Academic journal article Genetics

Genomic Trajectories to Desiccation Resistance: Convergence and Divergence among Replicate Selected Drosophila Lines

Academic journal article Genetics

Genomic Trajectories to Desiccation Resistance: Convergence and Divergence among Replicate Selected Drosophila Lines

Article excerpt

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UNDERSTANDING how adaptation proceeds in a population is important for predicting natural responses to impending stress that will challenge the adaptive potential of many species. Climate stress is a timely example: our ability to predict and respond to climate change effects on natural populations requires relevant models of how evolution proceeds, backed up by empirical evidence.

Aridity is one climate-imposed stress likely to increase in parts of the world as climate change proceeds. Many areas that currently experience reasonable rainfall are projected to become drier over the coming decades (Dai 2012), and some are already showing significant reductions in rainfall (e.g., southwestern Australia: Delworth and Zeng 2014). Dry conditions typically impose water balance stress on organisms, and, when there is no behavioral escape possible, the only opportunities for survival are tolerance or adaptation (Franks and Hoffmann 2012).

The potential for adaptation to desiccation stress varies greatly among species. Many plants show evidence of both plastic and adaptive responses to aridity (Des Marais and Juenger 2010; Byrne et al 2013; Juenger 2013). This has been harnessed in crop breeding through artificial selection for efficient water use, though increasing aridity will likely still challenge agriculture (Qureshi et al. 2013). In animals, there is extensive knowledge from Drosophila melanogaster about the potential for adaptation to numerous climate stresses (Hoffmann et al. 2003, 2005; Franks and Hoffmann 2012), and artificial selection experiments have demonstrated this species can readily evolve higher desiccation tolerance (Hoffmann and Parsons 1989a; Hoffmann etal. 2003) with reported heritabilities of around 60% (Hoffmann and Parsons 1989b; Kellermann et al. 2009). However, some related species respond quite differently when faced with dry conditions. The Australian rainforest endemics D. birchii and D. bunnanda both have zero, or very low, adaptive potential under extreme desiccation stress (Hoffmann et al. 2003; Kellermann et al. 2009), although they do show significant heritability under more moderate levels of stress (van Heerwaarden and Sgro 2014). The widespread species D. melanogaster and D. serrata, however, are able to adapt even to the more severe levels of desiccation (Hoffmann and Parsons 1989b; Blows and Hoffmann 1993).

Numerous subphenotypes have been linked to desiccation resistance in drosophilids, including cuticle composition (Rajpurohit et al. 2013); reduced water loss rate (Hoffmann and Parsons 1993; Gibbs et al. 2003); metabolic rate; glycogen, lipid and/or carbohydrate storage (Hoffmann and Harshman 1999); and sensing and signaling pathways (Telonis-Scott et al. 2012, 2016). Identifying the specific loci involved in adaptation to stress offers possibilities for assessing the generality of stress adaptation, both within, and across, species (Franks and Hoffmann 2012; Byrne et al. 2013). Knowing to what extent stress adaptation proceeds from predictable genes, gene families, or regulatory network modules will aid in predicting adaptive capacity for species in which experimental manipulation is not possible. Further to this, assessing how a population adapts to a stress at the genomic level has implications for population size dynamics and connectivity, which affect population resilience to other stresses and future adaptive capacity (Willi and Hoffmann 2009; Hoffmann and Sgro 2011).

This paper deals with the use of selection experiments to understand how, and where, adaptation proceeds across the genome, how repeatable it is, and how selection influences divergence or convergence across the genome. An evolve-andresequence (E&R) approach-where replicate lines from a common genetic background are characterized for genomic variation after a period of selection-is useful for studying these questions as a step toward understanding the generality of responses to climate stress. …

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