The treasure trove of yeast genera and species described by Johannes van der Walt (1925–2011)

Abstract

Yeast taxonomy and systematics have in recent years been dealt with intensively primarily by a small group of individual researchers with particular expertise. Amongst these was Johannes P. van der Walt, who had a major role in shaping our current understanding of yeast biodiversity and taxonomy. Van der Walt based his taxonomic studies not only on available cultures, but also by going into the field to isolate yeasts from various substrates. This pioneering work led to the discovery of many new genera and species, which were deposited in the Centraalbureau voor Schimmelcultures (CBS) collections for future studies in taxonomy, genomics, and industrial uses. These treasures collected during more than 60 years provide an outstanding legacy to the yeast community and will continue to exist in his absence. This contribution provides a comprehensive overview of the current nomenclatural and taxonomic status of the yeast genera and species introduced by van der Walt during his career.

Introduction

Johannes van der Walt passed away after a short illness on 13 November 2011. He will be remembered as a person very much interested in the biodiversity of yeasts, a passion which is apparent from the many yeast strains representing novel taxa that he isolated from various, mainly South African, sources.

The first yeast species that was isolated in South Africa was from an infected human nail and was described as Hanseniaspora guilliermondii by Adrianus Pijper (Pijper 1928), a pathologist practicing in Pretoria. The type strain of this species was deposited by Pijper in the yeast collection of the Centraalbureau voor Schimmelcultures (CBS), at that time located in Delft. The yeast collection had been transferred from Baarn to Delft after the appointment of Albert Jan Kluyver as Professor of Microbiology of the Technical University in Delft in 1921 (Samson et al. 2004), and came back together with the CBS filamentous fungal collection in Utrecht in 2000.

As a result of Pijper’s mediation, Johannes van der Walt started to study for his PhD in Delft under the guidance of Kluyver in 1949, obtaining his degree in 1952 for a thesis entitled “On the yeast Candida pulcherrima and its pigment pulcherrimine” (van der Walt 1952). It was also in Delft that van der Walt was instructed in the use of specific enrichment techniques for the isolation of soil-borne microorganisms. After his return to South Africa in 1952, van der Walt started to search for novel yeast species. Applying a wide range of enrichment methods, van der Walt and his collaborators spent almost 50 years hunting intermittently for new taxa associated not only with natural sources such as uncultivated grassland soils, arboricolous beetle infestations and other similar niches, but also manufactured products such as wine and beer. This broad-based survey led to the discovery of many novel sexual and asexual ascomycetous taxa and some of heterobasidiomycetous affinity. Some of these species are still only known from South African isolates.

Although originally trained in chemistry, van der Walt developed a great interest in the systematics, ecology, and genetics of yeasts. His interest in yeast systematics was a consequence having the CBS yeast collection close to his work-place in Delft, facilitating his study of these organisms. From that time, van der Walt maintained strong connections with the CBS, consulting their yeast taxonomists on taxonomic problems, and by depositing 492 strains in the collection. These strains formed the basis for 20 new genera and 109 new species. Because of his broad knowledge of enrichment techniques, but also of yeast systematics, van der Walt was invited to contribute several chapters to the second and third editions of The Yeasts: a taxonomic study (Lodder 1970, Kreger-van Rij 1984).

Van der Walt’s broad knowledge of yeasts and his discovery of previously unrecognized genera and species was much respected by other yeast taxonomists, who named four genera and four species in his honour: Vanderwaltia (Novak & Zsolt 1961; now included in Hanseniaspora), Waltomyces (Yamada & Nakase 1985; now included in Lipomyces), Waltiozyma (Muller & Kock 1986; now included in Wickerhamomyces), Vanderwaltozyma by Kurtzman (2003), Torulopsis vanderwaltii (Vidal-Leira 1966; reclassified by Yarrow & Meyer 1978, as Candida vanderwaltii), Kluyveromyces waltii (Kodama 1974; reclassified by Kurtzman 2003, as Lachancea waltii), Myxozyma vanderwaltii (Spaaij et al. 1993), and Bullera waltii (Sugita et al. 1999; reclassified by Wang & Bai 2008, as Derxomyces waltii).

