Genome evolution under asexuality
Final Report Abstract
Why sex involving high rates of outcrossing and recombination is the most prevalent form of reproduction instead of direct asexual reproduction without the costs of males still remains an open question. Within the scope of this funding period, I and coauthors investigated genomic consequences of asexual reproduction in natural populations by comparing genomes and transcriptomes of evolutionary replicates of sexual and asexual Timema species, as well as oribatid mites. We found that asexuality leads to less effective purifying selection, arrested GC-biased gene conversion, more segregating deleterious variants and lower levels of polymorphism in the asexual compared with sexual Timema sister species. This suggests, that theoretically predicted consequences of asexuality can indeed be found in nature. Moreover, contrary to other species with mostly single lineages to compare (e.g. rotifes and root-knot nematodes), we found no evidence for increased amounts of horizontally transferred genes or increased heterozygosity in the genomes of asexual Timema stick insects. Consistent with empirical evidence from other species, Timema transposable element (TE) load was similar between sister species, indicating no mutational meltdown through TE activity. As genome data generation took more time than expected, I extended the analyses to old asexual species (oribatid mites), where we found more effective selection in the asexuals compared to sexuals, contrary to all documented analyses of purifying selection in other organisms. It remains to be tested, if this is because of effective repair mechanisms and/or very large population sizes of asexual oribatid mites compared with sexuals. To disentangle other factors from the reproductive mode effect on TE dynamics, I inferred TE load through time in replicate sexual and asexual yeast strains originating from the same ancestor. I found that asexuality drives the reduction of TE load. We currently construct TE dynamic models parameterized with experimental data to check if the decline is consistent with theory. Moreover, we will follow up on RNA plasticity in Timema asexuals. My work on Timema was featured in Nature Research Highlights (https://www.nature.com/articles/d41586-018-04778-x), and the work together with the supervised PhD student Alexander Brandt was featured in press releases e.g. in the ‘Göttinger Tageblatt’ and radio coverage on DFunk Nova (http://www.goettinger-tageblatt.de/Campus/Goettingen/Hornmilben-Forschung).
Publications
- (2018) Consequences of Asexuality in Natural Populations: Insights from Stick Insects. Molecular biology and evolution 35 (7) 1668–1677
Bast, Jens; Parker, Darren J.; Dumas, Zoé; Jalvingh, Kirsten M.; Tran Van, Patrick; Jaron, Kamil S.; Figuet, Emeric; Brandt, Alexander; Galtier, Nicolas; Schwander, Tanja
(See online at https://doi.org/10.1093/molbev/msy058) - 2016. No accumulation of transposable elements in asexual arthropods. Mol. Biol. Evol. 33:697–706
Bast J, Schaefer I, Schwander T, Maraun M, Scheu S, Kraaijeveld K
(See online at https://doi.org/10.1093/molbev/msv261) - 2016. The Evolution and Maintenance of Sex. In: Encyclopedia of Evolutionary Biology. Oxford: Academic Press. p. 89–97
Jalvingh K, Bast J, Schwander T
- 2017. Effective purifying selection in ancient asexual oribatid mites. Nat. Commun. 8:873
Brandt A, Schaefer I, Glanz J, Schwander T, Maraun M, Scheu S, Bast J
(See online at https://doi.org/10.1038/s41467-017-01002-8)
