Dissertation
Genomic consequences of transitions to asexuality in the freshwater snail, Potamopyrgus antipodarum
University of Iowa
Doctor of Philosophy (PhD), University of Iowa
Autumn 2019
DOI: 10.17077/etd.005204
Abstract
What accounts for the predominance of sexual reproduction is a longstanding and still unresolved questions in evolutionary biology. The relatively high cost of sexual relative to asexual reproduction means that there must be profound benefits of sex. Theory holds that transitions to asexuality should have severe consequences on the efficacy of natural selection, in particular on the removal of deleterious mutations without meiotic recombination. What the genomic consequences of asexuality are is thus a critical component of understanding the maintenance of sex. Here I use the freshwater New Zealand snail, Potamopyrgus antipodarum to address how asexuality influences genome evolution. Specifically, for my dissertation research I used RNA-seq and whole-genome sequence data to evaluate 1) whether and to what degree sexuals and asexuals exhibit different patterns of gene expression 2) how reproductive mode (i.e. sex vs. asex) effects the accumulation of transposable elements (TEs) and 3) the effects of reproductive mode on the expansion of tandemly repeated ribosomal RNA genes (rDNA) and histones.
My results from gene expression comparisons of sexual vs. asexual lineages indicate that asexuals have more similar patterns of gene expression to closely related sexuals than other more distantly related asexuals, which is consistent with evidence of high population structure in P. antipodarum. This first look at gene expression in sexual vs. asexual P. antipodarum lays a groundwork for future investigations into the causes of asexuality. Using whole-genome sequence data from 26 (11 sexual and 15 asexual) lineages of P. antipodarum I tested the opposing hypotheses that transitions to asexuality will 1) lead to greater accumulation TEs relative to sexuals due to a reduced efficacy of selection to remove mildly deleterious TEs or 2) lead to a loss of TEs due to the reliance on sex to facilitate the spread of TEs in populations. While I found notable variation in TE loads across P. antipodarum, reproductive mode did not appear to be an influence on TE abundance. Instead, my results indicate that variation in TE family abundances is reflective of the recent evolutionary history of these sexual and asexual lineages. Because we know so little about how TEs evolve below the species level, my results are an important addition to improving our understanding of the influence of population structure and history in shaping TE variation. These results also demonstrate the need for large population-level genome sampling to detect potential effects of population structure and reproductive mode on TE evolution in P. antipodarum. In contrast to my findings of a likely influence of population structure on gene expression and TE abundance, I found a strong pattern of increased copy number for rDNA and histones – which I discovered are physically joined in Potamopyrgus – in asexual relative to sexual lineages. My results indicate that asexual P. antipodarum have repeatedly and rapidly accumulated greater amounts of this gene cluster, resulting in immense copy number variation across asexual lineages. The convergence on high rDNA-histone copy number in asexuals appears to be consequence of genome evolution under asexuality. This finding raises many new questions regarding genomic and phenotypic variation in asexual P. antipodarum as well as broader consequences of asexuality.
Details
- Title: Subtitle
- Genomic consequences of transitions to asexuality in the freshwater snail, Potamopyrgus antipodarum
- Creators
- Kyle E. McElroy
- Contributors
- Maurine Neiman (Advisor)John JM Logsdon (Committee Member)Bryant F McAllister (Committee Member)Douglas W Houston (Committee Member)Andrew Kitchen (Committee Member)
- Resource Type
- Dissertation
- Degree Awarded
- Doctor of Philosophy (PhD), University of Iowa
- Degree in
- Biology
- Date degree season
- Autumn 2019
- DOI
- 10.17077/etd.005204
- Publisher
- University of Iowa
- Number of pages
- xv, 219 pages
- Copyright
- Copyright 2019 Kyle E. McElroy
- Language
- English
- Description illustrations
- color illustrations
- Description bibliographic
- Includes bibliographical references (pages 216-219).
- Public Abstract (ETD)
Why sexual reproduction is so common despite its great costs relative to asexual (i.e. “clonal”) reproduction is a major question in evolutionary biology. Understanding how asexuality influences genome evolution is a crucial step towards an answer. I explored the genomic consequences of asexuality in the freshwater New Zealand snail Potamopyrgus antipodarum, which is characterized by populations of coexisting and otherwise similar sexual and asexual individuals, making this species an ideal natural system for testing hypotheses about the benefits of sex. I compared gene expression between sexuals and asexuals to identify genes associated with asexuality. Though I found little difference between sexual and asexual gene expression my results build on previous work showing that P. antipodarum gene expression variation reflects recent genetic history. I used whole-genome sequence data to ask whether sexual and asexual lineages differ in repetitive DNA burden, including but not limited to transposable elements (TEs), DNA sequences that move and replicate within genomes. While I found no evidence that reproductive mode has influenced TE abundance in P. antipodarum, I did find extensive intraspecific variation in TE load. This result demonstrates that understanding genome evolution requires a diverse and representative sample of individuals. I also uncovered a striking pattern of rapid and repeated increase in copy number for ribosomal RNA genes and histones in asexuals relative to sexuals. This finding sets the stage to address new and exciting questions about genomic variation in asexual P. antipodarum and in other asexual lineages.
- Academic Unit
- Biology
- Record Identifier
- 9983779698302771
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