonaceae (Begerow et al.  2004; Stoll et al. 2005).

Species identification and numbers

The diversity of smuts in the Ustilaginaceae on Poaceae encompasses over 530 species (Vánky 2011). Cryptic species are certain to be revealed when species complexes, e.g., Macalpinomyces eriachnes, are investigated. Vánky (2011) recognized approximately 170 species of Ustilago, which were delimited by host and spore morphology. It is likely the species number of Ustilago will decrease when generic concepts are resolved in the Ustilaginaceae. Species currently recognized as Ustilago will be transferred to new or other genera delimited by sorus morphology and host range. For example, U. maydis does not fit the concept of Ustilago s. str. and warrants transfer to the earliest valid genus, Mycosarcoma, when these closely related genera are resolved (McTaggart et al. 2012a; Stoll et al. 2005; Vánky and Lutz 2011; Piepenbring et al. 2002) (Table).

Table Ustilago. Details of the isolates used in the phylogenetic tree

Ex-type (ex-epitype) strains are bolded and marked with an * and voucher stains are bolded

Molecular phylogeny

Relationships between Ustilago and closely related genera are still unresolved, and Ustilago is polyphyletic (Fig. Ustilago). Systematic studies based on the nLSU or ITS regions of rDNA have assigned taxa within these closely related genera (Shivas et al. 2013a; Vánky and Lutz 2011; McTaggart et al. 2012c). Nuclear genes (EF1α, GPDH, RPB1, and RPB2), another ribosomal gene (SSU) and mating loci were explored as markers for the evolution of smut fungi in the Ustilaginaceae (Kellner et al. 2011; McTaggart et al. 2012a; Munkacsi et al. 2007). At this stage, these markers are not as widely used as ITS and LSU, which are recommended for species identification and generic placement, respectively.

Recommended genetic markers

• The large subunit (LSU) of nrDNA–generic level
• The internal transcribed spacer (ITS) of nrDNA–species level