Confirmed linkage for QTLs affecting muscling in Texel sheep on chromosomes 2 and 18

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Confirmed linkage for QTLs affecting muscling in
Texel sheep on chromosomes 2 and 18

• G.A. Walling, P.M. Visscher,
• G. Simm S.C. Bishop
• Roslin Institute (Edinburgh)
• University of Edinburgh
• Scottish Agricultural College

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Project Aims

• Started July 1999
• Detect and verify QTL for growth and carcass
  traits
• Investigate genotyping strategies

http//www.roslin.ac.uk/sheepmap
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Aims

• Use commercial UK Texel sheep
• Use candidate regions to Determine evidence for
  major genes
• Chromosome 2
• Chromosome 18

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Scientific Background

• Studies in Belgium, Australia and New Zealand
  suggest a major gene for muscling and fatness on
  chromosome 2
• Putative Callipyge and Carwell loci both increase
  muscling and are located on chromosome 18

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Animals

• Five large half-sib families
• 1999 348 progeny
• 2000 275 progeny
• Blood collected from all rams and progeny
• DNA extracted from blood

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Phenotypic Traits

• 8 Week Weight (8WW)
• Weight at ultrasonic scanning (ScanWT)
• Ultrasonic Muscle Depth (Mus)
• Ultrasonic Fat Depth (Fat)
• Muscle Depth adjusted for weight (MusWT)
• Fat Depth adjusted for weight (FatWT)

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Genotyping

• All 5 sires screened for
• 28 markers on chromosome 2
• 14 markers on chromosome 18
• Progeny genotyped for markers heterozygous in
  their sire

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Linkage Maps

• Produced using Cri-Map
• Final maps in good agreement with previous studies

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Statistical Analysis

• Traits corrected for fixed effects and
  covariates
• estimated using ASREML
• Inheritance probabilities calculated at 1cM
  intervals
• Regress adjusted phenotypes on inheritance
  probabilities
• at each location
• for each family

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Thresholds and Further Analyses

• Nominal P-value of 0.01
• Lander Kruglyak (1995)
• Significant results
• Compare with a 2 QTL model using grid search
  method
• Confidence intervals using bootstrapping

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Results Chromosome 2

• Significant effects clustered around two regions
• Fat traits around 170cM
• Muscle traits around 65cM
• Best model for weight traits was a QTL in each of
  these two regions
• P(2QTL vs. 1QTL) lt 0.05

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Chromosome 2 Fat Traits
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Chromosome 2 Muscle Traits
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Chromosome 2 Weight traits
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Chromosome 18 Results

• All results highlight one region affecting muscle
  and fatness
• 95 CI include Callipyge and Carwell loci

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Chromosome 18 Results
Callipyge Locus
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Results Effects

• Given in phenotypic standard deviations
• After removing fixed effects and covariates
• Chromosome 2
• Fat 0.66
• Muscle 0.61
• 8WW 0.75 ScanWT 0.69
• Chromsome 18
• Muscle 0.59
• Fat 0.60

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Conclusions

• Candidate region approach successful
• Identify loci in unrelated populations
• Beneficial approach when other methods are
  unavailable

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Conclusions

• Best explanation for chromosome 2 is 2 QTLs
• 65cM muscle growth
• 170cM fat growth
• Region on chromosome 18 affecting muscle and fat

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Chromosome 18 Callipyge or Carwell?

• Position almost identical to both loci
• Effects similar to Carwell not Callipyge
• Carwell not previously known to affect fatness
• Closely link locus affecting fatness
• Alternative Carwell allele affecting fatness

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Future Goals

• Improve estimate of position and effects
• Increase numbers by
• Collecting from more progeny
• Widen the study to other families
• Ultimate goal to identify the Carwell locus

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Acknowledgements

• The UK sheep genome mapping project is funded by