Tritirachiomycetes

Introduction

The six currently known Tritirachium species are all anamorphic (asexual) molds, without a known teleomorphic (sexual) stage. The type species, T. dependens Limber, was described from dead roots of Yucca treculeana from Cuba (Limber 1940); the same species has subsequently been found on a cellar wall in Belgium (Beguin 2010; Schell et al. 2011). While the precise role of Tritirachium species in the environment is not known, there is evidence that T. dependens is a potentially obligate associate of Penicillium species (molds in phylum Ascomycota), on which it depends for certain micronutrients. Two species; T. oryzae (Vincens) de Hoog and T. roseum J.F.H. Beyma have been isolated as saprotrophs (decomposers) from various indoor environments, but also as causal agents of infections on human cornea and scalp (Moraes et al. 2010; Rodrigues and Laibson 1975). Tritirachium cinnamomeum J.F.H. Beyma is represented only by one specimen that was isolated from a fly in the Netherlands and the remaining two species were just recently identified as new and need further studies (Schell et al. 2011). There is little information about the biology of the members of this fungal group and to date only the ones of potential medical importance have been studied in any detail.

Characteristics

Tritirachium species are hyphal in culture and the colonies are colored in shades ranging from pale pinkish or lilac to brown. The mycelium produces conidiophores, which have side branches with zigzag-like tips that are characteristic for the genus. The conidia produced on these conidiophores are hyaline and one-celled with variable sizes (2.5–4.2 × 2.0–2.9 µm). Transmission electron microscope studies revealed the presence of simple uniperforate septal pores and the absence of dolipores or Woronin bodies in the hyphae of Tritirachium (Schell et al. 2011)—characteristics of Basidiomycetes in subphylum Pucciniomycotina. There is some overlapping of morphological characteristics between the species currently placed in Tritirachium; therefore morphological observations alone may not be sufficient for diagnosing all members of this genus at the species level. 

Discussion of Phylogenetic Relationships

Until recently the genus Tritirachium was placed in phylum Ascomycota, primarily due to similarities in conidiophore morphology with other mold species in subphylum Pezizomycotina. However, very recent multi-gene analyses based on nuclear small and large subunits and translation elongation factor 1-alpha, revealed that most species currently placed in Tritirachium, including the type species, are, in fact, members of Pucciniomycotina. This result was further confirmed with the results of ultrastructural studies (Schell et al. 2011). The authors of this study introduced a new strongly supported monophyletic class Tritirachiomycetes Aime & Schell, a new order Tritirachiales Aime & Schell and a new family Tritirachiaceae Aime & Schell to accommodate these taxa. As with other deeper-level nodes in Pucciniomycotina, additional studies are still needed to reveal the relationship of Tritirachiomycetes to other classes within Pucciniomycotina. 

Other Names for Tritirachiomycetes

• Tritirachiales Aime & Schell 2011
• Tritirachiaceae Aime & Schell 2011

References

Beguin, H. 2010. Tritirachium egenum, a thiamine- and siderophore-auxotrophic fungal species isolated from Penicillium rugulosum. FEMS Microbiology Ecology 74: 165–173.

de Hoog, G.S. 1972. The genera Beauveria, Isaria, Tritirachium and Acrodontium gen. nov. Studies in Mycology 1: 1–41.

Limber, D.P. 1940. A new form genus of the Moniliaceae. Mycologia 32: 23–30.

MacLeod, D.M. 1954. Investigations on the genera Beauveria Vuill. and Tritirachium Limber. Canadian Journal of Botany 32: 818–890.

Moraes, R.N., M.C. Ribeiro, M.C. Nogueira, K.C. Cunha, M.M. Soares, M.T. Almeida . 2010. First Report of Tritirachium oryzae infection of human scalp. Mycopathologia 169: 257–259.

Rodrigues, M.M., P. Laibson. 1975. Exogenous corneal ulcer caused by Tritirachium roseum. American Journal of Ophthalmology 80: 804–806.

Schell, W.A., A.G. Lee, M.C. Aime . 2011. A new lineage in Pucciniomycotina: class Tritirachiomycetes, order Tritirachiales, family Tritirachiaceae. Mycologia 103: DOI: 10.3852/10-333.