The parallel evolution of dwarfism in Arctic charr is accompanied by adaptive divergence in mTOR-pathway gene expression

Abstract

Ecological factors have a major role in shaping natural variation in body size, although the underlying mechanisms are poorly understood. Icelandic Arctic charr (Salvelinus alpinus L.) populations represent an ideal model to understand body-size evolution, because adult dwarfism has arisen independently on multiple occasions in response to parallel environmental pressures. The mechanistic target of rapamycin (mTOR) pathway transmits signals from the environment to control cellular growth and is a primary candidate to be under selection for the dwarf phenotype. To test this hypothesis, we modified 'inputs' to this pathway in five dwarf and two generalist populations (with ancestral life history and body-size traits), using a standardized manipulation of food intake in a common environment. The skeletal muscle transcript levels of 21 mTOR-pathway genes were quantified in 274 individuals (∼6000 datapoints), and statistical modelling was used to elucidate sources of variation. Constitutive expression differences between populations were the main component of variation for around three-quarters of the studied genes, irrespective of nutritional-state and body-size phenotype. There was evidence for stabilizing selection acting among populations, conserving the nutritionally dependent regulation of pathway genes controlling muscle atrophy. There were three genes (mTOR, 4E-BP-1 and IGFBP4), where the expression variation between dwarf and generalist populations exceeded the between-population variation. Divergence in the expression of these candidate adaptive genes was most evident during a period of rapid growth following sustained fasting and was directionally consistent with their functions regulating growth and protein synthesis. We concluded that selection has operated efficiently to shape gene expression evolution in Icelandic charr populations and that the regulation of certain mTOR-pathway genes evolved adaptively in locations favouring dwarfism, resulting in reduced muscle protein accretion under growth-favouring conditions.

Dynamic transcriptional regulation of autocrine/paracrine igfbp1, 2, 3, 4, 5, and 6 in the skeletal muscle of the fine flounder during different nutritional statuses

“…Also in this study, both Igf1 and amino acids synergistically stimulate the expression of igfbp4 (Bower & Johnston 2010). In vivo studies on fish skeletal muscle have shown that igfbp4 is constitutively upregulated during nutritionally favorable anabolic periods , Bower et al 2008, Macqueen et al 2011, coinciding with the present results. Interestingly, when different populations of the Arctic charr were assessed, dwarf fish expresses less igfbp4 in muscle in basal conditions than normal-sized fish, and during refeeding, the expression of this binding protein was twofold lower in dwarfs than in normal-sized Arctic charr (Macqueen et al 2011).…”

“…In the muscle of fine flounder, igfbp2 mRNA levels decrease during fasting and return to basal levels during long-term refeeding, in agreement with a previous report on the muscle of rainbow trout . In contrast, other recent reports have shown that igfbp2 expression increases during fasting, supporting the negative role of this binding protein (Bower et al 2008, Macqueen et al 2011.…”

“…Interestingly, when different populations of the Arctic charr were assessed, dwarf fish expresses less igfbp4 in muscle in basal conditions than normal-sized fish, and during refeeding, the expression of this binding protein was twofold lower in dwarfs than in normal-sized Arctic charr (Macqueen et al 2011). In the fine flounder, igfbp4 expression is also similar to that of igf1 long-term fasting and refeeding but not during short-term refeeding, suggesting that Igfbp4 may be a pro-myogenic molecule, as has been suggested previously in fish (Bower et al 2008, Macqueen et al 2011.…”

“…Previous studies on the biology of Igfbps in fish have shown that Igfbps regulate embryonic development (Duan et al 1999, Kajimura et al 2005) and exert ligand-dependent and -independent mechanisms (Dai et al 2010, Zhong et al 2011, as well as showing that Igfbps are modulated by Gh treatment (Duan et al 1999, Cheng et al 2002, Chen et al 2004, Pedroso et al 2009b, oxygen availability (Kajimura et al 2005, Rahman & Thomas 2011, Sun et al 2011, reproduction , Chen et al 2010, and nutrition , Bower et al 2008, Pedroso et al 2009a, Peterson & Waldbieser 2009, Amaral & Johnston 2011, Macqueen et al 2011. Nevertheless, studies on local muscle-derived igfbps (autocrine/ paracrine) have been scarce and have recently shown contrasting results, reflecting a complex physiology of Igfbps and suggesting that intricate patterns of regulation have evolved between and within teleost lineages , Bower et al 2008, Amaral & Johnston 2011, Macqueen et al 2011.…”

The developmental transcriptome of contrasting Arctic charr (Salvelinus alpinus) morphs

“…We also discovered genetic separation in immunological genes ( MHCIIα and cath2 ) between morphs in Iceland and within the lake 52 , consistent with ecologically driven evolution of immune functions. Recently qPCR analyses showed that expression of mTOR pathway components in skeletal muscle correlates with the SB-charr form in Iceland 54 , but it is unknown whether there is genetic differentiation in those genes or upstream regulators.…”