The Function of Plastic Reproductive Behaviour in Drosophila melanogaster

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Carazo Ferrandis, Pau
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Phenotypic plasticity is a widespread phenomenon across the tree of life, with far reaching consequences for ecological and evolutionary processes. In species facing strong sexual selection and marked variation in the socio-sexual context in which they reproduce, adaptive plasticity in reproductive behaviour is expected to evolve. In this thesis, we aimed to contribute to this area of evolutionary biology research using the model organism Drosophila melanogaster. The results of this thesis offer a twist to the steadily growing literature of ageing via sensory perception by showing that socio-sexual cues so far documented to accelerate ageing mostly bear fitness benefits in ecologically relevant scenarios. This suggests that plastic responses previously documented as maladaptive may in fact be by-products of widespread adaptive plasticity. First, we found that plastic responses induced by perception of female cues, previously reported to accelerate ageing, mostly yield lifetime fitness benefits by improving male reproductive success when competing against other males. Interestingly, these lifetime fitness benefits did not appear to be driven by clear short-term effects, but rather by an accumulation of benefits of small magnitude along the life of males. Using mathematical simulations parameterised with observed empirical data, we also show that variation in population mating rate across time and/or space can be a critical determinant for the evolution of male plastic responses to the presence of female cues, which suggests that such plasticity may have evolved in other iteroparous species with similar life-history patterns as Drosophila melanogaster. In addition, we show that, because fitness returns of plastic responses depend on male condition (so that high condition males mostly reap reproductive benefits whereas low condition males are more likely to suffer reproductive and actuarial costs), such plasticity can significantly strengthen sexual selection. Finally, we found that perception of dead conspecifics, also previously reported to accelerate ageing, actually triggers highly plastic responses that are rapidly induced and can be immediately reversed, and consist of females exposed to dead conspecifics repeatedly increasing their reproductive investment, both with respect to the quantity and quality of their offspring. We suggest that such modulation of reproductive investment represents an adaptive terminal investment strategy, whereby females shift their reproductive investment towards current reproduction as they perceive a threat to their survival. Taken together, these results suggest that Drosophila uses socio-sexual cues in order to display adaptive reproductive plasticity in response to demographic fluctuations that are common in the wild. The results from this thesis hopefully contribute towards a more comprehensive understanding of adaptive plastic responses in reproductive behaviour.
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