Page 553 - Anatomy and Physiology of Farm Animals, 8th Edition

538 / Anatomy and Physiology of Farm Animals

Sex Chromosomes the adenohypophysis. Another hypotha-
lamic peptide, gonadotropin inhibitory
VetBooks.ir Unlike mammals, in birds the female is the hormone (GnIH), inhibits gonadotropin

synthesis and release. Ultimately, these
gender that is the heterogametic sex (carry-
ing one copy of each of two different sex hormones and the circadian system of the
chromosomes). Female birds carry Z and W birds ensure that when the young are
sex chromosomes, and males carry two cop- hatched there is enough food available for
ies of the Z chromosome. Normally, females them to be raised. Increasing periods of
can contribute either Z or W to progeny, light are associated with increased GnRH
whereas males can contribute only Z. and subsequent increased levels of FSH and
LH. In the male, this manifests as the begin-
ning of spermatogenesis with testicular
Reproduction and Photoperiods enlargement and development in birds
that are seasonal breeders. Male domestic
Reproductive processes in all female birds chickens (roosters) will reach sexual
are subject to regulation by changes in maturity and begin spermatogenesis when
photoperiods (period of light exposure per raised with either a long photoperiod (14
day). Increasing periods of light, such as or more hours of light per day) or a short
during the spring, induce hormonal photoperiod (such as 8 hours per day).
changes in wild female birds to initiate egg However, spermatogenesis is delayed with
production and mating behavior. Longer shortened photoperiods. Mature males
periods of light are associated with the must be maintained with appropriate pho-
release of gonadotropin‐releasing hor- toperiods for maximal fertility.
mone (GnRH) from the hypothalamus, Breeds of domestic chickens selected
which in turn stimulates the release of LH for high egg production will begin egg
from gonadotrope cells in the adenohy- laying if they reach maturity during
pophysis. Two forms of GnRH have been either spring (increasing day length) or
found in avians. GnRH‐I (Glu‐His‐Trp‐ fall (decreasing day length). However,
Ser‐Tyr‐Gly‐Leu‐Gln‐Pro‐Gly) differs chickens raised during the spring reach
from the mammalian GnRH (Glu‐His‐ maturity and begin laying at earlier
Trp‐Ser‐Tyr‐Gly‐Leu‐Arg‐Pro‐Gly) by ages. Egg size is reduced when smaller,
only one amino acid and is considered to younger chickens begin laying, so pro-
be the physiologically relevant form of ducers often manipulate photoperiods
GnRH in avians. GnRH‐II (Glu‐His‐Trp‐ to delay the beginning of laying until
Ser‐His‐Gly‐Trp‐Tyr‐Pro‐Gly) appears to birds are of an appropriate size. After
be an ancient form of GnRH that has laying has begun, reductions in photo-
been structurally conserved throughout periods will lower egg production, as
evolution and has a role in neural func- compared to egg production with a
tion but not on the gonadotrope cells in constant photoperiod.