The growth and metabolome of Saccharomyces uvarum in wine fermentations are strongly influenced by the route of nitrogen assimilation.

Nitrogen is a critical nutrient in beverage fermentations, influencing fermentation performance and formation of compounds that affect organoleptic properties of the product. Traditionally, most commercial wine fermentations rely on Saccharomyces cerevisiae but the potential of alternative yeasts is increasingly recognised because of the possibility to deliver innovative products and process improvements. In this regard, Saccharomyces uvarum is an attractive non-traditional yeast that, while quite closely related to S. cerevisiae, displays a different fermentative and aromatic profile. Although S. uvarum is used in cider-making and in some winemaking, better knowledge of its physiology and metabolism is required if its full potential is to be realised. To address this gap, we performed a comparative analysis of the response of S. uvarum and S. cerevisiae to 13 different sources of nitrogen, assessing key parameters such as growth, fermentation performance, the production of central carbon metabolites and aroma volatile compounds. We observed that the two species differ in the production of acetate, succinate, medium-chain fatty acids, phenylethanol, phenylethyl acetate, and fusel/branched acids in ways that reflect different distribution of fluxes in the metabolic network. The integrated analysis revealed different patterns of yeast performance and activity linked to whether growth was on amino acids metabolised via the Ehrlich pathway or on amino acids and compounds assimilated through the central nitrogen core. This study highlights differences between the two yeasts and the importance that nitrogen metabolism can play in modulating the sensory profile of wine when using S. uvarum as the fermentative yeast.

Beyond Saccharomyces pastorianus for modern lager brews: Exploring non-cerevisiae Saccharomyces hybrids with heterotic maltotriose consumption and novel aroma profile.

Non-domesticated, wild Saccharomyces yeasts have promising characteristics for beer diversification, particularly when used in the generation of de novo interspecific hybrids. A major motivation for the current work was the question whether attractive novel Saccharomyces interspecific hybrids can be created for the production of exotic lager beers without using the genomic resources of the ale yeast Saccharomyces cerevisiae. Importantly, maltotriose utilization is an essential characteristic typically associated with domesticated ale/lager brewing strains. A high-throughput screening on nearly 200 strains representing all eight species of the Saccharomyces genus was conducted. Three Saccharomyces mikatae strains were able to aerobically grow on maltotriose as the sole carbon source, a trait until recently unidentified for this species. Our screening also confirmed the recently reported maltotriose utilization of the S. jurei strain D5095T. Remarkably, de novo hybrids between a maltotriose-utilizing S. mikatae or S. jurei strain and the maltotriose-negative Saccharomyces eubayanus strain CBS 12357T displayed heterosis and outperformed both parents with regard to aerobically utilizing maltotriose as the sole source of carbon. Indeed, the maximum specific growth rates on this sugar were comparable to the well-known industrial strain, Saccharomyces pastorianus CBS 1513. In lager brewing settings (oxygen-limited), the new hybrids were able to ferment maltose, while maltotriose was not metabolized. Favorable fruity esters were produced, demonstrating that the novel hybrids have the potential to add to the diversity of lager brewing.

The evolution and role of the periplasmic asparaginase Asp3 in yeast.

The study of nitrogen assimilation in yeast is of interest from genetic, evolutionary, and biotechnological perspectives. Over the course of evolution, yeasts have developed sophisticated control mechanisms to regulate nitrogen metabolism, with domesticated lineages sometimes displaying particular specialisation. The focus of this study was on assimilation of asparagine, which is a significant nutritional source for some alcoholic fermentations. We were particularly interested in ASP3, which encodes a periplasmic asparaginase and that was proposed to have been acquired relatively recently in S. cerevisiae by horizontal gene transfer. We examined 1680 S. cerevisiae genome assemblies to evaluate the distribution and evolutionary trajectory of ASP3. Our findings suggest an alternative hypothesis that ASP3 is an ancient Saccharomyces gene that has generally been lost over the course of evolution but has been retained in certain fermentative environments. As asparagine is the major nitrogen source in apple juice, we explored whether the presence of ASP3 would confer a growth advantage. Interestingly, we found that although ASP3 enhances growth when asparagine is the sole nitrogen source, the same effect is not seen in apple juice. These data indicate that growth in pure culture may not reflect the original selective environment for ASP3+ strains and highlight the role that complex regulation may play in optimising nitrogen assimilation in yeasts.

Domestication-induced reduction in eye size revealed in multiple common garden experiments: The case of Atlantic salmon (Salmo salar L.).

Domestication leads to changes in traits that are under directional selection in breeding programmes, though unintentional changes in nonproduction traits can also arise. In offspring of escaping fish and any hybrid progeny, such unintentionally altered traits may reduce fitness in the wild. Atlantic salmon breeding programmes were established in the early 1970s, resulting in genetic changes in multiple traits. However, the impact of domestication on eye size has not been studied. We measured body size corrected eye size in 4000 salmon from six common garden experiments conducted under artificial and natural conditions, in freshwater and saltwater environments, in two countries. Within these common gardens, offspring of domesticated and wild parents were crossed to produce 11 strains, with varying genetic backgrounds (wild, domesticated, F1 hybrids, F2 hybrids and backcrosses). Size-adjusted eye size was influenced by both genetic and environmental factors. Domesticated fish reared under artificial conditions had smaller adjusted eye size when compared to wild fish reared under identical conditions, in both the freshwater and marine environments, and in both Irish and Norwegian experiments. However, in parr that had been introduced into a river environment shortly after hatching and sampled at the end of their first summer, differences in adjusted eye size observed among genetic groups were of a reduced magnitude and were nonsignificant in 2-year-old sea migrating smolts sampled in the river immediately prior to sea entry. Collectively, our findings could suggest that where natural selection is present, individuals with reduced eye size are maladapted and consequently have reduced fitness, building on our understanding of the mechanisms that underlie a well-documented reduction in the fitness of the progeny of domesticated salmon, including hybrid progeny, in the wild.