The widespread occurrence among eukaryotes of sex and of mobile DNA sequences requires an evolutionary explanation, since both appear to reduce individual fitness. Both phenomena have been hypothesized to provide fitness advantages to populations, but such explanations require rather than explain the initial establishment of mobile elements and genes for sex. Genes encoding sexuality may invade asexual populations as molecular parasites, whose success then allows mobile elements to spread as parasites of sexual genomes. The prediction that mobile elements can invade only sexual populations was tested using isogenic sexual and asexual populations of Saccharomyces cerevisiae and the retrotransposon Ty3. Active Ty3 elements more consistently invaded sexual than asexual populations. In subsequent experiments involving selection on media containing ethanol as a carbon source or $ beta$-glycerophosphate as a limiting phosphorus source, transposition by galactose-induced Ty3 elements produced none of the mutations involved in adaptation to these media, and conferred no adaptive advantage among competing populations. The mean copy numbers of two mobile elements were unchanged by long-term sexual or asexual propagation of Chlamydomonas reinhardtii populations, because transposition by these elements occurred very rarely or had no effect on fitness. Sexual and asexual S. cerevisiae populations did not differ in their adaptation to galactose media, but sexual populations maintained on glucose had higher growth rates on both media than did asexual populations maintained on glucose, implying that selection against deleterious mutations was more effective in sexual populations.
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:QMM.40475 |
| Date | January 1996 |
| Creators | Zeyl, Clifford. |
| Contributors | Bell, Graham (advisor), Green, David (advisor) |
| Publisher | McGill University |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Format | application/pdf |
| Coverage | Doctor of Philosophy (Department of Biology.) |
| Rights | All items in eScholarship@McGill are protected by copyright with all rights reserved unless otherwise indicated. |
| Relation | alephsysno: 001537185, proquestno: NN19789, Theses scanned by UMI/ProQuest. |
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