title: Evolution and plasticity in geothermal three-spined sticklebacks (Gasterosteus aculeatus)
Creators: name: Smith, Bethany
Publisher: name: University of Glasgow
Year: 2024

description: Ectotherms are expected to be particularly vulnerable to climate change driven increases in temperature. Understanding how populations adapt to novel thermal environments will be key for informing mitigation plans. I took advantage of threespine stickleback (Gasterosteus aculeatus) populations inhabiting adjacent geothermal and ambient habitats to test for evidence of adaptive divergence and plasticity.
In the first data chapter, Chapter 2, I performed a reciprocal transplant experiment to assess the effects of transplantation and morphological variation on growth and survival. I found evidence for adaptive morphological divergence, as growth (length change) in the non-native habitat was found to relate to head, posterior and total body shape. Higher growth in fish transplanted to a nonnative habitat was found to be associated with shape profiles closer to that of the native fish. The consequences of transplantation were asymmetric with ambient sourced fish transplanted to the geothermal habitat suffering from lower survival rates and greater parasite prevalence than geothermal sourced fish transplanted to the ambient habitat. I also found evidence for divergent shape allometries that related to growth. My findings suggest that wild populations can adapt quickly to thermal conditions. However, immediate transitions to warmer conditions may be particularly difficult.
Whole transcriptome gene expression can provide a wholly objective insight into divergence and plasticity in a population. In the second data chapter, Chapter 3, I used an F1 generation of geothermal and ambient sticklebacks reared at 12°C and 18°C to test for evidence of robust divergence in gene expression and divergence in plasticity. I also used F1 hybrids, generated from crossing geothermal and ambient sticklebacks, to assess the genomic and regulatory mechanisms behind geothermal-ambient divergence. I found a small number of genes related to neuron development and functioning to be downregulated in the brains of geothermal fish at both rearing temperatures. Additionally, I found a large number of genes to be plastic in the brains of geothermal fish, largely relating to metabolism. I found little evidence of additive variation or cis-regulatory divergence, suggesting that there is little evolutionary divergence between geothermal and ambient fish. However, I found evidence of potential reproductive isolation between geothermal and ambient sticklebacks in the form of a high degree of transgressive expression in hybrid fish.
In my final data chapter, Chapter 4, I assessed for consistent divergence in gene expression and adaptive plasticity across three population pairs of geothermal and ambient sticklebacks. I found evidence of consistent divergence across two populations, largely in genes relating to glucose metabolism. I also found evidence of a consistent divergent plastic response in geothermal sticklebacks involving an upregulation in genes relating to the innate and adaptive immune system.