In the early years of taxonomy, a group of scientist that included Johannes van der Walt, Nico van Uden and Herman J. Phaff (Fig. 1), as well as Lynford J. Wickerham, made a huge contribution by using phenotypic characteristics of morphology and physiology for the description of novel genera and species. At that time, these features were considered as important for generic assignment and species distinction. One relevant practical contribution for species characterization was the introduction of the Diazonium Blue B (DBB) test by van der Walt & Hopsu-Havu (1976). In cases where the sexual cycle of a yeast was unknown, this DBB test could be used by yeast taxonomists to determine whether the yeast had an ascomycetous or basidiomycetous affinity. Basidiomycetous yeasts gave a dark red colour reaction when the DBB solution was applied, while this reaction was absent in ascomycetous yeasts.

Fig. 1.

Asci with ascospores of Kluyveromyces polysporus (Vanderwaltozyma polyspora) After Barnett et al. (2000). A. YM agar, 16 d. B. McClary acetate agar, 2 weeks. Bar = 5 μm.

Since the 1970s, following the trends set for bacterial taxonomy, molecular criteria such as mol% G+C and DNA-DNA hetero-duplex formations, and later gene sequencing, were introduced for the yeasts. Today, the introduction of novel species is predominantly based on molecular information obtained by sequencing one or several genes. This evolution in yeast taxonomy can be reconstructed from the five monographs on yeasts that have been published over the years (Lodder & Kreger-van Rij 1952, Lodder 1970, Kreger-van Rij 1984, Kurtzman & Fell 1998, Kurtzman et al. 2011).

Yeast Genera

Between 1964 and 1995, twenty novel yeast genera were introduced by van der Walt (Table 1). The first of these genera was Dekkera. Species of this genus are known as spoilage organisms of soft drinks and alcoholic beverages (Dequin et al. 2003, Dufour et al. 2003, Stratford & James 2003). Besides Dekkera, seven more genera were introduced by van der Walt as single author. Of the remaining genera, eight were published in collaboration with researchers at CBS and four with other authors.

Table 1 Genera introduced by van der Walt and co-authors.

As a consequence of the application of DNA sequence comparisons, eight of these genera were not accepted in the most recent edition of The Yeasts (Kurtzman et al. 2011), but were reduced to synonymy (Table 1). The generic name Debaryozyma (van der Walt & Johannsen 1978) was not accepted because the proposal of Lodder & Kreger-van Rij (1978) to conserve the name Debaryomyces was approved (Greuter et al. 1988) The monospecific genus Wingea is not now retained because the type species of this genus was phylogenetically shown to belong in Debaryomyces (Suzuki et al. 2012). Further, since the ex-type culture Aessosporon was found to mate with strains of Sporidiobolus salmonicolor (Sampaio 2011, unpubl.), this generic name can be considered a synonym of the earlier Sporidiobolus. The status of the genus Entelexis is uncertain; Lachance et al. (2011) commented on this in a discussion of Candida magnolia (previouslyTorulopsis magnoliae), since that species was indicated as the type species of Entelexis by van der Walt & Johannsen (1973).

Yeast Species

Van der Walt was (co-)responsible for the introduction of 109 novel yeast species during the period 1956 to 1999 (Table 2). Of the taxa compiled in Table 2, 30 species were described by van der Walt alone, 15 in collaboration with co-authors at the CBS, and the remaining species with mycologists in other countries. Most of the type strains of these species are isolates from South African sources, and only 20 are from elsewhere. Thirty types were isolated from soil in different localities of South Africa; eight came from vegetable material; 35 from insect-related sources such as frass, tunnels or insect guts; ten are from processed food products such as beer, wine, and buttermilk; and three are from lichens.

Table 2 Species introduced by van der Walt and co-authors.

