Analyzing regions in the genome with low levels of recombination helps understand the prevalence of sexual reproduction. Here, I show that variability in regions of reduced recombination in Drosophila can be explained by interference among strongly deleterious mutations; selection becomes progressively less effective in influencing the behaviour of neighbouring sites as the number of closely linked sites on a chromosome increases. I also show that the accumulation of loss-of-function mutations on the neo-Y chromosome of Drosophila miranda is compatible with a model of selection against such mutations alone, without the need to invoke the action of selective sweeps. I describe the discovery of two new sex-linked genes in the plant Silene latifolia, SlCyt and SlX9/SlY9. SlCyt has been recently translocated from an autosome to the X and shows signs of a selective sweep. Its possible role in having caused recombination arrest between the evolving X and Y chromosome is discussed. SlX9 still has an intact Y-linked copy that is presumably functional. Nucleotide diversity at SlY9 is very low, whereas SlX9 has an unusually high diversity and shows signs of introgression from S. dioica into S. latifolia, but the effect of this seems very localized.

Orientador: Diogo Cavalcanti Cabral-de-Mello / Resumo: Uma característica comum da evolução dos cromossomos sexuais é a acumulação de DNAs repetitivos, que estão envolvidos em sua diversificação e degeneração. Em gafanhotos o sistema ancestral dos cromossomos sexuais é o XO, mas em algumas espécies os cromossomos sexuais neo-XY surgiram por translocação Robertsonian entre o X ancestral e um par autossômico. Esse é o caso do Ronderosia bergii (Acrididae: Melanoplinae) que adicionalmente também apresentou uma grande inversão pericentrica no neo-Y, fazendo com que essa seja uma espécie singular para o entendimento da evolução dos cromossomos sexuais. Nesse estudo foi caracterizado os DNA satélites e Elementos de Transposição da espécie, com enfoque no entendimento das diferenças entre macho e fêmea e que estão putativamente associados aos cromossomos sexuais. Foi encontrado um total de 54 famílias de DNAs satélites e 56 famílias de TEs. Os DNAsat foram 13,5% mais abundantes no genoma do macho, enquanto os TEs foram apenas 1,02% mais abundantes no genoma da fêmea, evidenciando alta amplificação dos DNAsat no cromossomo neo-Y e menor envolvimento dos TEs na diferenciação dos cromossomos sexuais. A alta diferenciação entre os cromossomos sexuais é observada pela amplificação de múltiplos DNAsat no neo-Y com ocorrência de famílias exclusivamente mapeadas nesse cromossomo, entretanto, algum grau de homologia foi observado. Os dados do mapeamento dos DNAsat evidenciou um alto turnover dos neo-cromossomos sexuais em R.bergii em nível intra... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: A common characteristic along sex chromosome evolution is the accumulation of repetitive DNAs, which could account for their diversification and degeneration. In grasshoppers the ancestral sex chromosome system is XO, but in some species neo-XY sex chromosome emerged by Robertsonian translocation with an autosomal pair. This is the case of the Melanoplinae Ronderosia bergii that additionally present a large pericentric inversion in the neo-Y, making this a singular species to understand evolution of sex chromosomes. Here we characterized the satellite DNAs and Transposable Elements of the species focused in the understanding of differences between male and female genomes that are putatively associated with sex chromosomes. We found a total of 54 satDNA families and 56 families of TEs. The satDNAs were 13.5% more abundant in male genomes, while TEs were only about 1.02% more abundant in female genome, evidencing high amplification of satDNAs on neo-Y chromosome and minor role of TEs in sex chromosome differentiation. The high differentiation between the sex chromosomes is demonstrated by amplification of multiply satDNAs in neo-Y and occurrence of families exclusively mapped on this chromosome, although some degree of homology was noticed. Our data of chromosomal mapping of satDNAs evidenced high turnover of neo-sex chromosomes in R. bergii at intrapopulation level, caused by multiply paracentric inversions and other rearrangements like amplifications and transpositions. Final... (Complete abstract click electronic access below) / Mestre