Ogataea thermophila sp. nov., the teleomorph of Candida thermophila

Abstract

Ascospore formation was observed in the type strain of Candida thermophila Shin K-S, Shin YK, Yoon and Park on some yeast sporulation media. In addition, a further sporulating strain was found that proved to be conspecific with C. thermophila on the basis of sequences of the D1/D2 domain of the large-subunit (26S) rRNA gene and the internal transcribed spacer (ITS)1–5.8S rRNA gene – ITS2 region. Therefore, Ogataea thermophila Péter, Tornai-Lehoczki, Shin K-S & Dlauchy sp. nov. is proposed as the teleomorph of C. thermophila. The type strain is Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T.

yeast, Candida thermophila, Ogataea thermophila

Introduction

The genus Ogataea was proposed by Yamada (1994) to accommodate some nitrate-assimilating methylotrophic Pichia species on the basis of the partial 18S and 26S rRNA gene sequences of the investigated strains. According to Kurtzman & Robnett (1998), a more robust dataset would be required to substantiate the circumscription of the genus. Morais (2004) listed the teleomorphic species belonging to the Ogataea clade (some of them still as Pichia species), and were of the opinion that it would be surprising if a phylogeny based on many genes should result in major changes in membership. Since then, two additional ascospore-forming yeast species of this group, Pichia siamensis (Limtong, 2004) and P. thermomethanolica (Limtong, 2005), have been described.

Candida thermophila was described by Shin (2001), on the basis of a strain isolated from soil in Korea. Another strain of the species was reported as the causative agent of catheter-related fungemia by Bar-Meir (2006). Although the type strain of C. thermophila, as it grows below 20°C, is not thermophilic according to the definitions of Mouchacca (1997) and Watson (1987), remarkably, Shin (2001) reported that its maximal growth temperature was 50–51°C. This characteristic is uncommon among yeasts. Although sexual reproduction was not observed by Shin (2001), we found ascospores in the type strain of C. thermophila on some yeast sporulation media. In addition, another sporulating, methanol-assimilating yeast strain was recovered from rotten willow wood in Hungary that, according to the sequences of the D1/D2 domain of the large-subunit (LSU) (26S) rRNA gene and its internal transcribed spacer (ITS)1–5.8S rRNA gene – ITS2 region, is conspecific with C. thermophila. Therefore in this article, we propose Ogataea thermophila sp. nov. as the teleomorph of C. thermophila.

Materials and methods

The strains investigated in this study were the type strain of C. thermophila (Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T [JCM, Japan Collection of Microorganisms, RIKEN, Wako, Saitama 351–0198, Japan; KCCM, Korean Culture Center of Microorganisms, Seoul, Korea; KCTC, Korean Collection for Type Cultures, Biological Resource Center, KRIBB, Daejeon 305–806, Korea]). Isolated from soil in Korea (Shin, 2001), and one strain, NCAIM Y.01538 (earlier tentatively named as ‘Pichia salicis’), recovered from rotten willow (Salix alba) wood in Budapest, Hungary. The strains were characterized with standard methods described by Yarrow (1998). The D1/D2 domain of the LSU (26S) rRNA gene of the Hungarian isolate was sequenced as described by Kurtzman & Robnett (1998). The ITS regions (ITS1, 5.8S rRNA gene and ITS2) were amplified and sequenced, from both strains, using ITS1 (5′-TCCGTAGGTGAACCTGCGG-3′) and ITS4 (5′-TCCTCCGCTTATTGATATGC-3′) primers.

Results and discussion

It was recognized that strain NCAIM Y.01538, which readily formed easily liberating hat-shaped ascospores on several media, shared an identical D1/D2 LSU rRNA gene sequence with the type strain of C. thermophila, suggesting, according to the guidelines of Kurtzman & Robnett (1998), that they may be conspecific. (The sequence of the D1/D2 domain of the 26S rRNA gene of strain NCAIM Y.01538 was deposited in the GenBank database under accession number AF403148.) However, strain NCAIM Y.01538 exhibited some physiologic differences when compared to the data published by Shin (2001) for the type strain of C. thermophila (Table 1). It is worth mentioning that the ability of the type strain of C. thermophila to grow at 50°C was confirmed in this study as well, whereas the upper temperature limit of growth for strain NCAIM Y.01538 was found to be very near to this value (49°C). As the ITS regions are believed to evolve more rapidly than the coding regions of the rRNA genes (White, 1990; Chen, 2000), in order to substantiate further the conspecificity of the two strains, the ITS1–5.8S rRNA gene – ITS2 regions from both strains were amplified and sequenced. The generated sequences were deposited in the GenBank database. The GenBank accession numbers of the ITS1–5.8S rRNA gene – ITS2 region for NCAIM Y.01538 and C. thermophila (Y94^T=JCM 10994^T=KCCM 50661^T) are EF064155 and EF064156, respectively. The amplicons of the two strains were of the same size, and between the 701-bp fragments, only one substitution was found (in the ITS1 region). As strains with fewer than 1% nucleotide differences in the ITS1 and ITS2 regions overall (Sugita, 1999) are usually conspecific (Chen, 2001), we conclude that strain NCAIM Y.01538 is conspecific with the type strain of C. thermophila, despite the differences in their physiological properties.

