Loss of Elongation-Like Factor 1 Spontaneously Induces Diverse, RNase H-Related Suppressor Mutations in .

A healthy individual may carry a detrimental genetic trait that is masked by another genetic mutation. Such suppressive genetic interactions, in which a mutant allele either partially or completely restores the fitness defect of a particular mutant, tend to occur between genes that have a confined functional connection. Here we investigate a self-recovery phenotype in , mediated by suppressive genetic interactions that can be amplified during cell culture.

A Potential Case of Reinforcement in a Facultatively Sexual Unicellular Eukaryote.

The origin of a new species requires a mechanism to prevent divergent populations from interbreeding. In the classic allopatric model, divided populations evolve independently and accumulate genetic differences. If contact is restored, hybrids suffer reduced fitness and selection may favor traits that prevent mistakes in mating, a process known as reinforcement.

Evolution: a collection of misfits.

Different strains of one genetic model species after another are turning out to have limited abilities to interbreed, as if they were on the way to becoming different species. Are model organisms aberrations, or are the first steps in speciation easier than they seem?

Rapid evolution of cheating mitochondrial genomes in small yeast populations.

Outcrossed sex exposes genes to competition with their homologues, allowing alleles that transmit more often than their competitors to spread despite organismal fitness costs. Mitochondrial populations in species with biparental inheritance are thought to be especially susceptible to such cheaters because they lack strict transmission rules like meiosis or maternal inheritance. Yet the interaction between mutation and natural selection in the evolution of cheating mitochondrial genomes has not been tested experimentally.

Life-history evolution and density-dependent growth in experimental populations of yeast.

We studied the evolution of the correlation between growth rate r and yield K in experimental lineages of the yeast Saccharomyces cerevisiae. First, we isolated a single clone every approximately 250 generations from each of eight populations selected in a glucose-limited medium for 5000 generations at approximately 6.6 population doublings per day (20 clones per line × 8 lines) and measured its growth rate and yield in a new, galactose-limited medium (with ∼1.

The yield of experimental yeast populations declines during selection.

The trade-off between growth rate and yield can limit population productivity. Here we tested for this life-history trade-off in replicate haploid and diploid populations of Saccharomyces cerevisiae propagated in glucose-limited medium in batch cultures for 5000 generations. The yield of single clones isolated from the haploid lineages, measured as both optical and population density at the end of a growth cycle, declined during selection and was negatively correlated with growth rate.

Prezygotic isolation between Saccharomyces cerevisiae and Saccharomyces paradoxus through differences in mating speed and germination timing.

Although prezygotic isolation between sympatric populations of closely related animal and plant species is well documented, far less is known about such evolutionary phenomena in sexual microbial species, as most are difficult to culture and manipulate. Using the molecular and genetic tools available for the unicellular fungus Saccharomyces cerevisiae, and applying them to S. paradoxus, we tested the behavior of individual cells from sympatric woodland populations of both species for evidence of prezygotic isolation.

Evolutionary genetics: desperate times call for more sex.

A new study has found that strains of the fungus Aspergillus nidulans produce more of their spores sexually in environments where they are less fit, resembling a hypothesized transitional stage in the evolution of sex.

Yeast sex: surprisingly high rates of outcrossing between asci.

Background: Saccharomyces yeasts are an important model system in many areas of biological research. Very little is known about their ecology and evolution in the wild, but interest in this natural history is growing. Extensive work with lab strains in the last century uncovered the Saccharomyces life cycle.

The role of sex in fungal evolution.

Sex in fungi is often associated with dispersal and dormancy, but in many species is not required for reproduction, and its evolutionary genetic role is unclear. Sex can accelerate adaptation to a new environment, and recombination in Saccharomyces cerevisiae, though historically rare, has had a prominent role in the origins of many sequenced strains, in particular the origin of clinical strains from domesticated ancestors. Sex and recombination have recently been discovered in several human pathogens that were long thought to be asexual, but so far there is no compelling evidence that it plays an important genetic role in their adaptation.

Evolutionary genetics: a piggyback ride to adaptation and diversity.

Competition between adaptive mutations in different asexual lineages limits the rate of adaptation. But additional adaptive mutations can occur in lineages that already have one, altering the dynamics of evolving asexual populations.

A short history of recombination in yeast.

Despite it being the darling of fungal genomics, we know little about either the ecology or reproductive biology of the budding yeast, Saccharomyces cerevisiae, in nature. A recent study by Ruderfer et al. estimated that the ancestors of three S.

Experimental evolution with yeast.

