Limits...
Yeast sex: surprisingly high rates of outcrossing between asci.

Murphy HA, Zeyl CW - PLoS ONE (2010)

Bottom Line: Extensive work with lab strains in the last century uncovered the Saccharomyces life cycle.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.

View Article: PubMed Central - PubMed

Affiliation: Wake Forest University, Winston-Salem, North Carolina, United States of America.

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.

Show MeSH

Related in: MedlinePlus

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.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC2864747&req=5

pone-0010461-g002: 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.

Mentions: In individual ascus-to-ascus trials, we found high frequencies of outcrossing and no significant differences between intra- and inter-species trials (within S. cerevisiae- 40%, within S. paradoxus- 43%, interspecies- 42%; see Figure 2). In these trials, since there were 8 spores in total, there were four possible mating events. Our assay could only determine whether or not outcrossing occurred, not the number of times. Therefore, when we observed outcrossing, it meant that at least one and up to four matings were between asci. An observation of 40% of trials exhibiting outcrossing can be interpreted as anywhere from 10–40% of possible matings being between spores from different asci. Since we were unable to determine the actual number of matings, we used the most conservative estimate of each outcrossed colony (only one mating) in our statistical analyses. Using this assumption, the observed rate of outcrossing in the individual trials were S. cerevisiae- 10%, S. paradoxus- 11%, and interspecies- 10.5%.


Yeast sex: surprisingly high rates of outcrossing between asci.

Murphy HA, Zeyl CW - PLoS ONE (2010)

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.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2864747&req=5

pone-0010461-g002: 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.
Mentions: In individual ascus-to-ascus trials, we found high frequencies of outcrossing and no significant differences between intra- and inter-species trials (within S. cerevisiae- 40%, within S. paradoxus- 43%, interspecies- 42%; see Figure 2). In these trials, since there were 8 spores in total, there were four possible mating events. Our assay could only determine whether or not outcrossing occurred, not the number of times. Therefore, when we observed outcrossing, it meant that at least one and up to four matings were between asci. An observation of 40% of trials exhibiting outcrossing can be interpreted as anywhere from 10–40% of possible matings being between spores from different asci. Since we were unable to determine the actual number of matings, we used the most conservative estimate of each outcrossed colony (only one mating) in our statistical analyses. Using this assumption, the observed rate of outcrossing in the individual trials were S. cerevisiae- 10%, S. paradoxus- 11%, and interspecies- 10.5%.

Bottom Line: Extensive work with lab strains in the last century uncovered the Saccharomyces life cycle.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.

View Article: PubMed Central - PubMed

Affiliation: Wake Forest University, Winston-Salem, North Carolina, United States of America.

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.

Show MeSH
Related in: MedlinePlus