experimental breeding programs

You did not hijack it just took it in a needed direction .
I found this on the web
5 2
Avian genetic diversity: Domesticated species
GENETIC DIVERSITY IS CONSIDERED crucial to the
continued survival of a species, be it wild or domestic.
Such within-species diversity has been
the raw material of agriculturists over millennia.
In response to selective breeding and the differential
survival of less fit animals, preferred traits
have been accentuated and clustered to produce
distinct breeds and varieties of the modern domesticated
species (NRC 1993). In more recent
times, researchers have deliberately isolated
various mutations in specialized stocks, permitting
the systematic study of such mutations and
promoting a better understanding of the normal
function of the affected genes.
The totality of wild and domesticated species
form the gene pool or genetic resources base
necessary for the survival of the species. The
genes and genotypes present in this pool represent
genetic resources which are accessible and
can be exploited by biologists and breeders. In
this report, we emphasize "genetic stocks" which
have been bred for specific traits and genes in
contrast to breeds in which the individual birds
have many traits in common and can generally
be maintained with randomly breeding populations.
Genetic stocks are typically selected for
traits of special interest to breeders and geneticists.
Many of them are reproductively, physically,
or physiologically compromised, and require
special care in breeding and management,
even for maintenance or conservation purposes.
Target species
While the AGRTF recognizes the need for conservation
of undomesticated avian species, this report
primarily addresses the need for conservation
of specialty stocks of domesticated species,
particularly chicken, turkey, and Japanese
quail. A limited number of waterfowl (duck and
goose) genetic stocks and semi-domestic gamebird
stocks (ring-necked pheasant and bobwhite
quail) have been developed and will be noted in
this report. Noted below are salient features of
the most widely used domesticated species that
have the greatest need for conservation of genetic
stocks.
Chicken
First domesticated over 6,000 years ago, the
chicken (Gallus gallus or G. domesticus) presents
by far the greatest amount of genetic diversity of
the domesticated avian species, with over 400
identified genetic variations (SOMES 1988). Many
are showcased in the more-than 100 recognized
chicken breeds and commercial varieties, which
variously integrate most of the naturally occurring
mutations affecting size, body type, production
characteristics, posture, color, feather
structure and location, comb shape, and behavior
(see Figures 1 and 2 for wild- and domestictype
chickens). Some of the most extreme variants
include: the tiny, short-legged Japanese
Bantam; the tall, aggressive Old English Game
Figure 1. Red Jungle Fowl rooster from UCD 001
(Photo courtesy of J. Clark, University of CaliforniaЦDavis).
6 popular with researchers or hobbyists than the
chicken or the Japanese quail, at least six
breeds are still kept for exhibition and a few
unique research stocks have been developed
(Box 5), including several commercial-type longterm
selected and randombred-control lines kept
at Ohio State University (see survey results, Appendix
2, Tables 2.1 and 2.2). Perhaps as a consequence
of the few researchers studying the
turkey, relatively few mutations (49) have been
reported in the turkey compared to the chicken
and Japanese quail (SOMES 1988).
Japanese quail
Gaining in popularity as an experimental animal
in both research and education, the Japanese
quail (Coturnix japonica) is a small, early maturing,
highly efficient egg and meat producer. Until
recently, the Japanese quail was classified as a
hatch, and grow into fully functional
males (OLSEN 1965). The existence of
this line has given rise to the notion
that genetic imprinting does not exist
in birds, although this conclusion must
be tentative in the absence of any formal
investigation of imprinting in the
unique parthenogenetic stock. However,
this parthenogenetic stock exists
precariously at only two research stations
in the world (the University of
Guelph (Ontario, Canada) and the University
of Oman).
Fowl; the light-weight Singlecomb
White Leghorn hen that
can lay more than 300 eggs in
her first year of production; and
the large Rock-Cornish commercial
meat chicken, with its phenomenal
rate of growth and
well-fleshed carcass. These are
all thought to share a common
ancestor in the Red Jungle Fowl
(G. gallus gallus) (Figure 1)
which is still found wild in parts
of India and Southeast Asia
(CRAWFORD 1990; FUMIHITO et al.
1994), although some poultry
specialists believe that several
other jungle fowl species (G. sonnerati, G. lafayettei,
and G. varius) also contributed to the ancestral
gene pool (CRAWFORD 1990).
