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Selected Publication:

Type of publication: PhD Thesis
Type of document:

Year: 2015

Authors: Tobler, Raymond

Title: Adaption to thermal stress in experimental Drosophila populations.

Other title: Adaption to thermal stress in experimental Drosophila populations

Source: PhD-Arbeit, Vet. Med. Univ. Wien, pp. 85.


Advisor(s):

Schlötterer Christian

Vetmed Research Units:
Institute of Population Genetics


Abstract:
An outstanding challenge in contemporary evolutionary biology is to characterize the genetic basis of adaptive evolution. Fundamental questions include determining the number and effect size of loci under selection, the relative importance of functional vs. regulatory changes, the prevalence of epistasis and pleiotropy, and the contribution of standing variation relative to de novo mutations in sexually reproducing populations. Prior to the 21st century, answers to these types of questions had been difficult to come by due logistical constraints in the acquisition of population genetic information and the lack of suitable study systems that permit biological replication and control of selective and demographic variables. However, the recent advent of next generation sequencing in conjunction with experimental evolution has ushered in an exiting new era of adaptation research: it is now possible to quantify the genome-wide response to adaptation across successive generations in real-time for replicated populations exerted to selective pressures specified by researchers. Continuing innovations in NGS methodologies have recently allowed experimental evolution to expand beyond haploid systems to multicellular, sexually reproducing diploid species. One particularly promising approach is the combination of pooled sequencing (i.e. Pool-Seq) of whole diploid populations undergoing experimental evolution, which are contrasted across generations or between different selective regimes to identify the genomic targets of adaptive evolution. This system, dubbed evolve and resequence (E&R), has mostly been used on the genetic model organism Drosophila melanogaster and has already yielded a number of illuminating results. For instance, classical hard sweeps, which are common in haploid systems, are rare in sexually reproducing systems due to selection acting predominantly on standing variation rather than new beneficial variants. However, Drosophila E&R studies routinely report vastly more selected loci than are expected under population and quantitative genetic theory, suggesting that these studies are plagued by large numbers of false positives. Such problems seriously limit identification of the causal loci driving the adaptive response and must be resolved if the full power of E&R studies is to be realised. In this thesis I discuss results from an ongoing E&R study where replicated D. melanogaster populations were subjected to two opposing cyclic thermal habitats (either hot or cold), with genomic and phenotypic changes being contrasted between the evolved and starting populations. In the first empirical chapter we show, for the first time, that most false positives in Drosophila E&R studies are probably caused by a combination of large segregating inversions and long-range linkage between neutral variants and strongly selected beneficial loci. These findings provide an empirical basis for the implementation of improved identification of causative loci in future E&R studies. In the second empirical chapter we demonstrate that changes in fitness, developmental time and thermal stress resistance observed in each of the hot and cold environments were partly driven by rapid thermal fluctuations common to both environments, and that this parallel response acted in conjunction with habitat-specific adaptation. Hence, this study shows that thermal variation – which is ubiquitous in nature but absent from most experimental designs – can be an important component of the adaptive response in ectothermic species. Taken together, the two studies in this thesis provide an important step toward improved E&R experimental designs and should inspire further experimental work investigating the largely overlooked role of thermal variation in adapting populations.


Publication(s) resulting from University thesis:

Tobler, R; Hermisson, J; Schlötterer, C (2015): Parallel trait adaptation across opposing thermal environments in experimental Drosophila melanogaster populations. Evolution. 2015; 69(7):1745-1759

Tobler, R; Franssen, SU; Kofler, R; Orozco-Terwengel, P; Nolte, V; Hermisson, J; Schlötterer, C (2014): Massive habitat-specific genomic response in D. melanogaster populations during experimental evolution in hot and cold environments. Mol Biol Evol. 2014; 31(2):364-375
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