Browsing by Subject "Multicellularity"

Now showing 1 - 3 of 3
  • Thumbnail Image
    Item
    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.
  • Thumbnail Image
    Item
    Constructing individuality: Ecological and evolutionary dynamics in experimental transitions to multicellularity
    (2016-12) Rebolleda-Gomez, Maria
    The evolution of multicellularity transformed the history of life on our planet. Multicellularity involves the reorganization of previously autonomous cells into a more complex organism, however the ecological and evolutionary consequences of this reorganization remain poorly understood. This work explores experimentally the implication of reorganization for environmental change, as well as the costs and benefits of this transition in novel environments (UV radiation, and spatial structure), using a simple experimental system that evolved in our laboratory as the result of selection for larger size. Ten replicate populations of the unicellular strain Saccharomyces cerevisiae evolved under selection for faster settling, and selection in all of these populations resulted in the evolution of incipient multicellular phenotypes. Here, I evaluate how and under what conditions does adaptation occurs at a multicellular vs. unicellular levels. I expected that multicellularity would facilitate adaptation to UV radiation because multiple cells provide layers of protection for the internal cells. However, I found that the costs of a larger size are pervasive and adaptation at a multicellular level requires strong selective pressures. This work highlights the importance of space and the organization of cells as a consequence of the evolution of multicellularity: I show here how this organization has the potential to create novel ecological opportunities, new challenges for adaptation (like strong local competition) and different cellular micro-environments providing the potential for cellular differentiation.
  • Thumbnail Image
    Item
    Experimental evolution of increased size and complexity in Anabaena variabilis
    (2014-05) Jacobsen, Kristin Alexa
    The evolution of multicellularity has occurred over 25 times in the history of life. Previously, we have shown the evolution of multicellular traits can readily be observed in laboratory populations across model unicellular organisms like yeast, chlamydomonas, and E. coli. Cyanobacteria are the oldest multicellular organisms, dating back 3.5 billion years. Many species appear morphologically unchanged, suggesting they have remained primitively multicellular. Are they incapable of evolving increased complexity? Model prokaryote Anabaena is a filamentous cyanobacteria, predating fossil records, existing as single strands or loose mats with three distinct cells types. Rapid settling was used to select for increased size advantage. Response to selection resulted in dramatic size increase; microscopic strands became inseparable macroscopic aggregates. Anabaena also became more complex; growth rate increased, two distinguishable morphologies developed, and growth and reproduction patterns changed. This shows that Anabaena, although primitively multicellular for billions of years, rapidly evolves increased size and complexity.