Genomic instability in an interspecific hybrid of the genus Saccharomyces: a matter of adaptability.

Ancient events of polyploidy have been linked to huge evolutionary leaps in the tree of life, while increasing evidence shows that newly established polyploids have adaptive advantages in certain stress conditions compared to their relatives with a lower ploidy. The genus Saccharomyces is a good model for studying such events, as it contains an ancient whole-genome duplication event and many sequenced Saccharomyces cerevisiae are, evolutionary speaking, newly formed polyploids. Many polyploids have unstable genomes and go through large genome erosions; however, it is still unknown what mechanisms govern this reduction. Here, we sequenced and studied the natural S. cerevisiae × Saccharomyces kudriavzevii hybrid strain, VIN7, which was selected for its commercial use in the wine industry. The most singular observation is that its nuclear genome is highly unstable and drastic genomic alterations were observed in only a few generations, leading to a widening of its phenotypic landscape. To better understand what leads to the loss of certain chromosomes in the VIN7 cell population, we looked for genetic features of the genes, such as physical interactions, complex formation, epistatic interactions and stress responding genes, which could have beneficial or detrimental effects on the cell if their dosage is altered by a chromosomal copy number variation. The three chromosomes lost in our VIN7 population showed different patterns, indicating that multiple factors could explain the mechanisms behind the chromosomal loss. However, one common feature for two out of the three chromosomes is that they are among the smallest ones. We hypothesize that small chromosomes alter their copy numbers more frequently as a low number of genes is affected, meaning that it is a by-product of genome instability, which might be the chief driving force of the adaptability and genome architecture of this hybrid.

Aneuploidy and Ethanol Tolerance in Saccharomyces cerevisiae.

Response to environmental stresses is a key factor for microbial organism growth. One of the major stresses for yeasts in fermentative environments is ethanol. Saccharomyces cerevisiae is the most tolerant species in its genus, but intraspecific ethanol-tolerance variation exists. Although, much effort has been done in the last years to discover evolutionary paths to improve ethanol tolerance, this phenotype is still hardly understood. Here, we selected five strains with different ethanol tolerances, and used comparative genomics to determine the main factors that can explain these phenotypic differences. Surprisingly, the main genomic feature, shared only by the highest ethanol-tolerant strains, was a polysomic chromosome III. Transcriptomic data point out that chromosome III is important for the ethanol stress response, and this aneuploidy can be an advantage to respond rapidly to ethanol stress. We found that chromosome III copy numbers also explain differences in other strains. We show that removing the extra chromosome III copy in an ethanol-tolerant strain, returning to euploidy, strongly compromises its tolerance. Chromosome III aneuploidy appears frequently in ethanol-tolerance evolution experiments, and here, we show that aneuploidy is also used by natural strains to enhance their ethanol tolerance.

Lactose: the milk sugar from a biotechnological perspective.

Lactose is a very important sugar because of its abundance in the milk of humans and domestic animals. Lactose is a valuable asset as a basic nutrient and the main substrate in fermentative processes that led to the production of fermented milk products, such as yogurt and kefir. In some instances, lactose also can be a problem as the causative agent of some diseases, such as lactose intolerance and galactosemia, or for being a by-product generated in huge amounts by the cheese industry. The study of the biochemical reactions leading to the synthesis and assimilation of lactose has provided valuable models for the understanding of biosynthetic and catabolic processes. Lactose-hydrolyzing enzymes are structurally and phylogenetically related to different types of beta-galactosidases and bacterial cellobiases involved in the enzymatic degradation of cellulose. Biotransformation of lactose, by either enzymatic or fermentative procedures, is important for different types of industrial applications in dairy and pharmaceutical industries.

Cloning and molecular characterization of the delta6-desaturase from two echium plant species: production of GLA by heterologous expression in yeast and tobacco.

The synthesis of GLA (delta6,9,12-1-8:3) is carried out in a number of plant taxa by introducing a double bond at the delta6 position of its precursor, linoleic acid (delta9,12-18:2), through a reaction catalyzed by a delta6-desaturase enzyme. We have cloned genes encoding the delta6-desaturase (D6DES) from two different Macaronesian Echium species, E. pitardii and E. gentianoides (Boraginaceae), which are characterized by the accumulation of high amounts of GLA in their seeds. The Echium D6DES genes encode proteins of 438 amino acids bearing the prototypical cytochrome b(5) domain at the N-terminus. Cladistic analysis of desaturases from higher plants groups the Echium D6DES proteins together with other delta6-desaturases in a different cluster from that of the highly related delta8-desaturases. Expression analysis carried out in E. pitardii shows a positive correlation between the D6DES transcript level and GLA accumulation in different tissues of the plant. Although a ubiquitous expression in all organs is observed, the transcript is particularly abundant in developing fruits, whereas a much lower level is present in mature leaves. Functional characterization of the D6DES gene from E. gentianoides has been achieved by heterologous expression in tobacco plants and in the yeast Saccharomyces cerevisiae. In both cases, overexpression of the gene led to the synthesis of GLA. Biotechnological application of these results can be envisaged as an initial step toward the generation of transgenic oleaginous plants producing GLA.