Physiological constraints and their role in generating phenotypic diversity
Ian A. Johnston, Daniel J. Macqueen, Bjarni K. Kristjánsson, Michael A. BellBob Devlin and Vera L.A. Johnston
The optimum muscle fibre size hypothesis
• There is an optimum fibre diameter which minimisesthe energy costs of maintaining ionic homeostasis
• Fibre size optimisation (FSO) is achieved by altering thelifetime production of muscle fibres resulting in variationin fibre number
• The mechanisms are common to those generating otherforms of phenotypic diversity making FSO an interestingmodel
- developmental plasticity (?)- epigenetics- selection
Diffusional constraints
Energetic constraints
Cost ∝ (S/V)
Trade-off
Arctic charr and stickleback models
Rapidly evolving population Loberg Lake, Alaska
Spring-fed streamHrauná (H)Spring-fed pondsÁlftavatn (A)Kaldárbotnar (K)Miðhúsaskógur (M)Straumsvík (S)Silungapollur (P)Sílatjörn (J)
Ian A. Johnston, Bjarni K. Kristjánsson, Charles G. P. Paxton, Vera L. A. Vieira, Daniel J. Macqueen& Michael A. Bell (2012) Universal scaling rules predictevolutionary patterns of myogenesis in species with indeterminate growth. Proc. R. Soc. B 279, 1736 2255-2261
• Rapid evolution is consistent with strong selection for FSO
• Suggests changes in the frequency of a relatively few alleles oflarge effect
• Gene flow between small- and large-bodied populations residing insympatry may attenuate FSO
How best to investigate the mechanism(s) ?
Lack of genome sequence and reference transcriptome is a handicap
Sequencing of RAD libraries might be a cost effective approach
Important to use the power provided by multiple populations
Transgenic Coho salmon model
The strain M transgenic coho salmon used werederived from Chehalis River strain produced usingthe OnMTGH1 construct comprised of 320 bp ofsockeye salmon metallothionein-B promoter fusedto the 5’-UTR region of the full-length type-I growthhormone (GH) gene and the terminator from thesame species courtesy of Bob Devlin
WT Wild type 2 years oldTF GH-transgenic fed to satiation
1 year old. Higher appetite,food intake and foraging activity
TR GH-transgenic fed same rationas WT 2 years old. Growth sameWT but higher foraging activity
Nutritional restriction in transgene led to about35% fewer fibres produced at the same body sizein the transgene
FSO allows shift in energy allocation
Transcriptomes currently being examinedAn epigenetic mechanism?
Fork length (mm)
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Marine stickleback model
High and low fibre numberphenotypes
Both found in tidal reaches
Similar maximum length
Fibre phenotypes imperfectlymap on to armour platingphenotypes
Only small numbers studiedto date
PlansCategorise morphs in terms ofmetabolic rate
Sequenced genome so perhapsa better genetic model than charr?