Date of Degree
MS (Master of Science)
John M. Logsdon, Jr.
How asexual reproduction affects genome evolution, and how organisms that are ancestrally sexual alter their reproductive machinery upon becoming asexual are both central unanswered questions in evolutionary biology. While these questions have been addressed to some extent in organisms such as asexual clams, rotifers, ostracods, arthropods, and fungi, the most powerful and direct tests of how sex and its absence influence evolution requires direct comparisons between closely related and otherwise similar sexual and asexual taxa. Here, I quantify the rates and patterns of molecular evolution in the meiosis-specific genes Msh4, Msh5, and Spo11 in multiple sexual and asexual lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail. Because asexual P. antipodarum reproduce apomictically (without recombination), genes used only for meiosis should be under relaxed selection relative to meiosis-specific genes in sexual P. antipodarum, allowing me to directly study how asexuality affects the evolution of meiosis-specific genes. Contrary to expectations under relaxed selection, I found no evidence that these meiosis-specific genes are degrading in asexual P. antipodarum; instead they display molecular patterns consistent with purifying selection. The presence of intact meiosis-specific genes in asexual P. antipodarum hints that the asexuals may maintain the ability to perform meiosis despite reproducing apomictically. Asexual meiotic capability suggests that some meiotic components may persist or acquire a new role in these asexuals.
How asexual reproduction affects genome evolution, and how ancestrally sexual organisms alter their reproductive machinery upon becoming asexual are central unanswered questions in evolutionary biology. While these questions have been addressed to some extent in different asexual species, the most powerful tests of how sex and its absence influence evolution requires direct comparisons between closely related and otherwise similar sexual and asexual taxa. I address these questions by studying the evolution of genes critical to sexual reproduction in sexual and asexual lineages of Potamopyrgus antipodarum, a New Zealand freshwater snail. Because genes used only for sexual reproduction should have little or no functional relevance for asexuals, I can use this approach to study the effects of asexuality on the evolution of genes required only for sexual reproduction, which has implications for genome evolution in the absence of sex. I discovered that genes necessary for sexual reproduction show no evidence for being lost in asexual P. antipodarum. These results suggest asexuality may result in the loss of some, but not all, components of sexual reproductive machinery or that not enough time has passed for meiosis genes to be lost. Comparing evolutionary patterns between sexual and asexual organisms will help the scientific community understand the benefits and limitations of reproducing sexually. Understanding the consequences of sexual reproduction will illuminate fundamental biological and human health questions, ranging from the importance of biological diversity to the source and transmission of disease to dynamics between humans (who reproduce sexually) and parasites (which are often asexual).
publicabstract, Asexual reproduction, Evolution, Meiosis genes
vii, 45 pages
Includes bibliographical references (pages 39-45).
Copyright 2015 Christopher Steven Rice