Turkey
The one commercially important avian species
originating in North America, the domestic turkey
of commerce, is the product of hybridization
between two subspecies of turkey: the domesticated
Meleagris gallopavo gallopavo from Central
America and the wild M. g. sylvestris from the
eastern United States (CRAWFORD 1990). From
these hybrids, birds were selected for size, tameness,
carcass yield, and rapid growth, resulting
in several distinct breeds and varieties (Figure
3). The modern commercial or exhibition turkeys
are large, slow-maturing birds with a much
lower reproductive potential than chicken or
Japanese quail (at one generation per year for
the turkey). Although this species is far less
Box 5. Parthenogenetic turkeys
PERHAPS THE MOST SPECTACULAR use of turkeys
in experimental biology was the
study of meiosis, fertilization, and early
embryonic development with a strain
of parthenogenetic turkeys. In the
1960s and 1970s, M.W. Olsen of the
United States Department of Agriculture
Agricultural Research Center at
Beltsville developed a line of turkeys
in which an embryo would form in 30
to 50% of the unfertilized eggs (parthenogenesis).
Most of these embryos
die, but a small proportion of them
(about 0.5%) continue to develop,
Figure 3. Flock with different turkey breeds (Photo
courtesy of F.A. Bradley, University of CaliforniaЦDavis).
Figure 2. White Leghorn rooster from UCD 003 (Photo
courtesy of J. Clark, University of CaliforniaЦDavis).
7 subspecies of the common European quail (C.
coturnix). It is now classified as a distinct species
because of the nonhybridization of the two in the
wild or in captivity (CHENG and KIMURA 1990).
According to all available documentation, the
domestic Japanese quail strains used in meat
and egg production (even in Europe) are descended
from C. japonica, which is still found in
small wild populations in Japan. While gaining
popularity as a food animal in the US, its small
size has limited its use as a meat- or egg-producing
animal to specialty markets. However,
the Japanese quail has other qualities that make
it ideally suited for research. Usually reaching
sexual maturity by six weeks of age, the females
often lay an egg a day for several months. The
males are aggressive breeders and maintain high
fertility even when housed with four or more
hens. The early maturity and short incubation
interval (16 to 17 days) permit as many as five
generations in a single year, in contrast to the
slower-maturing chicken (one to two generations
per year) or the even slower maturing and less
productive turkey (one generation per year). The
quail is sometimes called the mouse of the bird
world, since it has become extremely popular as
a model species for biological research in several
fields, including toxicology, cell biology, nutrition,
and selective animal breeding. Although
most researchers use unselected or randombred
birds, over 100 mutations are known in this species,
including many affecting feather color and
shape (Figures 4 and 5), and several causing
embryo-lethal deformities (SOMES 1988; CHENG
and KIMURA 1990). At present, most of these mutant
strains are only maintained at the University
of British Columbia or by hobbyists. Two
drawbacks with this species are that the usual
productive life of an individual bird is quite
short, frequently less than one year, and, unlike
the chicken, close inbreeding is not tolerated.
Thus, only one moderately inbred line exists (at
the University of British Columbia).
Duck
Almost all of the 15 or so domestic duck breeds
recognized today are descended from the wild
mallard duck (Anas platyrynchus platyrynchus),
the exception being the Muscovy duck (Cairina
moschata) (LANCASTER 1990). In addition to the
different plumage patterns and colors, a variety
of body types and behavioral traits are found
among the duck breeds, ranging from the boatshaped,
vocal Call ducks to the cane-shaped Indian
Runner ducks. Only 22 mutations have
been described in the domestic duck, most involving
feather color or pattern (LANCASTER
1990). As such, these traits have been used in
defining breed and variety standards, especially
among the more ornamental duck breeds, such
as the Call, Indian Runner, Crested, Cayuga,
and Swedish. While such breeds are usually
only kept by hobbyists, a few are important in
commercial meat production, particularly White
Pekins, Rouens, and, in some areas, Muscovy or
Muscovy-domestic duck hybrids.
Goose
Six recognized domestic goose breeds were derived
from the western Greylag goose of Europe
(Anser anser anser). Several other breeds are
thought to have descended from the smaller
Swan goose of central Asia (A. cygnoides). The
African breed is believed to be derived from a
Figure 4. Japanese quail silver mutation from UBC SI
(Photo courtesy of K. Cheng, University of British Columbia).
