Rapid evolution of phenotypic divergence in fish populations

The main objective was to exploit Arctic charr populations in Iceland to investigate the parallel evolution of phenotypic traits in different populations subject to similar environments as well as the role of intraspecific competition in producing phenotypic divergence. The insulin-like Growth Factor hormone axis regulating development and growth and the formation of skeletal muscle fibres was investigated.

The project found that evolution has repeatedly acted on the life time production of muscle fibres in different fish populations to optimise muscle fibre size to minimise the energetic costs of ionic homeostasis. The evolution of the dwarf phenotype associated with volcanic spring habitats was associated with a reduced muscle fibre number and adaptive changes in a limited number of genes controlling growth.

Evidence was obtained for the parallel evolution of dwarfism and muscle fibre loss in Arctic charr populations from volcanic lava spring habitats, likely due to common selective pressures. Dwarf charr were discovered in lava fields only 220 years old, indicating the very rapid evolution of this phenotype. Laboratory rearing experiments under optimal environmental conditions coupled with extreme value analysis of wild populations established that the greatest muscle fibre loss accompanying dwarfism occurred in isolated allopatric habitats. Populations from habitats with potential migration routes to other waterways, or which contained sympatric non-specialist morphs, exhibited a less extreme form of dwarfism and muscle fibre loss. A plausible explanation is that selection for small body size required to exploit the subterranean network of larval fissures present in spring habitats was counteracted by limited gene flow from non-specialist morphs. The results on muscle fibre loss conform to expectations of the “optimal fibre number” hypothesis, which envisages a trade-off between the requirement to avoid diffusional constraints while maximising fibre diameter to minimise the significant costs of ionic homeostasis to the energy budget. The offspring of field collected dwarf, pelagic and non-specialist morphs were reared under the conditions of the natural spring habitat (3-4oC) or at the higher temperature (6oC) experienced by non-specialist sympatric morphs. Cranial morphology was investigated using landmark analysis and multivariate statistical approaches. Variations in head shape were found to be largely genetic. Rearing temperature affected head morphology in each morph, but not in the direction required to explain the phenotypic differences between morphs in the wild. 42 cDNAs were sequenced (ca. 30,000 bp) and submitted to GenBank, including Arctic charr full-coding sequences of IGF-I, IGF-II, 7 IGFBPs, an IGFBP-protease uncharacterised outside mammals (PAPP-A) and 8 novel akirin genes involved in IGF-signalling. 48 Arctic charr qPCR assays were designed and used in published studies, including 41 for IGF-mTOR- or related pathways and 7 references genes used for data normalisation. Comprehensive characterisation of genes of interest was necessary to elucidate their functions and phylogeny, leading to substantial discoveries in the fields of vertebrate akirin and calpain gene-family research. Objective 2. Populations of charr were sampled from across Iceland and subjected to a standardised manipulation of food-intake in a common environment, altering nutritional ‘inputs’ to the IGF-mTOR pathway. The skeletal-muscle expression of 21 IGF-mTOR-pathway genes were quantified in 280 fish across the experiment, representing 5 dwarf and 2 generalist populations (with non-derived life-history and body-size traits) that have been genetically-isolated since the end of the Pleistocene. Statistical modelling elucidated sources of expression variation across ca. 6000 datapoints. There was evidence for adaptive differences in gene expression between dwarf and generalist charr for a small subset of mTOR-pathway genes. Divergence candidate adaptive gene expression was evident during a period of rapid growth following sustained-fasting and was directionally consistent with their functions controlling protein-synthesis. It was concluded that the regulation of key mTOR-pathway genes evolves adaptively in locations favouring dwarfism, resulting in reduced muscle protein-accretion compared to generalist populations receiving an equal input of nutrients. The results were also consistent with natural selection being important in shaping gene expression evolution in Arctic charr population independent of phenotypic dwarfism.