Adaptation to environmental unpredictability in rotifers: an experimental evolution approach

Organisms living in changing environments might develop evolutionary responses in order to deal with environmental unpredictability. It is remarkable that selection due to environmental unpredictability can act on multiple traits of the life cycle to reduce the impact of environmental variability. Cyclically parthenogenetic rotifers inhabiting time varying water bodies are expected to develop evolutionary responses to cope with environmental unpredictability. In this thesis the first objective was to study the adaptive response of life-history traits related to diapause in the rotifer Brachionus plicatilis using an experimental evolution approach. This experimental approach has been recognised as a powerful tool to detect short-term evolutionary response under controlled conditions. Laboratory populations of B. plicatilis were subjected to two contrasting selective regimes (predictable vs unpredictable) during eight cycles of selection. These laboratory populations showed rapid adaptation to unpredictable environments, displaying a divergent response in those life-history traits. Populations subjected to the unpredictable selective regime showed both lower hatching fractions of diapausing eggs and earlier sex initiation, suggesting that bet hedging strategies underlie adaptation to environmental unpredictability in these organisms. The second objective was to elucidate the genomic basis of adaptation to environmental unpredictability of B. plicatilis populations, using genomic technologies. Genotyping by sequencing (GBS) was performed on clones from both selective regimes after the seventh cycle of selection and bioinformatics analyses also used GBS data from field populations. A total of 6,107 single nucleotide polymorphisms (SNPs) were identified and genotyped. Three SNPs strongly shifted their allele frequencies in response to environmental unpredictability being candidates to be under selection in these environments and five SNPs associated with the two life-history traits related to diapause were identified. The third objective was to explore the genetic expression in diapausing eggs produced by the same rotifer populations at the end of the evolution experiment, which were indeed subjected to conditions either promoting or blocking hatching. As a result of RNA-seq, a total of 3,068 differentially expressed genes were identified in the diapausing eggs. The comparative transcriptome analyses revealed that genes related with diapause maintenance and termination are differentially expressed under the two selective regimes. This study extends the knowledge of the complex molecular and cellular events that take place during diapause. In conclusion, this thesis supports that laboratory populations of B. plicatilis can develop rapid evolutionary responses to cope with environmental unpredictability and contributes to the empirical evidence of bet hedging