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Chapter 30 : Why Bother with Sex? Answers from Experiments with Yeast and Other Organisms
Category: Fungi and Fungal Pathogenesis
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This chapter concentrates on those experiments that employ fungi such as Saccharomyces cerevisiae. S. cerevisiae has the attractive microbial experimental properties mentioned earlier and further is extremely well genetically characterized, and may be genetically manipulated and dissected. These facts mean that S. cerevisiae is often the organism of choice for many researchers interested in population genetics and evolution. The spo11 spo13 system was developed in the S. cerevisiae Y55 background. Starting with a prototrophic haploid which was ho (unable to switch mating type), the URA3 gene was first precisely deleted using PCR-based gene targeting. Competition experiments were thus conducted, under a variety of conditions, to discern the relative fitness of the wild-type and asexual sporulator. Overall the spo11 spo13 system seems ideally suited to applications concerning the maintenance of sex. This system has no effect on mitotic fitness or sporulation rates (at least in the Y55 genetic background) and can hold all factors constant apart from recombination, random assortment, and syngamy. The rapid and consistent increase in fitness in the harsh environment points toward the more efficient incorporation of beneficial mutations as the most likely candidate for sex’s benefit.
Each individual in this simulation contains two homologous chromosomes, each with 10 loci with an initial value of one. Each population comprises 10,000 individuals. An individual’s fitness is simply the sum of values at each locus, and so there are no dominant/recessive or epistatic effects. All individuals undergo mitotic divisions with random mutation. Every generation, each allele has a 1% chance of mutating—if an allele is chosen for mutation, the value mutated to is drawn randomly from a normal distribution with a mean of 0.01 and variance of 0.1. Thus, most mutations are detrimental but beneficial ones do occur, with larger beneficial mutations being increasingly rare. Selection, which is simply reproduction weighted by fitness, occurs after mutation. The sexual populations have the option of also reproducing via a meiotic division with recombination, which produces two haploid gametes. Recombination occurs at five random points along the chromosomes. The haploid gametes then mate at random to reconstitute diploids once again. There is no timing cost to reproducing sexually. Both sexual and asexual populations were propagated for 700 mitotic generations, and sex was imposed every 25 mitotic generations in the sexual line. The mean and standard error of fitness were recorded for both populations and are plotted (black, sexual; gray, asexual). Replicate runs produced very similar results.
A generalized life cycle of S. cerevisiae. A diploid cell will divide mitotically when provided with sufficient nutrients and produce cells that, barring de novo mutation, are genetically identical to the mother (top left). If starved, a diploid that is heterozygous at the mating-type locus (MAT) will begin a single meiotic division. Recombination occurs after DNA duplication. The chromosomes are then randomly assorted among four haploid spores. The spores form a distinctive tetrad structure and are encapsulated within an ascus. Each spore is one of two mating types (a or α), which is dictated by the idiomorph at the MAT locus. These spores germinate when nutrients are encountered again, and will mate with cells of the opposite mating type to reconstitute a diploid (bottom left).
An overview of the construction protocol for the spo11 spo13 system used by Goddard et al. ( 25 ). Solid boxes represent the genotypes of strains constructed at each step. Genes in uppercase denote wild-type alleles, while those in lowercase represent mutants, with the Greek delta symbol indicating a precise deletion of the gene. The status of only the mating type (either a or α), mating type switching (HO), a component of uracil anabolism (URA3), SPO11, and SPO13 loci are shown, and these strains are assumed to be of the wild type at all other loci. A detailed explanation may be found in the text.
The change in natural logarithm of fitness of asexual and sexual populations of yeast in benign and harsh environments. Points represent fitness measurements for individual populations with twice log-likelihood error bars (these approximate 95% confidence limits); the error bars for the benign treatment are plotted but are mostly too small to be discriminated. The fitted model for the harsh environment is plotted for asexual (—) and sexual (•) treatments (parameters: a 1 = 0.761, a 2(asexual) = −5.287, a 2(sexual) = −4.901). Asexual strains in the benign environment are represented by a square (▪); sexual strains in a benign environment are represented by a diamond (♦). Reproduced from reference 25 with slight modification.
Relevant studies that have directly tested hypotheses concerning the maintenance of sex a