Transposable elements in sexual and asexual animals
by Jens Bast
Date of Examination:2015-01-30
Date of issue:2015-01-15
Advisor:Dr. Marina Schäfer
Referee:Prof. Dr. Stefan Scheu
Referee:PD Dr. Mark Maraun
Persistent Address:
http://dx.doi.org/10.53846/goediss-4860
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Abstract
English
The prevalence of sexual reproduction among eukaryotes despite its marked disadvantages is one of the most elusive problems in evolutionary biology. The observation that most asexual eukaryotes are recent offshoots scattered throughout the eukaryotic tree of life has led to the common assumption that asexual species are bound for early extinction. Numerous ecological and genetic models seek to explain this pattern. One prominent set of models predict that mutation accumulation leads to the extinction of asexual lineages. Even so, for mutation accumulation to outweigh the cost of sex, these models require unrealistically high mutation rates. Potentially however, transposable elements (TEs) pose a threat for newly arisen asexual lineages. Transposable elements are replicating independently from the host's cell cycle and can induce deleterious effects on the host. Theory predicts that upon the switch to asexuality, TEs should accumulate because of inefficient purifying selection, potentially driving the lineage to extinction. However, interests of hosts and TEs should align in asexuals, leading to the evolution of less harmful TE dynamics over time. This thesis investigated these predictions by identifying TEs in a range of animal species with different lineage age and reproductive mode using complete genome data. Overall, there was no evidence for TE accumulation in independent lineages that recently switched to asexuality (Leptopilina and Daphnia). However, certain TE families (Gypsy) were more abundant in asexuals, which might reflect the early stages of TE increase in these asexual taxa. By contrast, species under prolonged asexuality, i.e. the apomictic bdelloid rotifer Adineta vaga and automictic oribatid mites, harbored only few and mostly inactive TEs. The purge of TEs from populations over time might require certain prerequisites, such as large population size and effective host defence. Moreover, this thesis argues that TE dynamics in recently arisen asexual lineages are also affected by the mechanism through which the transition to asexuality is achieved. The results of this thesis suggest that the early extinction of asexual populations is unlikely to be caused by an overall expansion of TE copies in their genomes, but potentially by certain TE types if not contained. Furthermore, TEs might be cleared from asexual species over time in at least some cases.
Keywords: reproductive mode; parthenogenesis; asexual reproduction; transposable elements; mutation accumulation; evolutionary genomics; cost of sex