WHO elements - A new category of selfish genetic elements at the borderline between homing elements and transposable elements.

Homing genetic elements are a form of selfish DNA that inserts into a specific target site in the genome and spreads through the population by a process of biased inheritance. Two well-known types of homing element, called inteins and homing introns, were discovered decades ago. In this review we describe WHO elements, a newly discovered type of homing element that constitutes a distinct third category but is rare, having been found only in a few yeast species so far. WHO elements are inferred to spread using the same molecular homing mechanism as inteins and introns: they encode a site-specific endonuclease that cleaves the genome at the target site, making a DNA break that is subsequently repaired by copying the element. For most WHO elements, the target site is in the glycolytic gene FBA1. WHO elements differ from inteins and homing introns in two fundamental ways: they do not interrupt their host gene (FBA1), and they occur in clusters. The clusters were formed by successive integrations of different WHO elements into the FBA1 locus, the result of an 'arms race' between the endonuclease and its target site. We also describe one family of WHO elements (WHO10) that is no longer specifically associated with the FBA1 locus and instead appears to have become transposable, inserting at random genomic sites in Torulaspora globosa with up to 26 copies per strain. The WHO family of elements is therefore at the borderline between homing genetic elements and transposable elements.

Population genomics of the pathogenic yeast Candida tropicalis identifies hybrid isolates in environmental samples.

Candida tropicalis is a human pathogen that primarily infects the immunocompromised. Whereas the genome of one isolate, C. tropicalis MYA-3404, was originally sequenced in 2009, there have been no large-scale, multi-isolate studies of the genetic and phenotypic diversity of this species. Here, we used whole genome sequencing and phenotyping to characterize 77 isolates of C. tropicalis from clinical and environmental sources from a variety of locations. We show that most C. tropicalis isolates are diploids with approximately 2-6 heterozygous variants per kilobase. The genomes are relatively stable, with few aneuploidies. However, we identified one highly homozygous isolate and six isolates of C. tropicalis with much higher heterozygosity levels ranging from 36-49 heterozygous variants per kilobase. Our analyses show that the heterozygous isolates represent two different hybrid lineages, where the hybrids share one parent (A) with most other C. tropicalis isolates, but the second parent (B or C) differs by at least 4% at the genome level. Four of the sequenced isolates descend from an AB hybridization, and two from an AC hybridization. The hybrids are MTLa/α heterozygotes. Hybridization, or mating, between different parents is therefore common in the evolutionary history of C. tropicalis. The new hybrids were predominantly found in environmental niches, including from soil. Hybridization is therefore unlikely to be associated with virulence. In addition, we used genotype-phenotype correlation and CRISPR-Cas9 editing to identify a genome variant that results in the inability of one isolate to utilize certain branched-chain amino acids as a sole nitrogen source.

Identification of European isolates of the lager yeast parent Saccharomyces eubayanus.

Lager brewing first occurred in Bavaria in the 15th century, associated with restrictions of brewing to colder months. The lager yeast, Saccharomyces pastorianus, is cold tolerant. It is a hybrid between Saccharomyces cerevisiae and Saccharomyces eubayanus, and has been found only in industrial settings. Natural isolates of S. eubayanus were first discovered in Patagonia 11 years ago. They have since been isolated from China, Tibet, New Zealand, and North America, but not from Europe. Here, we describe the first European strains UCD646 and UCD650, isolated from a wooded area on a university campus in Dublin, Ireland. We generated complete chromosome level assemblies of both genomes using long- and short-read sequencing. The UCD isolates belong to the Holarctic clade. Genome analysis shows that isolates similar to the Irish strains contributed to the S. eubayanus component of S. pastorianus, but isolates from Tibet made a larger contribution.

Evolution of the Genetic Code in the Ascoideales (CUG-Ser2) Yeast Clade: The Ancestral tRNA-Leu(CAG) Gene Is Retained in Most Saccharomycopsis Species but Is Nonessential and Not Used for Translation.

