Identification and Molecular Characterization of Novel Mycoviruses in Saccharomyces and Non-Saccharomyces Yeasts of Oenological Interest

Wine yeasts can be natural hosts for dsRNA, ssRNA viruses and retrotransposon elements. In this study, high-throughput RNA sequencing combined with bioinformatic analyses unveiled the virome associated to 16 Saccharomyces cerevisiae and 8 non-Saccharomyces strains of oenological interest. Results showed the presence of six viruses and two satellite dsRNAs from four different families, two of which-Partitiviridae and Mitoviridae-were not reported before in yeasts, as well as two ORFan contigs of viral origin. According to phylogenetic analysis, four new putative mycoviruses distributed in Totivirus, Cryspovirus, and Mitovirus genera were identified. The majority of commercial S. cerevisiae strains were confirmed to be the host for helper L-A type totiviruses and satellite M dsRNAs associated with the killer phenotype, both in single and mixed infections with L-BC totiviruses, and two viral sequences belonging to a new cryspovirus putative species discovered here for the first time. Moreover, single infection by a narnavirus 20S-related sequence was also found in one S. cerevisiae strain. Considering the non-Saccharomyces yeasts, Starmerella bacillaris hosted four RNAs of viral origin-two clustering in Totivirus and Mitovirus genera, and two ORFans with putative satellite behavior. This study confirmed the infection of wine yeasts by viruses associated with useful technological characteristics and demonstrated the presence of complex mixed infections with unpredictable biological effects.

The Species-Specific Acquisition and Diversification of a K1-like Family of Killer Toxins in Budding Yeasts of the Saccharomycotina

Killer toxins are extracellular antifungal proteins that are produced by a wide variety of fungi, including Saccharomyces yeasts. Although many Saccharomyces killer toxins have been previously identified, their evolutionary origins remain uncertain given that many of these genes have been mobilized by double-stranded RNA (dsRNA) viruses. A survey of yeasts from the Saccharomyces genus has identified a novel killer toxin with a unique spectrum of activity produced by Saccharomyces paradoxus. The expression of this killer toxin is associated with the presence of a dsRNA totivirus and a satellite dsRNA. Genetic sequencing of the satellite dsRNA confirmed that it encodes a killer toxin with homology to the canonical ionophoric K1 toxin from Saccharomyces cerevisiae and has been named K1-like (K1L). Genomic homologs of K1L were identified in six non-Saccharomyces yeast species of the Saccharomycotina subphylum, predominantly in subtelomeric regions of the genome. When ectopically expressed in S. cerevisiae from cloned cDNAs, both K1L and its homologs can inhibit the growth of competing yeast species, confirming the discovery of a family of biologically active K1-like killer toxins. The sporadic distribution of these genes supports their acquisition by horizontal gene transfer followed by diversification. The phylogenetic relationship between K1L and its genomic homologs suggests a common ancestry and gene flow via dsRNAs and DNAs across taxonomic divisions. This appears to enable the acquisition of a diverse arsenal of killer toxins by different yeast species for potential use in niche competition.

Viral dsRNA in the wine yeast Saccharomyces bayanus var. uvarum

The presence of viral dsRNA (L and M fractions) in the cryophilic yeast Saccharomyces bayanus var. uvarum is documented here for the first time. Sixty-eight strains of different origins were analyzed. Most of them did not carry dsRNA; the L fraction was found in seven strains, while 11 strains had both L and M fractions. The size of the L fraction was invariable (4.5 kb), as in the cultured yeast Saccharomyces cerevisiae. In contrast to L-dsRNA, the M fraction varied in size from ca. 1.2 to 1.8 kb. In total, seven different M-dsRNA types were recognized (M1-M3 and M8-M11), predominantly among French wine strains of S. bayanus var. uvarum. Phenotypic analysis revealed that the M-dsRNAs found were cryptic and may represent mutant forms of killer plasmids.

[Molecular polymorphism of viral dsRNA of yeast Saccharomyces paradoxus]

An analysis of 53 strains of yeast Saccharomyces paradoxus (YSP) of different geographic origins enabled us, for the first time, to find viral double-stranded RNA (L and M fractions) in YSP and to study natural polymorphism. As in the cultured Scerevisiae, the size of L dsRNA was constant (4.5 kb). The size of minor M dsRNA varied from 1.5 to 2.4 k.b. In YSP, we determined 7 types of M dsRNA (M1-M7), which were not connected with the source of isolation or geographic origin of the host strains.