Figure 5. Japanese quail porcupine mutation from
UBC PC-WB (Photo courtesy of K. Cheng, University
of British Columbia).
8 hybrid between these two species (HAWES 1990).
Strict herbivores, geese have a long history of
domestication, but their delayed maturity (two
years) and low egg production rate make them
less attractive as an experimental animal or as a
commercially viable species (Box 6). However,
due to the increasingly diverse consumer groups
in the US and Canada, formerly noncommercial
species are becoming popular on a small scale
for specialty markets. One example is the demand
from the Asian markets for a smaller, less
fatty meat goose. Until now, Europeans and
North Americans have traditionally raised Embden
geese for market purposes. This large-bodied,
fatty bird is not well suited to the method of
cooking employed by Asian chefs. Therefore, waterfowl
suppliers are now starting to grow the
smaller Chinese geese for this market.
Gamebirds
Several species of game birds are commonly
bred commercially or by hobbyists, including
many subspecies of the Ring-necked pheasant
(Phasianus colchicus) and the Bobwhite quail
(Colinus virginianus). Nine color mutations have
been identified in the pheasant, along with several
affecting skin color and feather structure,
and 11 that produce biochemical polymorphisms
(SOMES 1990). For most populations, very little
selective breeding or inbreeding is deliberately
practiced, and the development of gamebird
stocks for genetic research is unusual. Exceptions
include the now-extinct inbred pheasant
lines developed at the University of CaliforniaЦ
Davis (WOODARD et al. 1983) and the Bobwhite
and pheasant blood-type variants currently kept
at Northern Illinois University (JARVI et al. 1996).
Types of genetic stocks
For the purposes of this report, genetic stocks
are classified into four categories that reflect the
genetic composition and type and the breeding
system used to maintain them.
Х Randombred
Х Highly inbred
Х Long-term selected
Х Mutant (including cytogenetic variants
and transgenics)
We are primarily concerned in this report
with conservation of genetic stocks developed for
research purposes, which include all of these
categories. Conservation of genetic stocks in the
different categories present different challenges
for successful conservation, including: high embryonic
mortality, low viability, poor reproductive
traits, pronounced susceptibility to one or
more diseases, large and deleterious genetic
load, poor response to specific environmental
stressors, poor recovery of cryopreserved semen,
and need for a very large gene pool (more than
100 birds per generation).
Randombred lines are maintained as relatively
large populations of birds (usually over 100) in
which little, if any, selection of breeding stock is
done by the curator. Quite simply, the number
of progeny from each male or female depends on
the reproductive success of that
bird at the time the eggs are collected
to reproduce the population.
Such randombred stocks
are generally kept as closed
flocks, although new bloodlines
may be introduced to the population
to improve the vigor of the
flock, particularly if inbreeding
depression is observed. The
birds may be reared and bred in
a single large enclosure, with all
males having access to all females.
This is a common for
pheasants, ducks, geese, some
chickens, and naturally breeding
turkey stocks. Alternatively,
the birds may be randomly segregated
into smaller floor pens
or randomly paired or grouped
in cages, as is common with
and improving egg production with
controlled lighting and trapnesting.
Early studies with Pilgrim geese (the
only goose breed that shows strong
sexual dimorphism) included a demonstration
of increased egg production
with selection (MERRITT 1962), and use
of light control to increase egg production.
More recently, artificial insemination
techniques have been improved
(GRUNDER and PAWLUCZUK 1991) and
geese have been shown to lack endogenous
viruses (i.e., viral DNA integrated
into the host bird chromosomes) of the
avian leukosis type (GRUNDER et al.
1993). Unfortunately, with the loss of
funding from the Canadian government
in April of 1997, these stocks
were either eliminated or dispersed.
Box 6. Research with goose breeds in Canada
WHILE GEESE ARE NOT COMMONLY used for
experimental purposes, a relatively
large experimental population of geese
was maintained at the Center for Food
Animal Research (CFAR) in Ottawa. Several
distinct stocks of Chinese, Embden,
and Chinese X Pilgrim hybrid
geese were developed in Ottawa to
study production traits and DNA fingerprinting
patterns (GRUNDER et al.
