Browsing by Subject "Experimental evolution"

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    Comparing the effects of sexual and asexual reproduction during long term adaptation of Baker's yeast Saccharomyces cerevisiae
    (2013-12) Chang, Yao-chung
    If the ancestors of extant organisms were well adapted to a particular environment and that environment reoccurs, the ability to recapture prior adaptations could increase the efficacy of subsequent evolution. The ability to reacquire ancestral adaptation is defined as reversible evolution. Sexual reproduction may facilitate reversibility, as it maintains and increases the genetic variation of populations more efficiently than asexual reproduction. Recombination is likely to be important if multiple genes affect the selected trait. I chose to examine the reversibility of cell size adaptations since cell size regulation in yeast involves complex gene interactions. To quantify the effect of reproduction on the reversibility of yeast cell size, I employed a long-term (1300 generations) selection experiment. In the first 500 generations (Chapter 1), adaptation to a stable and benign abiotic environment occurred for all populations. Sexual reproduction was beneficial for such adaptation. During the next 400 generations (Chapter 2), populations under selection for small cell size had evolved delayed reproduction with the budding of daughter cell timed to occur immediately prior to the selection event. In Chapter 3, following selection for larger cells, the rapid reversibility of delayed reproduction occurred within 100 generations. In chapter 4, the development of multicellular yeast clusters was observed after continued selection (300 generations) for larger single cells. Prior reproductive history played a prominent role in influencing the evolution of multicellularity. Previously asexual populations were substantially larger (44-70%) than previously sexual ones.