Metschnikowia mating genomics

Yeast sexes: mating types do not determine the sexes in Metschnikowia species

Although filamentous Ascomycetes may produce structures that are interpreted as male and female gametangia, ascomycetous yeasts are generally not considered to possess male and female sexes. In haplontic yeasts of the genus Metschnikowia, the sexual cycle begins with the fusion of two morphologically identical cells of complementary mating types. Soon after conjugation, a protuberance emerges from one of the conjugants, eventually maturing into an ascus. The originating cell can be regarded as an ascus mother cell, hence as female. We tested the hypothesis that the sexes, female or male, are determined by the mating types. There were good reasons to hypothesize further that mating type α cells are male. In a conceptually simple experiment, we observed the early stages of the mating reaction of mating types differentially labeled with fluorescent concanavalin A conjugates. Three large-spored Metschnikowia species, M. amazonensis, M. continentalis, and M. matae, were examined. In all three, the sexes were found to be independent of mating type, cautioning that the two terms should not be used interchangeably.

Yeasts from tropical forests: Biodiversity, ecological interactions, and as sources of bioinnovation

Tropical rainforests and related biomes are found in Asia, Australia, Africa, Central and South America, Mexico, and many Pacific Islands. These biomes encompass less than 20% of Earth's terrestrial area, may contain about 50% of the planet's biodiversity, and are endangered regions vulnerable to deforestation. Tropical rainforests have a great diversity of substrates that can be colonized by yeasts. These unicellular fungi contribute to the recycling of organic matter, may serve as a food source for other organisms, or have ecological interactions that benefit or harm plants, animals, and other fungi. In this review, we summarize the most important studies of yeast biodiversity carried out in these biomes, as well as new data, and discuss the ecology of yeast genera frequently isolated from tropical forests and the potential of these microorganisms as a source of bioinnovation. We show that tropical forest biomes represent a tremendous source of new yeast species. Although many studies, most using culture-dependent methods, have already been carried out in Central America, South America, and Asia, the tropical forest biomes of Africa and Australasia remain an underexplored source of novel yeasts. We hope that this review will encourage new researchers to study yeasts in unexplored tropical forest habitats.

The evolving species concepts used for yeasts: from phenotypes and genomes to speciation networks

Here we review how evolving species concepts have been applied to understand yeast diversity. Initially, a phenotypic species concept was utilized taking into consideration morphological aspects of colonies and cells, and growth profiles. Later the biological species concept was added, which applied data from mating experiments. Biophysical measurements of DNA similarity between isolates were an early measure that became more broadly applied with the advent of sequencing technology, leading to a sequence-based species concept using comparisons of parts of the ribosomal DNA. At present phylogenetic species concepts that employ sequence data of rDNA and other genes are universally applied in fungal taxonomy, including yeasts, because various studies revealed a relatively good correlation between the biological species concept and sequence divergence. The application of genome information is becoming increasingly common, and we strongly recommend the use of complete, rather than draft genomes to improve our understanding of species and their genome and genetic dynamics. Complete genomes allow in-depth comparisons on the evolvability of genomes and, consequently, of the species to which they belong. Hybridization seems a relatively common phenomenon and has been observed in all major fungal lineages that contain yeasts. Note that hybrids may greatly differ in their post-hybridization development. Future in-depth studies, initially using some model species or complexes may shift the traditional species concept as isolated clusters of genetically compatible isolates to a cohesive speciation network in which such clusters are interconnected by genetic processes, such as hybridization.

Catching speciation in the act-act 2: Metschnikowia lacustris sp. nov., a sister species to Metschnikowia dekortorum

The isolation of a single yeast strain in the clade containing Metschnikowia dekortorum, in the Amazon biome of Brazil, incited us to re-examine the species boundaries within the clade. The strain (UFMG-CM-Y6306) was difficult to position relative to neighbouring species using standard barcode sequences (ITS-D1/D2 rRNA gene region). Mating took place freely with α strains of M. bowlesiae, M. dekortorum, and M. similis, but two-spored asci, indicative of a fertile meiotic progeny, were formed abundantly only with certain strains of M. dekortorum. Accordingly, we examined mating success among every phylotype in the clade and constructed a phylogeny based on a concatenation of 100 of the largest orthologous genes annotated in draft genomes. The analyses confirmed membership of the Amazonian isolate in M. dekortorum, but also indicated that the species should be subdivided into two. As a result, we retain three original members of M. dekortorum in the species, together with the new isolate, and reassign six isolates recovered from Mesoamerican lacustrine habitats to Metschnikowia lacustris sp. nov. The type is UWOPS 12-619.2^T (isotype CBS 16250^T). MycoBank: MB 833751.