1994). The stocks included standard
breed control strains, stocks selected
for multiple traits, and the unselected
reference strains (SOMES 1988). As with
chicken and turkey research in the early
part of this century, most studies with
the relatively undeveloped pure and
crossed goose varieties have been related
to agricultural objectives, including
methods of rearing broiler geese
9 Japanese quail. A minimum of 25 pairs, usually
more than 150 birds, is needed to keep inbreeding
at a minimum. These stocks are often kept
as a source of "normal" control birds, and also
function as a resource stock, from which inbred
or selected stocks can be derived or qualitative
mutations isolated.
Inbred lines are produced by breeding together
close relatives for many generations, resulting in
increasingly homozygous and homogeneous
progenies. Different types of mating schemes are
used, depending on how rapidly the researcher
is attempting to approach complete homozygosity.
Disregarding parthenogenesis, the most
rapid inbreeding is produced by father-daughter,
mother-son, or brother-sister (full-sib) matings.
These breeding schemes can also be used to expose
deleterious recessive traits or to fix preferred
or beneficial single-gene traits in a population.
Unfortunately, even in the absence of major
genetic defects, the fertility and viability of
the inbred offspring are almost always lower
than the more outbred parent strain, a characteristic
called inbreeding depression. If selection
and breeding strategies do not compensate for
this decline, inbreeding depression can result in
the extinction of the line within a few generations.
This is a particular concern in lines propagated
by full-sib matings which also have large
genetic loads (many deleterious alleles). However,
once the lethal and sub-vital alleles have
been purged from the inbred strain, it can theoretically
be bred to essentially complete homozygosity
while maintaining reasonable reproductive
performance traits (fertility, egg hatchability, egg
production rates, viability, etc.). Such genetically
uniform stocks can then be used as a standard
genetic background in the study of individual
genes and gene complexes. Particularly
useful inbred lines are
those which have been bred for
contrasting phenotypes due to
allelic differences at single loci.
These lines, having the same
genetic background for practically
all loci except for the alleles
of interest, are called
congenic lines. They are used to
study single-gene effects on productivity,
for molecular characterization
of genes affecting developmental
traits or disease
resistance, and many other uses
in basic biological and biomedical
research (ABPLANALP 1992).
Highly inbred genetic stocks are invaluable in
a wide range of research fields, particularly
genomics (gene mapping) and immunogenetics
(Box 7). A good example of the usefulness of inbred
strains in genomics is the mapping of classical
mutations. While over 80 classically identified
genetic mutations have been assigned to the
chicken linkage map, only a few have been located
on the molecular map. This is due to the
lack of genetic characterization of the exhibition
breeds and lines in which most of these mutations
are found. Such nonuniform genetic backgrounds
make them difficult to use in matings
designed to integrate the maps. In contrast,
congenic lines, mentioned above, are uniquely
useful for such genetic mapping. The integration
of genes in exhibition breeds into defined inbred
lines would provide the necessary uniformity for
molecular mapping of these traits.
Long-term selected stocks are the result of
many generations of testing and selective breeding
for traits governed by multiple genes (the socalled
quantitative or polygenic traits). Many valued
heritable characteristics in the poultry
breeds belong in this category. These include egg
production rate, egg size, feed efficiency, fertility,
hatchability, viability, disease resistance, body
size and shape, and behavioral characteristics.
To change the population mean for one or more
of these quantitative traits requires rigorous
testing and ranking of the individuals and family
groups for the traits-of-interest each generation,
followed by selective breeding of the higherranked
individuals and families to produce the
next generation. Many factors can affect the rate
of improvement in response to selection including
1) degree of heritability of the trait or traits
involved, 2) selection stringency, 3) level of in-
Box 7. Highly inbred stocks in immunogenetics
BASIC INFORMATION ABOUT FACTORS controlling
disease resistance in the chicken
has been gathered largely from studies
with congenic strains of chickens
(birds with identical, highly inbred
backgrounds but different major histocompatibility
complex (MHC)
haplotypes; ABPLANALP 1992). A number
of these congenic strains have been
developed at the University of CaliforniaЦ
Davis, the USDA Avian Disease and
Oncology Laboratory in East Lansing,
MI, the University of New Hampshire,
and Iowa State University. Researchers
have shown how each MHC-haplotype
could directly affect the resistance of
a bird to a variety of different diseases,
including coccidiosis, NewcastleТs disease,
and the tumor-inducing viruses
that cause MarekТs disease and lymphoid
leukosis. The congenic MHC
strains, most requiring at least ten generations
of back-crossing and bloodtesting
to develop, are key resources
required for furthering our understanding
of the way the MHC genes function.
