Genetics of Local Adaptation in theThree-spined Stickleback
Year: 2013
Type: dissertation
Abstract: Spatial differentiation in phenotypic traits is commonly observed in the wild, but both the proximate (cf. environmental vs. genetic) and ultimate (cf. adaptive vs. stochastic) causes underlying this differentiation often remain obscure. Studies focussed on the genetic basis of this differentiation can inform us about these issues, especially if the genetic variants under investigation can be linked with information about their functional role(s) and/or gauged against expectations derived from evolutionary null models. However, due to the difficulties in deciphering and studying the genetic basis of phenotypic variability and differentiation in quantitative traits especially in marine vertebrates the occurrence and scale of local adaptation in them is still poorly understood. Yet, identifying patterns of adaptive divergence and the ecological factors that have contributed to them is essential for understanding how natural selection can maintain local adaptation in the face of gene flow. In this thesis I used a genome-wide set of candidate gene-based microsatellite markers, in combination with quantitative genetic approaches, to explore the patterns of adaptive diversity and divergence among stickleback populations from a variety of habitats ranging from global to local geographic scales. Through comparisons of several independent, isolated pairs of marine and freshwater populations, I found that selection is acting on many genomic regions harbouring genes whose putative functions are related to a wide variety of physiological processes. I also found indications that adaptation to freshwater environments may have been achieved through different genetic pathways in different populations. Importantly, the design of my study was such that alternative demographic explanations for observed patterns could be excluded. Focussing on populations within the physically continuous, yet environmentally heterogeneous marine habitat, I further investigated whether selection is acting strongly enough to promote adaptive population structuring despite high gene flow. Signatures of selection were detected in several candidate genes, along with clear evidence for adaptive differentiation in a phenotypic trait (lateral plate number). Analysis of population structure with only these outlier loci uncovered a higher degree of differentiation than was evident in neutral loci, and in some cases, patterns of adaptive differentiation were correlated with environmental variables likely to act as selective agents in the marine environment (viz. salinity and temperature). Evidence for local adaptation among Baltic Sea sticklebacks was confirmed in a common garden experiment, which demonstrated a loss of fitness in populations native to low salinity regions when exposed to high salinity treatments. Overall, the results from this thesis point to the conclusion that adaptive genetic and phenotypic differentiation is common, even in continuous marine habitats lacking obvious physical barriers to dispersal and gene flow. These results are particularly noteworthy, firstly from the perspective that earlier studies conducted using neutral marker genes have largely overlooked the patterns and magnitude of divergence, and secondly due to the comprehensive geographic coverage of the investigations.
Source: Työväentutkimus Vuosikirja
Author: Jacquelin DeFaveri
Language: English
Subfield: Genetics
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2156750420