1

Divergent physiologic characteristics of the strains NCAIM Y.01538 and Candida thermophila Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T

	NCAIM Y.01538	Candida thermophila (Y94T=JCM 10994T=KCCM 50661T=KCTC 17233T)
Fermentation of α, α-trehalose	−	ws
Assimilation of carbon compounds
d-Galactose	ws or l	−
Sucrose	+	−
Maltose	+	−
Methyl-α-d-glucoside	+	−
Cellobiose	−	+
Salicin	−	+
Melezitose	+	−
d-Gluconate	−	+
Succinate	s	−

w, weak; s, slow; l, latent.

*

Data from Shin (2001). All of these results were confirmed during this study, except for d-gluconate assimilation, which was found to be negative. (Trehalose fermentation was found to be ws or negative.)

Later, we observed ascospore formation in the type strain of C. thermophila on some sporulation media, although on most culture media tested it was much less vigorous than in the case of NCAIM Y.01538. Both strains sporulated on cornmeal, potato–dextrose and yeast extract–malt extract (YM) agars. Therefore, on the basis of the type strain of C. thermophila and strain NCAIM Y.01538, we propose O. thermophila sp. nov., the teleomorph of C. thermophila. The genus Ogataea, instead of Pichia, was selected to accommodate the teleomorph of C. thermophila because the type strain of C. thermophila on the phylogenetic trees given by Shin (2001), Morais (2004) and Limtong (2004, 2005) is well nested in the clade including O. minuta, the type species of the genus.

Latin description of O. thermophila Péter, Tornai-Lehoczki, Shin K-S & Dlauchy sp. nov.

Status ascigerus Candida thermophila Shin K-S, Shin YK, Yoon et Park. Descriptio est idem ac descriptio Candidae thermophilae (Shin K-S, Shin YK, Yoon et Park) his rebus exceptis: Asci conjugati vel inconjugati et deliquescentes. 2–4 ascosporas piliformes habent. Species est homothallica. Trehalosum (exigue, lente variabile) fermentatur. d-Galactosum (exigue, lente vel tarde variabile), sucrosum (variabile), maltosum (variabile), α-methyl-d-glucosidum (variabile), cellobiosum (variabile), salicinum (variabile), melezitosum (variabile), d-gluconicum (variabile) succinatum, (lente variabile) assimilantur.

Typus stirps: Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T.

Description of O. thermophila Péter, Tornai-Lehoczki, Shin K-S & Dlauchy sp. nov.

Anamorph: Candida thermophilaShin (2001) International Journal of Systematic and Evolutionary Microbiology 51 : 2167–2170.

The description is identical to that of C. thermophila (Shin, 2001), except for the following.

Either conjugation between independent cells or between the parent cell and its bud precede ascospore formation, or asci are unconjugated. Two to four hat-shaped ascospores are formed in each deliquescent ascus (Fig. 1). The occurrence of heterogamous conjugation suggests homothallism. Sporulation was observed in case of both strains on cornmeal, potato–dextrose and YM agars, after an incubation period of 7–14 days at 25°C. Trehalose fermentation is slow and weak or absent. The following carbon sources are assimilated: d-galactose (weak slow or latent and variable), sucrose (variable), maltose (variable), methyl-α-d-glucoside (variable), cellobiose (variable), salicin (variable), melezitose (variable), d-gluconate (variable), and succinate (slow and variable).

1

Ogataea thermophila (Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T). Ascosporulating culture on potato–dextrose agar, 14 days, 25°C. Bar=10 μm.

Type strain: Y94^T=JCM 10994^T=KCCM 50661^T=KCTC 17233^T.

Acknowledgements

This research was partly supported by the Bolyai János Research Scholarship of the Hungarian Academy of Sciences.

Author notes

Editor: Cletus Kurtzman