Taxonomy and physiological characterisation of Scheffersomyces titanus sp. nov., a new D-xylose-fermenting yeast species from China

Three strains of a d-xylose-fermenting yeast species were isolated from the host beetle Dorcus titanus collected from two different localities in Henan Province, Central China. These strains formed two hat-shaped ascospores in conjugated and deliquescent asci. Multilocus phylogenetic analysis that included the nearly complete small subunit (SSU), the internal transcribed spacer (ITS) region and the D1/D2 domains of the large subunit (LSU) rDNAs, as well as RNA polymerase II largest subunit (RPB1) gene demonstrated that these strains represent a novel yeast species belonging to the genus Scheffersomyces. The phylogenetic analysis based on the nucleotide sequences of the xylose reductase (XYL1) gene supported the view that the new strains could be grouped as a unique species. Although this new species is highly similar to Scheffersomyces stipitis-like yeasts in terms of nrDNA sequences and morphological and physiological characteristics, the species can be clearly differentiated from its close relatives on the basis of the sequences of XYL1 and RPB1. Therefore, a novel yeast species, Scheffersomyces titanus sp. nov., is proposed to accommodate these strains. The type strain is NYNU 14712T (CICC 33061T = CBS 13926T).

Scientific RepoRts | 6:32181 | DOI: 10.1038/srep32181 those of the genus Scheffersomyces. Molecular phylogenetic data indicated that these strains represent a novel species closely related to S. stipitis-like yeasts. In this study, we describe this new species as S. titanus sp. nov.

Results
Phylogenetic analysis. The sequences of nuclear rDNAs, including SSU, ITS and LSU, of the three strains of the new species were identical. This new species is closely related to the d-xylose-fermenting Scheffersomyces species, especially to S. stipitis and its close relatives, including S. henanensis, S. illinoinensis and S. segobiensis (Figs 1 and 2). The novel species differ from other four d-xylose-fermenting species in S. stipitis subclade by only 3 to 5 substitutions in the ITS region and the D1/D2 domain. As such, the species cannot easily be distinguished from other species in S. stipitis subclade by nDNA sequence analysis alone. The sequence of the easily amplified XYL1 has been used to recognise cryptic species in the Scheffersomyces clade 3,5-7 . In the XYL1 locus, the species differed significantly from those of other four Scheffersomyces species in the subclade by 5.6-7% sequence divergence (34-40 substitutions). This new species also differed from other four yeast species in the S. stipitis subclade by [11][12].5% sequence divergence (76-83 substitutions) in RPB1. These results showed that these strains are distinct from those of other species in this subclade.
Phylogenetic analysis was performed using the combined SSU, ITS, LSU and RPB1 sequences, and XYL1 gene sequences. In the multilocus phylogenetic tree, our strains clustered within the S. stipitis subclade as a separate branch with 92% bootstrap support (Fig. 1). The tree topology is in agreement with topologies obtained previously 3,5-7 . In the tree obtained with XYL1 gene sequences (Fig. 2), the novel strains were placed in a well supported branch basal to the four species of S. stipitis subclade which were similar to those obtained from the multilocus phylogenetic tree. Results of these analyses confirmed that the three strains represent a distinct taxa of the genus Scheffersomyces.
Morphology and Physiology. The new species presents morphological characteristics typical of S. stipitis, the type species of the genus Scheffersomyces 2 . In addition to cells that reproduced by multilateral budding and formed with pseudohyphae, the novel species produced two hat-shaped ascospores in a deliquescent ascus (Fig. 3). The asci were produced by conjugation between a cell and its bud or between independent cells which are usually observed in its closely related species, such as S. henanensis, S. segobiensis and S. stipitis 2,7 . The mode of conjugation suggests the species to be homothallic. Physiologically, this new yeast species is highly similar to its closely related species in the S. stipitis subclade. However, some phenotypic differences exist between the new species and its closely related species (Table 1). In practice, all of the strains of the new species can be distinguished from S. henanensis, S. illinoinensis, S. segobiensis and S. stipitis, their closest phylogenetic relatives, on the basis of sequence comparisons because differences in phenotypic characteristics are minor (Table 1). Figure 4 shows the kinetics of growth on 20 g L −1 glucose or d-xylose by the strain NYNU 14712 T . This strain exhibited a typical growth curve where the sugar is efficiently fermented. After the sugar is exhausted from the media, the produced ethanol starts to be consumed and used as a carbon source by the yeast. The strain grew well on both carbon sources and produced practically the same amount of biomass (Fig. 4a). However, the lower levels of ethanol were produced during aerobic growth on glucose or xylose by the strain NYNU 14712 T (Y E/glu = 0.28 ± 0.02 g ethanol g −1 sugar; Y E/xyl = 0.27 ± 0.03 g ethanol g −1 sugar) (Fig. 4c). As found typically for other d-xylose-fermenting yeasts 9,10 , this yeast has a clear preference for glucose uptake and fermentation. This characteristic is evident during batch fermentations of a mixture of 20 g L −1 glucose plus 20 g L −1 d-xylose, where glucose consumption occurs before d-xylose utilization when both sugars are present at the beginning of the fermentation (Fig. 5). Nevertheless, produced ethanol from a mixture of sugars containing glucose and d-xylose (Y E/sug ~ 0.30 g ethanol g −1 sugar) at yields similar to those reported for other d-xylose-fermenting Scheffersomyces yeasts 14 , S. titanus may provide a source of genes, enzymes and/or sugar transporters to engineer strains for efficient ethanol production from renewable biomass.

