the optimum muscle fibre size hypothesis johnston talk.pdftransgenic coho salmon model the strain m...

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?