“If a bud be torn from the branch of a tree and cut out and planted in the earth . . . . it will grow, and become a plant in every respect like its parent. This evinces that every bud of a tree is an individual vegetable being; and that a tree therefore is a family or swarm of individual plants . . . . [and] the shoot is a succession of individual vegetable members.”
Erasmus Darwin (1800) in Phytologia, page 2.
Erasmus Darwin (1800) in Phytologia, page 2.
Scientific Inpiration |
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My interests involve investigating the gaps in our understanding of the evolution of clonality and inbreeding as it relates to the dead-end hypothesis. The dead-end hypothesis suggests that clonal and obligate selfing species are on a trajectory for exinction. Yet, clonality and obligate self fertilization is common in the plant kingdom. I am intrigued by the physical limitations of plants (i.e. sessile) because the evolutionary developments over time in plants is vast. How exactly does a plant solve complex problems when they are largely unable to move when selective pressures become too strong? How does a plant evolve complex traits when decisions in mating, avoidance of predation and resource allocation is limited compared to other organisms?
I think the answer is somatic mutation! My PhD research focuses on the effects of stress on the accumulation of mutations during vegetative growth. I am interested in the implications of mutations on subsequent generations and their potential to be heritable.
...But there are often more ways to break something than there are to fix it. Deleterious mutations are more common than beneficial ones but we cannot neglect that mutation is the source of variation. The question would then be, do we believe we have effectively assessed the potential for somatic mutations in plants? It is most likely that the majority of somatic mutations are overlooked due to selection eliminating. How exactly does a plant solve complex problems when they are largely unable to move when selective pressures become too strong?
Ultimately, I believe that plants are better at the evolution game than anticipated by previous research. I hope to focus my research career on the potential for adaptation and evolutionary mechanisms in plants.
Somatic Mutation Accumulation and Cell Lineage Selection |
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Mutation is the source of variation for evolution and adaptation, but organisms differ in the potential contribute to heritable variation. Contrary to most animals, the same cell lines responsible for vegetative growth are also responsible for gametes (pollen and ovules). Meristems (stem cells) are carried at the tips of plant stems. which promotes the accumulation of large numbers of somatic mutations that can be passed on to offspring in the next generation. However, there is a paradox because rates of mutation accumulation remain similar in plants and animals. The model of cell lineage selection (CLS) may address paradox and predicts the filtering of expressed deleterious somatic mutations and the retention of beneficial mutations. Cell lineage selection removes deleterious mutations and consequently shifts the distribution of mutation effects to allow a higher proportion of beneficial somatic variants to be transmitted to the next generation. I developed a novel application of the MuTec2 pipeline to evaluate somatic mutation accumulation in Mimulus guttatus. I found large numbers of low-frequency somatic variants, and smaller numbers of mutations at frequencies approaching 50%, which is consistent with the appearance of beneficial mutations driving a selective sweep of the meristem tissue. These phenotypic and genomic results from clonal propagation of Mimulus and our models of cell lineage selection support the hypothesis that plants have the potential for rapid adaptation to novel environments during vegetative growth.
J.A. Schwoch et. al. 2019 (in preparation)
J.A. Schwoch et. al. 2019 (in preparation)
Effects of Mutations on Their Progeny |
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I assessed the effects of somatic mutations can have on offspring fitness by inbreeding mutated clones in two ways. Autogamy (within flower fertilization) and geitonogamy (between flower fertilization of the same individual) will result in different displays of fitness. This is because if each stem can mutate independently of each other and carry unique mutations. The expected homozygosity will be higher in progeny that are the result of autogamy than geitonogamy. If accumulated somatic mutations are beneficial, progeny generated from autogamous crosses will have an increased fitness. I found that some stems display a prevalence of deleterious mutations while others produce offspring with increased fitness.
Cruzan, M.B., M.A. Streisfeld, and J.A. Schwoch. 2019.
Phenotypic effects of somatic mutation accumulation during vegetative growth. BioRXiv
(Link below)
Cruzan, M.B., M.A. Streisfeld, and J.A. Schwoch. 2019.
Phenotypic effects of somatic mutation accumulation during vegetative growth. BioRXiv
(Link below)