Studies with these stocks have already
given the primary poultry breeders
vital information to use in determining
the best of several alternative
breeding strategies to enhance disease
resis-tance potential of their production
stocks.
10
breeding, and 4) genetic variation in the original
source population. Selected stocks usually require
several generations to develop, tend to revert
towards the original stock values if the selection
pressure is lifted (i.e., if random or nonselected
pedigree reproduction is used), and
usually need to be reproduced in large numbers
(several hundred birds) each generation for the
best selection differential with minimized inbreeding.
Mutant stocks incorporate one or more of the
many single-gene mutations that have a major
effect on specific morphological or physiological
traits. These include variants (alleles) that affect
eggshell color, feather color or shape, skin color,
comb shape, metabolic function, major histocompatibility
complex (MHC) haplotype identity,
and pattern formation in the developing embryo.
The wide array of mutations affecting feather
color and shape are important for distinguishing
between breeds and varieties within breeds. In
the poultry industry, eggshell color, skin color,
feather color, and feathering rate mutations,
and, more recently, MHC types, have all played
important roles in the development of commercial
strains and varieties. Of particular interest
to biomedical researchers are those mutations
that cause disease conditions that mimic human
genetic disorders, including muscular dystrophy,
scoliosis, scleroderma, and a variety of developmental
mutations (usually lethal) that affect the
development of the face, limbs, integument, and
internal organs.
Cytogenetic variants are birds that have chromosomal
abnormalities, such as aneuploidy,
polyploidy, translocations, and large insertions
or deletions. A small number have been established
in the chicken, and these have provided
useful model systems for the study of meiosis,
inheritance, recombination, linkage, transcriptional
regulation, and gene dosage
effects. Such stocks include:
aneuploidy for the chromosome
encoding the MHC and
nucleolar organizer region
(NOR), complete triploidy (three
copies, instead of two, of all
chromosomes), large deletions
(the mPNU line, in which there
is segregation of an MHC/NOR
chromosome with a deleted
NOR), and various stocks carrying
translocations between
macrochromosomes (Box 8).
Transgenic stocks are
formed by inserting foreign
DNA, usually containing a gene of interest, into
one of the chromosomes of germline or somatic
cells. While some transgenic chickens have been
produced in the past few years (SALTER et al.
1986; 1987; SALTER and CRITTENDEN 1989), the
creation of transgenics is still very much experimental
in chickens and other avian species.
However, a number of research groups continue
to develop and refine transgenic methodologies,
and report promising advances in the production
of transgenic birds (SALTER et al. 1987; LOVE et
al. 1994; THORAVAL et al. 1995; MARUYAMA et al.
1998).
Research genetic stocks
Genetic stocks are used in three areas of research:
agricultural, biomedical, and basic or
fundamental biological research.
Agriculturally important avian genetic stocks
primarily include those selected for various production-
related characteristics (egg production,
body shape, feed-use efficiency, leg strength, disease
resistance). Another use for such stocks is
to provide a flexible, rapidly responding model
system for testing breeding techniques and systems
that might also be useful with large livestock
species (e.g., pigs, sheep, and cattle). These
stocks are particularly vulnerable to funding
cuts due to the long development period needed
for most selected stocks, and the relatively large
numbers that must be produced and monitored
annually to produce the selected population.
Biomedical research specifically uses animal
models for the study of various human diseases.
Avian models, mostly in the chicken, exist for
the autoimmune forms of vitiligo, scleroderma,
and thyroiditis, as well as for various developmental
defects, such as polydactyly, scoliosis,
and cleft palate. Genetic stocks are also used in
avian health research for studying the nature of
FROM THE MID-1960S TO THE 1980s, animal
genetics laboratories at Ohio State
University, the University of Minnesota,
and New Mexico State University developed
about 40 different chromosome
rearrangement strains in the
chicken (ZARTMAN 1971; WOOSTER et al.
1977; WANG et al. 1982). A number of
studies by these laboratories made important
contributions to our understanding
of chromosome behavior in
avian species (including recombination,
chromosome segregation, identification
of pseudoautosomal regions on
Box 8. Chicken chromosome rearrangement stocks
the sex chromosomes, and sources of
aneuploids). Unfortunately, with the
lack of support by various agencies
over the last ten years, over thirty of
these unique genetic resources were
irretrievably lost. The seven still in existence,
along with a recently isolated
spontaneous translocation, are currently
being maintained at the University
of Wisconsin. However, with departmental
reorganizations and budgetary
difficulties, these stocks are also
threatened.
