From environmental cue to phenotypic variation – a functional investigation of wing polyphenism in an emerging model species

Sammanfattning: Phenotypic plasticity is a ubiquitous feature of living organisms and enable individuals to adapt to changing environments. A particularly prominent example of plasticity is found in polyphenisms, where environmental cues received during development leads to the generation of discrete variation in populations. In this thesis, I have studied the mechanisms underlying wing polyphenism in the water strider Gerris buenoi in order to contribute to the understanding how environmental factors can act through growth regulatory systems to induce adaptive variation. Specifically, in Paper I, I investigated which environmental factors control wing morph determination in G. buenoi and found that this species most strongly responds to variation in photoperiod conditions, but also to crowding during the juvenile stages. Exposure to challenging nutritional conditions had no effect on wing morph frequencies. Further, I found that the nutrient sensitive insulin/insulin-like growth factor signaling pathway, which have been found to regulate wing polyphenism in species where nutrition is a determinant cue for wing morph induction, has no role in regulating G. buenoi wing polyphenism, an observation in line with data showing that wing morph determination is robust to variation in nutrient conditions. In Paper II, I explored a role for the developmentally important hormones ecdysone and juvenile hormone in G. buenoi wing polyphenism. Here, I used microinjections of 20-hydroxyecdysone and topical application of methoprene, as well as RNAi against hormone receptors for ecdysone and juvenile hormone. In these experiments, I found a small but significant effect of RNAi against the ecdysone receptor, indicating that ecdysone may play a role in wing morph induction. In Paper III, I used RNA sequencing to identify candidate growth regulatory pathways for wing morph induction by photoperiod and found a significant role for the conserved Fat/Hippo pathway in G. buenoi wing morph determination. Taken together, the results presented in this thesis suggest that evolution of genetic mechanisms underlying wing polyphenism may be constrained with regard to the particular environmental cue that is used to predict the future adaptive landscape. Further, the work presented in this thesis demonstrates the power in combining sequencing methods with functional genetic tools in order to more deeply characterize the causal basis to adaptive variation, an approach to ecological and evolutionary studies which I reviewed in Paper IV.