In the yeast genera Saccharomycopsis and Ascoidea, which comprise the taxonomic order Ascoideales, nuclear genes use a nonstandard genetic code in which CUG codons are translated as serine instead of leucine, due to a tRNA-Ser with the unusual anticodon CAG. However, some species in this clade also retain an ancestral tRNA-Leu gene with the same anticodon. One of these species, Ascoidea asiatica, has been shown to have a stochastic proteome in which proteins contain ∼50% Ser and 50% Leu at CUG codon sites, whereas previously examined Saccharomycopsis species translate CUG only as Ser. Here, we investigated the presence, conservation, and possible functionality of the tRNA-Leu(CAG) gene in the genus Saccharomycopsis. We sequenced the genomes of 23 strains that, together with previously available data, include almost every known species of this genus. We found that most Saccharomycopsis species have genes for both tRNA-Leu(CAG) and tRNA-Ser(CAG). However, tRNA-Leu(CAG) has been lost in Saccharomycopsis synnaedendra and Saccharomycopsis microspora, and its predicted cloverleaf structure is aberrant in all the other Saccharomycopsis species. We deleted the tRNA-Leu(CAG) gene of Saccharomycopsis capsularis and found that it is not essential. Proteomic analyses in vegetative and sporulating cultures of S. capsularis and Saccharomycopsis fermentans showed only translation of CUG as Ser. Despite its unusual structure, the tRNA-Leu(CAG) gene shows evidence of sequence conservation among Saccharomycopsis species, particularly in its acceptor stem and leucine identity elements, which suggests that it may have been retained in order to carry out an unknown nontranslational function.

Draft genome sequence of the yeast Schwanniomyces capriottii strain UCD805, isolated from soil in Ireland.

Schwanniomyces capriottii is a member of the Debaryomycetaceae family in the order Saccharomycetales. Here, we present the genome sequence of S. capriottii UCD805, which was isolated from soil in Dublin, Ireland. This genome is 12.2 Mb and was assembled into 14 scaffolds plus a mitochondrial genome scaffold.

Identification of a novel Candida metapsilosis isolate reveals multiple hybridization events.

Candida metapsilosis is a member of the Candida parapsilosis species complex, a group of opportunistic human pathogens. Of all the members of this complex, C. metapsilosis is the least virulent, and accounts for a small proportion of invasive Candida infections. Previous studies established that all C. metapsilosis isolates are hybrids, originating from a single hybridization event between two lineages, parent A and parent B. Here, we use MinION and Illumina sequencing to characterize a C. metapsilosis isolate that originated from a separate hybridization. One of the parents of the new isolate is very closely related to parent A. However, the other parent (parent C) is not the same as parent B. Unlike C. metapsilosis AB isolates, the C. metapsilosis AC isolate has not undergone introgression at the mating type-like locus. In addition, the A and C haplotypes are not fully collinear. The C. metapsilosis AC isolate has undergone loss of heterozygosity with a preference for haplotype A, indicating that this isolate is in the early stages of genome stabilization.

Draft Genome Sequence of the Yeast Torulaspora quercuum Strain UCD657, Isolated from Soil in Ireland.

Torulaspora quercuum is an ascomycete yeast first isolated in 2009. Here, we present the genome sequence of T. quercuum isolate UCD657, which was isolated from soil in Ireland. This genome is 10.4 Mb and was assembled into 8 chromosome-sized scaffolds of >1 Mb in size, plus a mitochondrial genome scaffold.

Draft Genome Sequence of the Yeast Blastobotrys aristata Strain UCD613, Isolated from Soil in Ireland.

Blastobotrys aristata is a member of the Trichomonascaceae family in the order Saccharomycetales. Here, we present the genome sequence of B. aristata UCD613, which was isolated from soil in Dublin, Ireland. This genome is 13.3 Mb and was assembled into 4 chromosome-size scaffolds of >2.2 Mb in size plus a mitochondrial genome scaffold.

Draft Genome Sequence of a Diploid and Hybrid Candida Strain, Candida sanyaensis UCD423, Isolated from Compost in Ireland.

Candida sanyaensis is a CUG-Ser1 clade yeast that is associated with soil. Assembly of short-read and long-read data shows that C. sanyaensis has a diploid and hybrid genome, with approximately 97% identity between the haplotypes. The haploid genome size is approximately 15.4 Mb.

Draft Genome Sequence of the Yeast Ogataea degrootiae Strain UCD465, Isolated from Soil in Ireland.

Ogataea degrootiae is an ascomycete yeast that was first isolated in the Netherlands in 2017. It is a member of the Pichiaceae clade. Here, we present the genome sequence of O. degrootiae UCD465, which was isolated from soil in Ireland. This genome is 14.6 Mb and haploid.

Draft Genome Sequence of the Birch Tree Fungal Pathogen Taphrina betulina UCD315.

Taphrina betulina is the ascomycete yeast that causes the formation of witches' brooms in birch trees. Here, we report the first draft genome sequence of T. betulina, from strain UCD315, isolated from soil in Ireland. The genome is haploid and 12.5 Mb long.