Marker-assisted selection may provide the opportunity to make significant
genetic gains in the improvement of economically important traits in livestock
(Soller & Beckmann, 1983; Smith & Simpson, 1986). Implementation
of this
approach will first require identification of candidate genes or anonymous
gene
markers associated with the traits of interest. Candidate genes are those
with a
known relationship between physiological or biochemical processes and an
economically important trait. In dairy cattle, genes associated with mammary
gland
growth, development and function are excellent candidate genes for milk
production
traits.
The polyamines are low molecular mass polycations that influence cell
proliferation and growth (Tabor & Tabor, 1984; Pegg, 1986). Ornithine
decarboxylase
(ODC, EC 4.1.1.17) catalyses the conversion of ornithine to putrescine,
the
rate-limiting step in polyamine biosynthesis (Pegg, 1986). The level of
ODC is
induced in quiescent cells exposed to a wide variety of stimuli such as
growth
hormone, corticosteroids, testosterone and growth factors (Tabor &
Tabor, 1984).
There is ample evidence that growth factors influence morphogenesis and
differentiation of the mammary gland (Imagawa et al. 1994). Trophic
hormones that
are associated with lactation, such as prolactin and growth hormone, are
also
required to induce differentiation of bovine mammary epithelial cells (Huynh
et al.
1991; Flint & Gardner, 1994), and both mitogenic and lactogenic effects
of prolactin,
insulin and hydrocortisone appear to be mediated through the polyamine
pathway
(Rillema et al. 1977; Bedford & Zadworny, 1990; Golden &
Rillema, 1993). Strange
et al. (1992) have shown that ODC is involved in synthesis of
a milk component; ODC
has elevated expression in the lactating mammary gland, which declines
sharply
after weaning. It is therefore possible that particular variants of ODC
could be
associated with increased mammary gland function and thus influence milk
related traits.
The objectives of the present study were to estimate allelic frequencies
of ODC
polymorphisms in Holstein bulls and to evaluate further the genotypic effects
of
ODC variants on milk production traits.