Download “Sex enhances adaptation by unlinking beneficial from detrimental

“Sex enhances adaptation by unlinking beneficial from detrimental mutations in experimental yeast populations” By Group 8 The Principles of the Paper
The paper aims to explore and understand the advantages of sexual over asexual reproduction. In
theory asexual reproduction should be favoured by species as it allows them to produce twice as
many members of the next generation with the same amount of work. This is referred to as the
two fold cost of sex (figure 1).
Figure 1: A diagram illustrating the two fold cost of sex with (a) being the sexual species and (b) being the asexual
Phylogenetic evidence shows that asexual species tend to last a shorter time than asexual species,
suggesting there must be additional benefits to sex that outweigh the twofold cost of sex. The
main benefit of sexual reproduction includes recombination, thereby avoiding Muller’s ratchet the continuous accumulation of detrimental mutations leading to an overall reduction in the
population’s fitness due to the lack of recombination.​
​
Asexual reproduction is heavily affected by
Muller’s ratchet. Once an individual has obtained a mutation, it will be passed onto all its
offspring. Over time, the effect(s) will be magnified as more mutations accumulate across the
genome.​
​
This paper focuses specifically on the accumulation of beneficial mutation and removal
of detrimental mutations, by studying mutation rates in both asexual and sexual colonies of yeast,
in differing environments. Using permissive and stressful environments, they were able to
observe the rates of accumulation and removal of mutations by exploiting two different types of
selection, purifying and directional selection. They used the two environments to try to
manipulate the proportion of beneficial and detrimental mutations that arose by subjecting the
yeast to different levels of environmental stress.
The Hypotheses
The ​
Weissman hypothesis (1889)​
proposes that two lineages of one species can come together
and exchange different mutations via recombination. This event produces a new lineage, possibly
carrying the advantageous mutations of the parental lineages. This gives an explanation for the
better relative fitness of sexual species versus asexual ones.
The ​
ruby in the rubbish hypothesis​
states that if a single beneficial mutation is acquired amongst
several deleterious mutations, in sexual species the beneficial mutation may be separated from
the detrimental ones due to recombination, but this would not occur in asexual populations .
The ​
Muller’s ratchet hypothesis​
suggest that asexual species accumulate irreversible deleterious
mutations. This gives an explanation for sexual reproduction being favoured over asexual
reproduction.
The ​
Fisher-Muller hypothesis (1930)​
explains that sexual reproduction favours a rapid introduction
of beneficial mutations within the population , proposing that sexual reproduction works on the
basis that selection operates on beneficial mutations, whilst it has no effect on detrimental
mutations.
How the Data was Collected
In this experiment, 300 mitotic generations of yeast comprising of sexual, asexual, wild type, and
mutator populations were used. First, assays were taken from permissive and stressful
environments to observe the effects of removing the MSH2 gene. A two-way ANOVA exploring the
investigated factors revealed that mutation rate had no large effect on fitness (F(1,10)= 13.87, P=
0.0681), whereas sexual status (P=0.0016) had a sustained effect (P<0.03 after 225 generations),
and a significant interaction between factors existed (P=0.0206). How the Data was Interpreted
Permissive environment
Lack of fitness gain suggests few beneficial mutations arose; directional selection was weak. No
significant difference was found between sexual and asexual populations in this environment,
even under elevated mutation rate. Sex had no effect under conditions where directional selection
is weakly acting on fitness.
Stressful environment
Beneficial mutations were generally incorporated, and directional selection was operating. Effects
of detrimental mutations were greater in stressful environment, and were removed by sex,
making adaptation easier.
Conclusion
They found that the data did not support the Fisher-Muller hypothesis as their data suggested that
sex served no advantage to the yeast that was in a stable environment (the yeast was well
adapted to it). The researchers found that sex unlinks detrimental mutation, whilst simultaneously
combining beneficial mutations, and allowing selection to act on both types of mutations more
effectively than on the asexual population, as proposed by Weismann hypothesis.