Testing the Major Theories Concerning the Evolution of Sex using Experimental Evolution
The evolution of sexual reproduction has been dubbed "the queen of problems in evolutionary biology". While there is a large amount of theory about the evolution of sex, there is relatively little well controlled empirical data. This thesis is an attempt to use an experimental evolution based approach in order to provide empirical insights into two different areas in the evolution of sex: the relative effects of beneficial and detrimental mutations on sex, and the effect of migration and gene flow on adaptation to new environments. In order to undertake this, a system comprising [i.e. comprised] of two Saccharomyces cerevisiae strains isogenic except for the ability to undergo sex was used. Theories about the evolution of sex can be broadly grouped into those which provide advantages for accumulating beneficial and clearing detrimental mutations. An empirical determination of their relative effects is useful in determining which theories are likely to be important to the evolution and maintenance of sexual reproduction. The first experiment consisted of propagating the sexual and asexual S. cerevisiae lines, as well as lines with increased mutation rates under both directional and stabilising selection for approximately 300 generations. The sexual lines increased in fitness significantly more quickly than the asexual lines under directional selection, regardless of mutation rate. In contrast, no lines, regardless of sexual status or mutation rate decreased or increased in fitness over the course of the experiment under stabilising selection, indicating that standard asexual selection was adequate to remove the vast majority of detrimental mutations. Thus in this experiment, sex is of much greater importance in accumulating beneficial mutations than in clearing detrimental mutations. The second experiment attempted to understand the effects of sexual reproduction and gene flow on adaptation. Predictions state that sexual organisms will be at a disadvantage to asexuals when adapting to multiple niches with migration between them, as maladapted hybrids are formed when mating occurs outside niches. To test this, sexual and asexual S. cerevisiae were adapted for approximately 350 generations to two differing environments, with varying rates of migration between them. In contrast to predictions, sexual lines showed higher adaptation to the new environments than asexuals irrespective of migration rate. The cause of this was investigated, and found to be caused by a loss of the trade off between the two environments in those treatments with high migration rates. This is interpreted as selection for generalists occurring in those lines which experience both environments. In summary, this thesis uses a powerful S. cerevisiae system in order to gain empirical insights into the evolution of sexual reproduction. The experiments further previous work with the same system into more complex and realistic scenarios, and provide novel empirical insights into the understanding of the evolution and maintenance of sex
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