The yeast community of Conotelus sp. (Coleoptera: Nitidulidae) in Brazilian passionfruit flowers (Passiflora edulis) and description of Metschnikowia amazonensis sp. nov., a large-spored clade yeast

Species of the nitidulid beetle Conotelus found in flowers of Convolvulaceae and other plants across the New World and in Hawaii consistently harbour a yeast community dominated by one or more large-spored Metschnikowia species. We investigated the yeasts found in beetles and flowers of cultivated passionfruit in Rondônia state, in the Amazon biome of Brazil, where a Conotelus species damages the flowers and hinders fruit production. A sample of 46 beetles and 49 flowers yielded 86 and 83 yeast isolates, respectively. Whereas the flower community was dominated by Kodamaea ohmeri and Kurtzmaniella quercitrusa, the major yeasts recovered from beetles were Wickerhamiella occidentalis, which is commonly isolated from this community, and a novel species of large-spored Metschnikowia in the arizonensis subclade, which we describe here as Metschnikowia amazonensis sp. nov. Phylogenetic analyses based on barcode sequences (ITS-D1/D2) and a multigene alignment of 11,917 positions (genes ura2, msh6, and pmt2) agreed to place the new species as a sister to Metschnikowia arizonensis, a rare species known only from one locality in Arizona. The two form sterile asci when mated, which is typical of related members of the clade. The α pheromone of the new species is unique but typical of the subclade. The type of M. amazonensis sp. nov. is UFMG-CM-Y6309^T (ex-type CBS 16156^T , mating type a), and the designated allotype (mating type α) is UFMG-CM-Y6307^A (CBS 16155^A ). MycoBank MB 833560.

Genomic content of a novel yeast species Hanseniaspora gamundiae sp. nov. from fungal stromata (Cyttaria) associated with a unique fermented beverage in Andean Patagonia, Argentina

A novel yeast species was isolated from the sugar-rich stromata of Cyttaria hariotii collected from two different Nothofagus tree species in the Andean forests of Patagonia, Argentina. Phylogenetic analyses of the concatenated sequence of the rRNA gene sequences and the protein-coding genes for actin and translational elongation factor-1α indicated that the novel species belongs to the genus Hanseniaspora. De novo genome assembly of the strain CRUB 1928T yielded a 10.2-Mbp genome assembly predicted to encode 4452 protein-coding genes. The genome sequence data were compared to the genomes of other Hanseniaspora species using three different methods, an alignment-free distance measure, Kr, and two model-based estimations of DNA-DNA homology values, of which all provided indicative values to delineate species of Hanseniaspora. Given its potential role in a rare indigenous alcoholic beverage in which yeasts ferment sugars extracted from the stromata of Cytarria sp., we searched for the genes that may suggest adaptation of novel Hanseniaspora species to fermenting communities. The SSU1-like gene encoding a sulfite efflux pump, which, among Hanseniaspora, is present only in close relatives to the new species, was detected and analyzed, suggesting that this gene might be one factor that characterizes this novel species. We also discuss several candidate genes that likely underlie the physiological traits used for traditional taxonomic identification. Based on these results, a novel yeast species with the name Hanseniaspora gamundiae sp. nov. is proposed with CRUB 1928T (ex-types: ZIM 2545T = NRRL Y-63793T = PYCC 7262T; MycoBank number MB 824091) as the type strain. Furthermore, we propose the transfer of the Kloeckera species, K. hatyaiensis, K. lindneri and K. taiwanica to the genus Hanseniaspora as Hanseniaspora hatyaiensis comb. nov. (MB 828569), Hanseniaspora lindneri comb. nov. (MB 828566) and Hanseniaspora taiwanica comb. nov. (MB 828567).