Discussion
In this study, the new d-xylose-fermenting yeast species S. titanus was described and illustrated based on morphological and molecular characters. Although this new species is highly similar to S. stipitis-like yeasts in nrDNA sequences, as well as morphology and physiological characteristics (Table 1), the species can be clearly differentiated from its close relatives, S. henanensis, S. illinoinensis, S. segobiensis and S. stipitis, by the sequences of XYL1 and RPB1 (Figs 1 and 2).
Kurtzman and Robnett (2013) compared the type species of 70 currently recognized genera by sequence divergence in the SSU and LSU rDNAs, EF-1a, RPB1 and RPB2 and found that the genus Scheffersomyces is polyphyletic 15 . The results also showed that S. spartinae is included in a clade with Spathaspora passalidarum, which is distinct from the type species S. stipitis, although the clade is weakly supported by statistical analyses 15 . Trehalose  Our results from the combined sequence comparison of SSU, ITS, D1/D2 LUS rDNAs and RPB1 indicated that the genus is not monophyletic; instead, the genus comprises two phylogenetically distinct groups on the tree (Fig. 1). The results strongly suggested that the genus Scheffersomyces should be categorised in the monophyletic group near the type species S. stipitis (Fig. 1). Another group consists of S. gosingicus and S. spartinae, which were previously considered as members of Scheffersomyces, may become representatives of a novel genus because their phylogenetic relationships within the genus were not supported by bootstrap (Fig. 1). Three strains of the new species were isolated from the beetles D. titanus (Lucanidae, Coleoptera) collected from two different regions in China. The repeated isolation of these yeast strains from D. titanus revealed that the species may be involved in a specific yeast-insect association. There are only two previous reports of Scheffersomyces species from China. These species include S. gosingica from forest soil in Taiwan Province 16 and S. henanensis from rotten wood in Henan Province 7 . Considering the number of Scheffersomyces species found in other countries and the small number of studies on Scheffersomyces species found in China, we believed that the discovery of S. titanus indicates the presence of other species belonging to the genus in this geographic area.
Ethanol production was observed in the fermentation assay. This finding confirmed that the new yeast species can ferment d-xylose to ethanol effectively. Although the molecular basis of efficient xylose fermentation is complex and not yet fully understood 17,18 , the discovery and analysis of new Scheffersomyces species that can ferment xylose may contribute important genomic information that could be applied to improve the efficiency of pentose assimilation by yeasts 19 , a significant limitation in cellulosic biofuel production. However, further studies have yet to determine whether the novel yeast species can be used directly to produce bioethanol from lignocellulosic hydrolysates, as revealed in S. stipitis 9,10 .

Methods
Yeast isolation and culture. Host beetles were collected from two different localities in Henan Province, Central China. The strain NYNU 14712 T was isolated from the gut of D. titanus (Coleoptera) collected from Baotianman Mountain (33°27′ N and 111°48′ E) in July 2014, and two other strains (NYNU 15875 and NYNU 15860) were found in the gut of the same insect collected from Funiu Mountain (32°45′ N and 113°30′ E) in August 2015. The two mountains were separated from one another by a distance of 50.2 km. The methods used to isolate the yeasts from the gut of insect have been described previously 20,21 . The insects were usually placed in Petri dishes for 1-3 days without food prior to dissection. Withholding food aids in eliminating certain contaminating organisms that may be isolated from the gut. The surface was disinfected by submersion in 95% ethanol for 1-2 min. The disinfected surface was then rinsed with 0.7% saline. The insect gut was removed aseptically under a dissecting microscope, and gut segments were streaked on acidified yeast extract-malt extract YM agar (0.3% yeast extract, 0.3% malt extract, 0.5% peptone, 1% glucose and 2% plain agar, adjusted to pH 3.5 with HCl) plates. The plates were then incubated at 25 °C for 3-4 days. The different yeast morphotypes were purified at least twice and stored in YM agar slants at 4 °C and in 15% glycerol at − 80 °C.