One of the highlights of his career was the isolation of a strain that produced asci with more ascospores than the normal 1–4 ascospores which he described as Kluyveromyces multisporus (now Vanderwaltozyma polyspora; Fig. 2). One of his new species, Saccharomyces inusitatus, is now considered to have a hybrid genome on the basis of DNA/ DNA reassociation experiments by A. Vaughan and A. Martini (Kurtzman et al. 2011) with high levels of similarity to both S. bayanus (94 %) and S. pastorianus (91 %).

Fig. 2.

Three famous yeast taxonomists (left to right): Herman J. Phaff, Nico van Uden, and Johannes P. van der Walt. Photograph taken in 1987 at the international symposium “The expanding realm of yeast-like fungi”, Amersfoort, The Netherlands,

Van der Walt introduced 16 new combinations of species of which the basionyms were described previously by other yeast taxonomists. As these species are not seen as species first introduced by van der Walt we have not included them in Table 2. These species names, introduced by van der Walt on basis of basionyms of other yeast taxonomist and presently recognized, are listed below:

• Ambrosiozyma monospora (Saito) Van der Walt 1972

• Ambrosiozyma platypodis (J.M. Baker & Kreger) Van der Walt 1972

• Cyniclomyces guttulatus (C.P. Robin) Van der Walt & D.B. Scott 1971

• Hyphopichia burtonii (Boidin et al.) Arx & Van der Walt 1976

• Kluyveromyces aestuarii (Fell) Van der Walt 1971

• Kluyveromyces dobzhanskii (Shehataet al.) Van der Walt 1971

• Kluyveromyces lactis (Dombrowski) Van der Walt 1986

• Kluyveromyces marxianus (E.C. Hansen) Van der Walt 1971

• Kluyveromyces wickerhamii (Phaff et al.) Van der Walt 1971

• Lodderomyces elongisporus (Recca & Mrak) Van der Walt 1971

• Myxozyma melibiosi (Shifrine & Phaff) Van der Walt et al. 1981

• Myxozyma mucilagina (Phaff et al.) Van der Walt et. al. 1981 Saccharomycopsis vini (Kreger-van Rij) Van der Walt & D.B. Scott 1971

• Torulaspora globosa (Klöcker) Van der Walt & Johannsen 1975

• Torulaspora microellipsodes (Osterwalder) Van der Walt & E. Johannsen 1975

• Yarrowia lipolytica (Wickerham et al.) Van der Walt & Arx 1980

Conclusions

Most of the species described early in his career by van der Walt were based on phenotypic features, and, as with genera, molecular data have led to the revision of the status of species described in the “pre-molecular era”. This is evident by comparing the initial status of the species with that in the present classification. From Table 2, it can be seen that 20 species were placed in synonymy with existing taxa, while 54 species were reassigned to different genera and are still recognized as well defined species. However, even after the addition of DNA sequence data, 34 species have retained their original status and stand as tribute to a great yeast taxonomist.

Even after his official retirement, van der Walt did not lose his passion for isolating interesting yeasts. For example, in 2010, over 20 years later, in collaboration with Teresa Coutinho, mating types of the presumed asexual species Candida deformans were isolated from lichens and soil (Groenewald & Smith, unpubl.). The last manuscript that he was actively involved with, resolving species within the Geotrichum/Galactomyces group (Groenewald et al. 2012), was possible because South African strains he isolated in 2009 had been sent to CBS.

The yeast community is indebted to van der Walt for his contribution to the yeast biodiversity and taxonomy over 63 years. It is also likely that further novel taxa remain to be discovered among the strains that he has deposited over the years, supporting the quotation of Pliny (23–79 AD) “Ex Africa semper aliquid novi”¹, a quotation that Johannes van der Walt was fond of citing.

On a personal note, one of us, M. T. S., who collaborated with van der Walt for many years adds: “Those who may have had the privilege to meet Johannes van der Walt or to collaborate with him, as I have, will definitely remember him not only from his taxonomic work, but will also remember him as an amiable person full with stories to tell while enjoying a fine dinnerwith a good glass of wine.”