Orpinomyces was the fourth genus to be discovered, and was a landmark genus in terms of being the first polycentric anaerobic fungal genus to be described. Polycentric anaerobic fungi have nuclei not only in their sporangia but also in their rhizomycelium. This movement of nuclei enables multiple sporangia to form, unlike with monocentric fungi which do not have nuclei in their rhizomycelium.
The genus Orpinomyces was named by Barr et al (1989), honouring the early work of Prof Colin Orpin. The first described Orpinomyces species in the literature was O. bovis (Barr et al, 1989). However, Akin and colleagues had previously described a polycentric isolate as Neocallimastix joyonii (Akin & Rigsby 1987; Akin et al. 1988). A few years later, Li & Heath (1991) compared both N. joyonii and O. bovis, and concluded that they represented the same genus and species. As such, O. joyonii is considered the type species (Ho & Barr et al, 1995), although technically speaking it has not been validly named.
At this point in time there is only one other recognised Orpinomyces species, O. intercalaris. The two Orpinomyces species can be physically differentiated in terms of the positioning of their sporangia (Ho & Barr, 1995). In O. joyonii the globose sporangia are on simple or compound sporangiophores (i.e. branched sporangial stalks). While with O. intercalaris the globose sporangia are intercalary (i.e. are enlargements of hyphal elements). There is no current consensus regarding type strains, as no type strains were clearly specified in the early literature (Ho & Barr, 1995). An RFLP method based on the LSU gene has been developed in order to help differentiate between the two different species (Dagar et al, 2011).
Within the family Neocallimastigaceae, Orpinomyces is phylogenetically most closely related to five other genera located in the same clade (when using LSU as a taxonomic marker): Aestipascuomyces, Feramyces, Neocallimastix, Pecoramyces and Ghazallamyces. Orpinomyces is unusual because it is the only genus within the clade that is polycentric, the others all being monocentric.
Images are shown above of a recently isolated Orpinomyces joyonii culture, strain D4A.
In liquid media, Orpinomyces exhibits thick cottony fungal growth (Figure 1). It produces large white (usually >1 cm diameter) colonies on agar roll tubes (Figure 2).
Microscopically, Orpinomyces produces polyflagellated zoospores (with about 14-24 flagella) (Figure 3). It has polycentric thalli with globose sporangia (Figure 4 & 5) and hyphae with tight constrictions (Figure 6).
In the literature it may appear that there is an available genome for Orpinomyces (i.e. strain C1A; Youssef et al, 2013), however, the strain used was later reclassified as Pecoramyces (Hanafy et al, 2017). As such, there is currently no publicly available sequenced Orpinomyces genome, however, several transcriptomes have been sequenced.
For Orpinomyces joyonii strain D4A that is pictured above, the LSU sequence is available in the NCBI database (accession number MH045490.1).