Caracterización del efecto de un mutante del bacteriófago Q8 en la adaptación a temperatura elevada

Abstract

RNA viruses rapidly adapt to environmental changes. They are unique in their evolutionary capacity, exhibiting high mutation rates, short generation times and large population sites, building heterogenous populations. All these features make these viruses optimal models in studies of experimental evolution. In this study we have analysed the evolutionary behaviour of bacteriophage Q8 populations when replication proceeds at above-optimal temperatures. The mutation A1088G has been identified in previous studies as a mutation that is only selected when the bacteriophage evolves at 43ºC. This mutation involves a change from aspartate to glycine at position 343 of the peptide chain of one of the viral proteins (A2), which could modify the infection cycle of the bacteriophage. In this work we attempt to clarify of the effect of this mutation on thermal adptation. To this end, we analyse the behaviour of RNA bacteriophague Q8 mutants harbouring the A1088G mutation (Q8A1088G) that co-propagate with two wild-type virus populations with differing degrees of heterogenity. The results obtained from the co-propagation experiments show that, although the mutation Q8A1088G provides some adaptative advantages compared to the clonal wild-type population. The competitive capacity of a viral population depends on the degree of heterogeneity characteristic of the population. This heterogeneity is not the only factor that improves viral fitness. Some mutations that emerge in the master sequences of the populations generated by propagating a clonal Q8A1088G population at 43ºC provide adaptative advantages that contribute to thermal adaptation. The chromatograms also show low-intensity polymorphisms corresponding to other mutations that, although they are not in sufficient proportion to form part of the master sequence of the new populations, can contribute to the increase in replicative capacity. The results obtained in this thesis show that the master sequence of a viral population does not represent its complex composition in terms of the mutant spectrum. This diversity has considerable influences on viral quasispecies evolution.