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Abstract:
Hybridization occurs when individuals of divergent populations or individuals of different species mate and produce viable hybrid offspring. Hybridization is an important factor in the evolutionary history of several populations, because it brings together alleles that have not been exposed to selection. Thus, hybrid adaptation to ancestral or new environmental conditions depends on the background and interaction of the parental genotypes. I am particularly interested in the performance of the first hybrid generation (F1 hybrids), especially when hybrids are viable and able to outperform one or both parents under different environmental conditions, a phenomenon known as heterosis. The aim of my thesis was to understand and identify mechanisms underlying heterosis. I used Saccharomyces yeasts as a model system due to their laboratory practicality, their ability to form viable hybrids and reliable fitness measurements. First, I competed a range of different F1 hybrids with their wild or domesticated parental populations; I identified prevalent heterosis for crosses between domesticated and wild populations of different yeast species but not for crosses between wild populations of the same yeast species. Thus, the environment from where parental strains were isolated seems to affect heterosis, and F1 hybrids with a domesticated background display more extensive heterosis. Domesticated yeasts are characterized by being highly heterozygous, which can potentially mask recessive deleterious alleles in the genome. When yeasts strains are brought to the laboratory they undergo an extreme form of inbreeding that induces a haploid spore to grow vegetatively, and allows mate-type switching followed by mating within the same haploid colony. This inbreeding process creates a complete homozygous monosporic clone with recessive deleterious alleles exposed. By crossing two domesticated monosporic clones derived from divergent populations several recessive deleterious alleles might be complemented and the F1 hybrid would have an advantage in comparison to its monosporic parents. Using monosporic clones as parental strains in heterosis studies may inflate heterosis measurements due to parental disadvantage and not the F1 hybrid advantage. Thus, I compared asexual fitness or growth of heterozygous yeast isolates with homozygous monosporic clones for both domesticated and wild yeast populations; I found that the monosporic cloning might explain some, but not all, of the heterosis seen, potentially accounting for the difference in heterosis between domesticated and wild yeast strains. Thus, heterosis is not solely explained by complementation of recessive deleterious alleles, and other mechanisms might affect the F1 hybrid advantage. I focus on heterosis at the transcriptome level and analyzed the transcription of a representative heterotic F1 hybrid relative to its parents in environments that favored one or the other parent. Hybrid transcription was varied and resembled the fitter parent in specific environments. Thus, at the transcriptome level, the F1 hybrid may repress potential deleterious alleles, making them recessive, and induce more advantageous alleles, making them dominant, by differentially transcribing its parental alleles. For the first time to our knowledge, multigenic heterosis at a transcriptome level was identified, which render the F1 hybrid better adapted than its parents to different environmental conditions. Heterosis studies in Saccharomyces yeasts, due to their simplicity, can evidence characteristics with an impact on heterosis while also tracing the evolutionary history of divergent populations. These types of studies have interesting applications agriculture sector where hybridization has been used for centuries to make higher yield crops and bigger cattle to fulfil human consumption needs.