No abstract

The SUC multigene family of the single-celled yeast Saccharomyces cerevisiae is polymorphic, with genes varying both in number and activity. All of the genes encode invertase, an enzyme that is secreted to digest sucrose outside of the cell. This communal endeavour creates the potential for individual cells to defect (cheat) by stealing the sugar digested by their neighbours without contributing the enzyme themselves. We measured the fitness of a defector, with a deleted suc2 gene, relative to an otherwise isogenic cooperator, with a functional SUC2 gene. We manipulated the level of social interaction within the community by varying the population density and found that the defector is less fit than the cooperator at low levels of sociality but more fit in dense communities. We propose that selection for antisocial cheating causes SUC polymorphism in nature. The infamous Prisoner's Dilemma game shows that social behaviour is generally unstable, and the success of both cooperation and defection can vary continuously in time and space. The variation in SUC genes reflects constant adaptation to an ever-changing biotic environment that is a consequence of the instability of cooperation. It is interesting that social interactions can have a direct effect on molecular evolution, even in an organism as simple as yeast.

BackgroundMatings between different Saccharomyces sensu stricto yeast species produce sexually sterile hybrids, so individuals should avoid mating with other species. Any mechanism that reduces the frequency of interspecific matings will confer a selective advantage. Here we test the ability of two closely-related Saccharomyces sensu stricto species to select their own species as mates and avoid hybridisation.ResultsWe set up mate choice tests, using five independently isolated pairs of species, in which individual germinating spores were presented with the opportunity to mate either with a germinating spore of their own species or with a germinating spore of the other species. For all five strain pairs, whether a S. cerevisiae or S. paradoxus occupies the role of "chooser" strain, the level of hybridisation that is observed between the two species is significantly lower than would be expected if mates were selected at random. We also show that, overall, S. cerevisiae exhibited a stronger own-species preference than S. paradoxus.ConclusionPrezygotic reproductive isolation is well known in higher organisms but has been largely overlooked in yeast, an important model microbe. Here we present the first report of prezygotic reproductive isolation in Saccharomyces. Prezygotic reproductive isolation may be important in yeast speciation or yeast species cohesion, and may have evolved to prevent wasted matings between different species. Whilst yeast has long been used as a genetic model system, little is known about yeast in the wild. Our work sheds light on an interesting aspect of yeast natural behaviour: their ability to avoid costly interspecific matings.

Complex life has arisen through a series of ‘major transitions’ in which collectives of formerly autonomous individuals evolve into a single, integrated organism. A key step in this process is the origin of higher-level evolvability, but little is known about how higher-level entities originate and gain the capacity to evolve as an individual. Here we report a single mutation that not only creates a new level of biological organization, but also potentiates higher-level evolvability. Disrupting the transcription factor ACE2 in Saccharomyces cerevisiae prevents mother–daughter cell separation, generating multicellular ‘snowflake’ yeast. Snowflake yeast develop through deterministic rules that produce geometrically defined clusters that preclude genetic conflict and display a high broad-sense heritability for multicellular traits; as a result they are preadapted to multicellular adaptation. This work demonstrates that simple microevolutionary changes can have profound macroevolutionary consequences, and suggests that the formation of clonally developing clusters may often be the first step to multicellularity.

Models of sexual selection predict that females use ornament size to evaluate male condition. It has also been suggested that ornament asymmetry provides females with accurate information about condition. To test these ideas we experimentally manipulated condition in the stalk-eyed £y, Cyrtodiopsis dalmanni, by varying the amount of food available to developing larvae. Males of this species have greatly exaggerated eyestalk length and females prefer to mate with males with wider eyespans. Our experiments show that male ornaments (eyestalks) display a disproportionate sensitivity to condition compared with the homologous character in females, and to non-sexual traits (wing dimensions). In contrast, in neither sex did asymmetry re£ect condition either in sexual ornaments or in non-sexual traits. We conclude that ornament size is likely to play a far greater role in sexual selection as an indicator of individual condition than does asymmetry.

Genetics and origin of a Drosophilamelanogaster population recently introduced to the Seychelles. Genet. Res. 40, 295-303. 26. Beppu, K., Kaneko, A., Toda, M.J., and Kimura, M.T. (1977). Methods in the studies of wild drosophilid fl ies in Hokkaido. 2. Key to species of Drosophilidae in Hokkaido, with a supplementary note on phylogeny.

Most models of speciation require gradual change and geographic or ecological isolation for new species to arise. Homoploid hybrid speciation occurred readily between Saccharomyces cerevisiae and Saccharomyces paradoxus. Hybrids had high self-fertility (about 82%), low fertility when backcrossed to either parental species (about 7.5%), and vigorous growth under different thermal environments that favored one or the other of the parental species. Extensive karyotypic changes (tetrasomy) were observed in the hybrids, although genic incompatibilities accounted for 50% of the variation in self-fertility.

It is commonly assumed that the world would be best off if everyone co-operates. Experimental and mathematical analysis of “co-operation” in yeast show why this isn't always the case.