Most animals and plants are sexual – they have a stage in their life-cycle at which genes from different individuals are recombined to produce genetically mixed progeny. Numerous evolutionary theories have been proposed for why sex is so prevalent, and these theories have important consequences for understanding many other fields, including speciation and molecular evolution. We study one notable and intriguing counter-example – the bdelloid rotifers – as a case study for gaining deeper understanding of the evolutionary causes and consequences of sex.
Our work has focused on whether asexual organisms diversify into species. It has been argued that ‘species’ are a feature of sexual organisms and that asexuals should not diversify into species. However, we showed that bdelloids have diversified into independently evolving species that experience divergent selection on feeding morphology in distinct habitats. The pattern of genetic diversity differs, however, from genetic diversity in monogonont rotifers (which have a sexual stage in their life-cycle). We have also found molecular evidence of inefficient natural selection in bdelloids, as predicted by evolutionary theory, and demonstrated the role of broad-scale dispersal in explaining bdelloid diversity patterns.
We are now seeking to understand how bdelloids interact with other organisms, such as parasites and symbionts, and the genetic mechanisms of adaptation to different environments. With Chiara Boschetti, Alan Tunnacliffe and colleagues at the University of Cambridge, we found that up to 10% of expressed genes in bdelloid rotifers come by horizontal transfer from bacteria, protists, plants and fungi. We published 3 new bdelloid rotifer genomes and confirmed that high prevalence of non-metazoan DNA is the most unusual feature (and not an artefact of contamination as found in some other cases).
Scanning electron micrographs of species of Rotaria and their jaws (trophi, not drawn to scale) by Diego Fontaneto
Media coverage and commentaries
The Times, ‘Creature that has not had sex for 100m years’
The Metro, ‘Not tonight, love, or for 40m years’.
Hillis, D.M. 2007. Asexual evolution: can species exist without sex? Current Biology. 17:R543-R544
Wilson, C.G., Nowell, R.W., Barraclough, T.G. 2018. Cross-contamination explains “inter and intraspecific horizontal genetic transfers” between asexual bdelloids rotifers. Current Biology 28:2436. (…but not the acquisition of non-metazoan DNA)
Nowell, R.W., Almeida, P., Wilson, C.G., Smith, T.P., Fontaneto, D., Crisp, A., Micklem, G., Tunnaclife, A., Boschetti, C., Barraclough, T.G. 2018. Comparative genomics of bdelloid rotifers: Insights from desiccating and nondesiccating species. PLoS Biology 16: e2004830
Eyres, I., Bochetti, C., Crisp, A., Fontaneto, D., Tunnacliffe, A. and Barraclough. T.G. 2015. Horizontal gene transfer in bdelloid rotifers is ancient, ongoing and more frequent in species from desiccating habitats. BMC Biology. 13:90 DOI 10.1186/s12915-015-0202-9
Tang, C.Q., Obertegger, U., Fontaneto, D., Barraclough T.G. 2014. Sexual species are separated by larger genetic gaps than asexual species in rotifers. Evolution. 68: 2901-2916
Boschetti C, Carr A, Crisp A, Eyres I, Wang-Koh Y, Lubzens E, Barraclough TG, Micklem G and Tunncliffe A. 2012. Biochemical diversification through foreign gene expression in bdelloid rotifers. PLoS Genet 8: e1003035 2.
Eyres, I., Frangedakis, E., Fontaneto, D., Herniou, E.A., Boschetti, C., Carr, A., Micklem, G., Tunnacliffe, A., Barraclough, T.G. 2012. Multiple functionally divergent and conserved copies of alpha tubulin in bdelloid rotifers. BMC Evol Biol 12:148
Fontaneto, D, Barraclough, T.G , Chen, K., Ricci, C., Herniou, E.A. 2008. Molecular evidence for broad-scale distributions in bdelloid rotifers: everything is not everywhere but most things are very widespread.Mol Ecol, 17: 3136 – 3146
Barraclough, T.G., Fontaneto, D., Ricci, C. and Herniou, E.A. 2007. Evidence for inefficient selection against deleterious mutations in cytochrome oxidase I of asexual bdelloid rotifers. Mol Biol Evol. 24:1952-1962.
Fontaneto, D., Herniou, E.A., Boschetti, C., Caprioli, M. Melone, G., Ricci, C. and Barraclough, T.G. 2007. Independently evolving species in asexual bdelloid rotifers. PLoS Biology. 5:e87