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Abstract
The field of genomics applies the dissection of genetic
differences towards an understanding of the biology of
complex traits. Quantitative trait loci (QTL) for testis
size, plasma FSH in boars, and body composition (backfat)
have been identified near the centromere on the X
chromosome in a Meishan-White Composite resource
population. Since thyroid function affects Sertoli cell
development and adult testis size in rodents, and
thyroxine-binding globulin (TBG) maps to this region on
the porcine X chromosome, TBG was a positional candidate
gene for testis size. We discovered a polymorphism in exon
2 of the porcine TBG gene that results in an amino acid
change of the consensus histidine to an asparagine. This single nucleotide polymorphism
(SNP) resides in the ligand-binding domain of the mature
polypeptide and the Meishan allele is the conserved allele
found in human, bovine, sheep and rodent TBG. Binding
studies indicate altered binding characteristics of the
allelic variants of TBG with the asparagine (White
Composite) isoform having significantly greater affinity
for thyroxine than the histidine (Meishan) isoform.
Alternate alleles in boars from the resource population
are also significantly associated with testis weight.
Therefore, this polymorphism in TBG is a candidate
for the causative variation affecting testis size in boars.
Key words:
Mechanisms of Hormone Action
Testis
Developmental biology
Follicle-stimulating hormone
Sertoli cells
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