Many of the difficulties of studying evolution in action can be surmounted using populations of microorganisms, such as yeast. A readily manipulated sexual system and an increasingly sophisticated array of molecular and genomic tools uniquely qualify Saccharomyces cerevisiae as an experimental subject. This minireview briefly describes some recent contributions of yeast experiments to current understanding of the evolution of ploidy, sex, mutation, and speciation.

Evolutionary genetics: choosing to evolve.

The evolution of mate choice is believed to be important in speciation. A recent experiment involving mating preference evolution in laboratory yeast populations supports theoretical predictions that this can occur without complete genetic isolation between populations, strengthening the case that ecological specialization as well as physical separation can lead to speciation.

Antagonism between sexual and natural selection in experimental populations of Saccharomyces cerevisiae.

Trade-offs between life-history components are a central concept of evolution and ecology. Sexual and natural selection seem particularly apt to impose antagonistic selective pressures. When sex is not integrated into reproduction, as in Saccharomyces cerevisiae, natural selection can impair or even eliminate it.

Nuclear-mitochondrial epistasis for fitness in Saccharomyces cerevisiae.

In addition to the familiar possibility of epistasis between nuclear loci, interactions may evolve between the mitochondrial and nuclear genomes in eukaryotic cells. We looked for such interactions in Saccharomyces cerevisiae genotypes evolved independently and asexually in the laboratory for 2000 generations, and in an ecologically distinct pathogenic S. cerevisiae strain.

The effects of sex and mutation rate on adaptation in test tubes and to mouse hosts by Saccharomyces cerevisiae.

Some hypotheses for the evolution of sex focus on adaptation to changing or heterogeneous environments, but these hypotheses have rarely been tested. We tested for advantages of sex and of increased mutation rates in yeast strains in two contrasting environments: a standard and relatively homogeneous laboratory environment of minimal medium in test tubes, and the variable environment of a mouse brain experienced by pathogenic strains. Evolving populations were founded as equal mixtures of sexual and obligately asexual genotypes.

The number of mutations selected during adaptation in a laboratory population of Saccharomyces cerevisiae.

There is currently limited empirical and theoretical support for the prevailing view that adaptation typically results from the fixation of many mutations, each with small phenotypic effects. Recent theoretical work suggests that, on the contrary, most of the phenotypic change during an episode of adaptation can result from the selection of a few mutations with relatively large effects. I studied the genetics of adaptation by populations of budding yeast to a culture regime of daily hundredfold dilution and transfer in a glucose-limited minimal liquid medium.

Capturing the adaptive mutation in yeast.

An accurate view of adaptive mutations is essential to evolutionary genetics, but their rarity makes them difficult to study. This can be partially overcome using the many tools of yeast genetics and the ability to study very large populations over many generations. Adaptation to laboratory environments has occurred primarily by chromosomal rearrangements, often involving retrotransposons and apparently selected for their effects on gene regulation.

Experimental studies on ploidy evolution in yeast.

Variation in the prominence of haploidy and diploidy is a striking feature of eukaryote life cycles that has not been explained from an evolutionary point of view. the ease with which ploidy and other variables of population genetics may be manipulated in yeast make Saccharomyces cerevisiae an excellent subject for experiments on the fitness effects of ploidy. Several hypotheses have been advanced to explain the emphasis on diploidy in plants and animals, and yeast experiments have been particularly informative for a few.

An evolutionary advantage of haploidy in large yeast populations.

Although seed plants and multicellular animals are predominantly diploid, the prominence of diploidy varies greatly among eukaryote life cycles, and no general evolutionary advantage of diploidy has been demonstrated. By doubling the copy number of each gene, diploidy may increase the rate at which adaptive mutations are produced. However, models suggest that this does not necessarily accelerate adaptation by diploid populations.

Conflicting levels of selection in the accumulation of mitochondrial defects in Saccharomyces cerevisiae.

The somatic accumulation of defective mitochondria causes human degenerative syndromes, senescence in fungi, and male sterility in plants. These diverse phenomena may result from conflicts between natural selection at different levels of organization. Such conflicts are fundamental to the evolution of cooperating groups, from cells to populations.

HETEROMORPHISM FOR A HIGHLY REPEATED SEQUENCE IN THE NEW ZEALAND FROG LEIOPELMA HOCHSTETTERI.

A satellite DNA sequence, Lhl, was cloned from the New Zealand endemic frog Leiopelma hochstetteri. Large tandem arrays of Lh1 were localized by in situ hybridization to the long arm of a small telocentric autosome in some individuals, but these arrays were absent from other individuals. Lh1 is also present in varying amounts on some supernumerary chromosomes in some individuals.