Yeast sex: surprisingly high rates of outcrossing between asci

Abstract

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. When nutrient limited, a diploid yeast cell will form four haploid spores encased in a protective outer layer called the ascus. Confinement within the ascus is thought to enforce mating between products of the same meiotic division, minimizing outcrossing in this stage of the life cycle.

Methodology/principal findings: Using a set of S. cerevisiae and S. paradoxus strains isolated from woodlands in North America, we set up trials in which pairs of asci were placed in contact with one another and allowed to germinate. We observed outcrossing in approximately 40% of the trials, and multiple outcrossing events in trials with three asci in contact with each other. When entire populations of densely crowded asci germinated, approximately 10-25% of the resulting colonies were outcrossed. There were differences between the species with S. cerevisiae having an increased tendency to outcross in mass mating conditions.

Conclusions/significance: Our results highlight the potential for random mating between spores in natural strains, even in the presence of asci. If this type of mating does occur in nature and it is between close relatives, then a great deal of mating behavior may be undetectable from genome sequences.

Figure 1. Saccharomyces life cycle.

A) When nutrient limited, diploid cells form four meiotic products, each surrounded by a spore wall, connected via bridges and all encased in one ascus . When conditions permit, spores germinate and bud or mate. Mating occurs between opposite mating types (a and α) and is either inbreeding (B–C) or outcrossing (D–E). B) Mating-type switching: after budding once, a haploid cell can switch its mating type through a highly coordinated gene conversion event , then mate with its daughter cell. C) Intratetrad mating: spores mate within an ascus. D) Intertetrad mating: spores from different asci mate. E) Free spores mate with one another at random.

Figure 2. Outcrossing in Saccharomyces yeasts.

Outcrossing frequency in individual ascus-to-ascus trials (I) and mass mating assays of dense cultures (M); black- assays with heterothallic strains; gray- assays with homothallic strains. Bars represent the overall average; diamonds represent the outcrossing frequency for a given strain combination. For the individual ascus-to-ascus trials, on average, 37 pairs of asci were assayed for each strain combination (one data point on graph); the conservative estimate, which assumes one mating per outcrossed colony, is plotted. In the mass mating assay, for each strain combination, one plate densely covered in asci was allowed to germinate and grow. This plate was sampled and an average of ∼300 resulting colonies were assayed to estimate the outcrossing frequency.

Figure 3. Outcrossing in individual three-way trials.

In each trial, three asci, each with a different antibiotic resistance, were placed in contact with one another. After germination and growth, outcrossing was assayed by replica plating to plates with different combinations of antibiotics. In total, 632 trials were conducted. The graph shows the proportion of those trials that contained the number of outcrossing events listed on the x-axis.

Figure 4. Photos of outcrossing between asci.

A–H: Representative photos of sporulated yeast responding to growth medium in a mass-mating assay. Arrows indicate instances of outcrossing. A) Sporulated yeast (two asci); B) ascus with germinating spores; C) four vegetative cells, previously spores from one ascus, two are mating; D–E) mating between germinating spores from two different asci; F–H) outcrossing and mating in a mass of germinating spores and asci.