A proteomics study on establishment and functionality of chimeric protein complexes in hybrid yeast species and their effect on reproductive isolation

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Organisms belonging to different species usually cannot mate with each other. However, in nature, if two species are sufficiently closely related, they can sometimes breed producing viable, although infertile, hybrids. A common place example of this condition is the 'mule', a cross of a male donkey and a female horse, which is viable but sterile. This Project aims to study the protein interactions which occur in hybrid species, and sets out to understand whether the two genetic information present in such hybrids acts and function as separate units or if there is a natural interaction between them. If the two set of parental proteins indeed cross-talk, we would like to quantify the effect of such communication in term of adaptation and fitness of the hybrid (i.e. Are the characteristic mule's stubbornness and bad-temper due to complex interactions between the horse's and donkey's proteomes? Is it because of such interactions that mules are fitter and more vigorous than their parents?). I propose to tackle this problem using hybrids of yeast species belonging to the genus Saccharomyces, the mules of the fungal world. S. cerevisiae, S. paradoxus, S. mikatae and S. bayanus can mate and the resulting hybrid cells, although abler to divide by budding, are unable to produce viable germs cells, also called spores. The availability of the complete genetic blueprint for all these species and their genetic versatility provide a great opportunity for the scientist to study molecular evolution and the establishment of genetic isolation barriers in these organisms. At this time no experimental studies looking at the protein network interaction and its effect on fitness in hybrids between species is available. We propose to look at the establishment of protein complexes, involved in different cell functions in hybrid yeast cell. A molecular tag will be attached to genes expressing the subunit A of the chosen complexes. The tagged subunit with all the interacting partners can then be isolated and annotated using biochemical and chemical strategies. The interacting proteins identified could either all belong to one parental species (the one in which the gene has been tagged), or to both parental types. So, this approach will tell us whether members of protein complexes, are able to interact naturally with the partners derived from both yeast parental species. We also propose to force the assembly of chimeric protein complexes in yeast hybrids. This will allow us to study the fitness of the resulting hybrids in poor and rich nutrition conditions, and to assess the different degree of complementation ability of the homologous protein (i.e. is protein A of species 1 able to functionally substitute protein A' from species 2 in the hybrid 1,2 cell?). This project represents the first attempt to investigate the connections of homologous proteins, deriving from two species, that are present simultaneously in the hybrid cell, and their impact on fitness and post-mating barriers of the two species considered.