Rotifer adaptation to environmental unpredictability
- Autores: Lluis Franch
- Directores de la Tesis: Manuel Serra (dir. tes.), M. Jose Carmona Navarro (codir. tes.)
- Lectura: En la Universitat de València ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: María Africa Gómez Gómez (presid.), J Francesc Mezquita Juanes (secret.), Luisa Orsini (voc.)
- Programa de doctorado: Programa Oficial de Doctorado en Biodiversidad
- Materias:
- Enlaces
- Tesis en acceso abierto en: TESEO
- Resumen
Environmental unpredictability is a phenomenon that occurs in any habitat, and is an important selection pressure for the evolution of adaptive responses in the organisms. Interestingly, the ponds located in the Mediterranean region are characterized by having a high degree of seasonality and uncertainty at various temporal scales. Unpredictability in natural populations can act on several organism traits, especially in those species with complex life histories. The rotifer Brachionus plicatilis is a zooplankton species that frequently inhabits the salt ponds of the Mediterranean region. It has a type of reproduction in which proliferation by parthenogenesis (asexual phase) combines with occasional bouts of male production and sexual reproduction; the latter resulting in diapausing egg production (sexual phase). This makes B. plicatilis populations inhabiting Mediterranean ponds a good model system to study the adaptation to environmental unpredictability.
In this thesis the first objective was to quantify the degree of environmental unpredictability of the studied Mediterranean ponds. In order to do that the variation in water-surface area during 27 years of a group of twenty saline water bodies was obtained from the scenes from the satellites Landsat 5 and 7. Different models for predictability estimation were developed here by considering how the variation in water-surface area could be relevant for the focus organism. The group of Mediterranean ponds studied were found to have a wide range of predictability values. A posteriori classification of the models for predictability estimation showed that some assumptions had negligible effects, while others can be associated with the species assemblages for which predictability needs to be assessed.
The second objective was to study variation in diapause-related life-history traits in B. plicatilis. To do this, nine populations from a set of Mediterranean saline ponds were studied and showed significant levels of within-population genetic variation for the following life-history traits: propensity for sex and for hatching fraction of diapausing eggs. The propensity for sex in rotifer populations, and hence the early investment in diapause, decreased with environmental predictability. This suggests a conservative, bet-hedging strategy. Diapausing egg hatching fractions showed intermediate values (from 44 to 88%) in all the studied populations, but this trait was not related to the level of environmental predictability. Rotifer populations are able to locally diverge in diapause-related traits within a small geographical range (240 km2) despite their potential for widespread genetic exchange through the passive dispersal of diapausing eggs.
The third objective was to unveil, at a genome wide scale, genotypes correlated to adaptation to local environmental unpredictability. The B. plicatilis genome was assembled in this thesis and its structural annotation yielded 54,725 predicted genes. Functions were tentatively assigned to 30% of them. Genotyping by sequencing (GBS), and the subsequent bioinformatics analyses in the 30 clones for each of the nine saline ponds studied provided a large number (4,543) of high quality single nucleotide polymorphisms (SNPs). A number of SNPs —most of them located within genes– showed higher between-population differentiation than expected by chance and were correlated with life-history traits and environmental parameters, so that they are candidates for diversifying selection for local adaptation. Unexpectedly, a large set of SNPs, more than half of them located within genes were found to present signatures of balancing/purifying selection in B. plicatilis. A number of genes were identified as strong candidates to be part of the genomic basis of local adaptation to fluctuating environments and constitute a database for future studies.
Overall this thesis supports the expectation that wild populations of B. plicatilis can develop evolutionary responses to face environmental unpredictability and contribute to the empirical evidence of bet hedging strategies.