Morphological, physiological and biochemical characteristics. Morphological and physiological
characteristics were examined in accordance with standard methods employed in yeast taxonomy 22 . Ascospore formation was determined for all new strains by first singly plating then crossing isolates in all possible combinations on YM, McClary's acetate, cornmeal and 5% malt extract agars. The cultures were incubated at 25 °C and examined weekly by microscopy for up to 28 days. Physiological and biochemical tests were performed by replica plating on solid and in liquid media 22 . Test samples were incubated at 25 °C and results were read weekly for up to 28 days. Ubiquinones were extracted and purified in accordance with the method described by Yamada and Kondo 23 with slight modifications and determined through HPLC, as described previously 24 . DNA amplification and sequencing. Genomic DNA was extracted using an Ezup column yeast genomic DNA purification kit in accordance with the manufacturer's protocol (Sangon Biotech, Shanghai, China). The concentration, integrity and purity of the total extracted DNA were confirmed through gel electrophoresis in 0.8% agarose in 0.5× Tris-Borate-EDTA (TBE). Nuclear rDNAs for SSU, ITS and D1/D2 LSU were amplified and sequenced, as described previously [25][26][27] . Two protein-coding genes, namely, RPB1 and XYL1, were amplified using the following degenerate primer pairs: RPB1-Af (5′ -GARTGYCCDGGDCAYTTYGG-3′ ) and RPB1-Cr (5′ -CCNGCDATNTCRTTRTCCATRTA-3′) for RPB1 28,29 ; XYL1-forward (5′ -GGTYTTYGGM TGYTGGAARSTC-3′ ) and XYL1-reverse (5′ -AAWGATTGWGGWCCRAAWGAWGA-3′) for XYL1 3,5 . The PCR conditions that were recommended in the references for each primer pair were employed. The purified PCR products were sequenced using a Dye terminator cycle sequencing kit (Applied Biosystems, Warrington, USA).

Phylogenetic analyses.
Comparisons with sequences from the international GenBank database (http://www.ncbi.nlm.nih.gov/) were conducted through BLASTN search 30 . Sequences were aligned using the multiple sequence alignment program CLUSTAL X 1.83 31 . Phylogenetic trees were constructed using MEGA software version 5.0 32 . The evolutionary distance data were calculated from Kimura's two-parameter model 33 in neighbour-joining analyses 34 . Confidence limits were estimated from bootstrap analysis (1000 replicates) 35 , and only values above 50% were recorded on the resulting trees. The sequences determined from this study, along with reference sequences obtained from GenBank are listed in Table S1.
Growth conditions and fermentation assays. The cells were grown on the YP medium (1% yeast extract and 2% peptone), adjusted to pH 5.0 with HCl and supplemented with 2% glucose and/or d-xylose. The cells were grown at 28 °C with shaking (160 rpm) in cotton-plugged Erlenmeyer flasks filled with the medium to 1/5 of the volume. The inocula for the growth assays were prepared by aseptically transferring a colony from a plate into 5 mL of glucose or xylose medium; growth was allowed to proceed to stationary phase for 2-3 days. The cells were then inoculated in the fresh media containing similar composition at a rate of 1%. Samples were obtained regularly and centrifuged at 5,000 × g for 1 min, and the supernatants were used to determine sugars and ethanol. Glucose and d-xylose levels were determined by HPLC (Waters 410, Milford, MA, USA) as described by Cadete et al. 14 . Ethanol was determined with alcohol oxidase (Sigma) and peroxidase (Sangon Biotech, Shanghai, China) as described previously 36 . Growth was followed by turbidity measurements at 570 nm after the medium samples were appropriately diluted in distilled water. The ethanol yields during growth on glucose (Y E/glu , g ethanol g −1 glucose), xylose (Y E/xyl , g ethanol g −1 xylose) or glucose plus d-xylose (Y E/sug, g ethanol g −1 sugar) were calculated taking into account the amount of sugar consumed at the point of maximum ethanol production. The reported values were the average ± mean deviations obtained from independent duplicate cultures and were analysed using the paired t-test.