Genetics of flystrike, dagginess and associated traits in New Zealand dual-purpose sheep : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Palmerston North, New Zealand
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Date
2013
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Massey University
Abstract
A literature review identified breech bareness, dagginess and fibre traits as potential
indirect indicator traits for flystrike. Dagginess (faecal accumulation) had the greatest
potential as an indirect indicator, and has been identified as an important trait itself.
Therefore flystrike and dagginess were investigated for their associations with fibre and
production traits. A genome-wide association study (GWAS) was performed to identify
regions under selection and associated with these traits. Finally, a genomic selection
(GS) analysis was performed for dagginess and dual-purpose production traits to
estimate molecular breeding values (MBVs) and to determine their impact on the New
Zealand dual-purpose selection index.
Heritability, genetic and phenotypic parameter estimations were performed on a
flystrike case-control dataset collected over 2 years. Flystrike had a heritability of 0.37,
and high genetic and phenotypic correlations with dag score and a high genetic
correlation with the coefficient of variation of fibre diameter. A similar analysis was
performed on an existing New Zealand sheep industry dataset of about 2 million
pedigree-recorded animals born between 1990 and 2008. The heritability for dag score
at 3 and 8 months (DAG3, DAG8) was 0.34 and 0.31 respectively. There were low or
nil genetic and phenotypic correlations of DAG3 and DAG8 with the other standard live
weight, fleece weight, reproduction and faecal egg count production traits or breech
bareness, fibre and wool traits.
A GWAS performed on an industry dataset of 8,705 genotyped animals, using
phenotype information on about 3 million pedigree-recorded animals, identified regions
on chromosome 6 and 15 associated with DAG3 and DAG8. The lambs from the
flystrike case-control dataset with SNPs imputed from 5K to 50K identified a number of
immune, diarrhoea and wool/hair growth genes associated with flystrike, dag score and
fibre traits in a GWAS. There were no similarities in the genes identified in the industry
or case-control GWAS; however, the SNP on chromosome 15 was re-identified in the
GS analysis for DAG8. The GS analysis showed that genomic predictions can be
Genetics of flystrike and dagginess in New Zealand dual-purpose sheep
performed for DAG3 and DAG8 and that using MBVs and modifying generation
interval can increase the rate of the genetic gain of the dual-purpose index by 84% per
year.
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Keywords
Flystrike, Myiasis, Sheep dags, Sheep breeding, Sheep genetics, Sheep disease prevention