11
mercial importance, such as the sex-linked gene
controling the rate of feather growth that has
been heavily utilized by modern chicken breeders
(Box 9).
In marked contrast to the general perception
that commercial poultry stocks all have a relatively
small and diminishing genetic base, some
researchers have reported the opposite. Specifically,
DUNNINGTON et al. (1994) used DNA fingerprinting
to measure variability among commercial
chicken breeding populations and concluded
that a considerable reservoir of genetic diversity
yet remained. IRAQI et al. (1991) reported a great
degree of polymorphism for endogenous viral (ev)
genes in five egg-type populations maintained by
an Israeli commercial breeder. AARTS et al. (1991)
also found variation for ev genes among and
within six WL and four medium-heavy brown
eggshell lines. While none of these methods specifically
reflects the variation remaining in genes
associated with economically important traits,
the recent substantial progress in the development
of the genetic map of the chicken (CHENG et
al. 1995; CHENG 1997) should soon lead to more
thorough and realistic assessment of the amount
of economic trait variability remaining in commercial
poultry populations.
Fancy breeds and mid-level
production stocks
For at least 50 years, poultry fanciers have been
the main conservators of the majority of the
disease resistance and effects of specific genes
on productivity under disease stresses.
Most of the genetic stocks are of value for
studying questions in basic biology that may
lead to more applied biomedical or agricultural
research, or by simply contributing to the knowledge
of how different biological systems function
in a wide variety of studies in the life sciences.
Some of the specialized stocks have been
derived directly from commercial chicken, turkey,
or Japanese quail lines, while others were
developed from special breeds, landraces, or
wild-types.
Commercial stocks
The commercial poultry stocks have made remarkable
genetic progress in the last 50 years
(Boxes 9, 10, and 11). At this time, selected
stocks used in commercial egg or meat production
must fit very specialized production criteria.
To develop these criteria, each breeding company
has identified particular commercial goals (egg
production, weight gain, feed conversion, carcass
characteristics, etc.) and seeks to meet them in
the shortest possible time (EMSLEY 1993). In this
way, the fundamental difference between basic
and applied research is highlighted. While a researcher
may have a preferred outcome for an
experiment, any result can provide useful information
to that researcher or others in the research
community. For the commercial breeder,
the only outcome that is acceptable is one that
improves the commercial product for the consumer,
and increases final profitability
for the producer
(HUNTON 1990).
From a commercial production
point of view, the loss of
unique avian germplasm has a
number of negative repercussions.
To start with, production
objectives and economic standards
are constantly changing,
particularly for meat production
birds. This means that agronomic
industries will continue
to need access to genetic diversity
to meet future market demands,
to adapt to adverse environmental
conditions, to fight
new diseases, and to meet the
demands for different nutritional
values. Thus, an effort
must be made to identify and
conserve all useful genetic resources
that could have comchicks
are all fast feathering. Such
chicks can be easily sexed at hatch time
by the relative feather growth by anyone
with a minimum of training. Previously
chicks were sexed using the vent
sexing method, whereby rudimentary
copulatory organs were examined to
determine sex. This was a costly procedure,
and at $0.03 per chick would
cost a hatchery $3,000 for every
100,000 chicks hatched. Over 600 million
egg-type chicks are hatched annually
in the US. If only half of these
are sexed by feather sexing, the
chicken industry saves over $9 million
per year. Broiler breeders are incorporating
this gene also, as sex-separate
rearing becomes more prevalent. With
over 9 billion broilers hatched in the
US each year, this also will have an economic
advantage to the industry.
Box 9. Economics of sex-linked genes and chicken genetics
IN 1908, SPILLMAN REPORTED that the female
was the heterogametic sex in
chickens (now described as ZW, as compared
to mammals where the male is
heterogametic, XY). This was based on
the finding that the barring gene was
inherited as a sex-linked gene, being
passed from the dam to her sons. This
early finding has played an important
role in commercial poultry breeding,
as many lines are now sexed at hatch
time using the sex-linked rate-of-feathering
gene. This gene influences development
of the early wing feathers
in the chick. If the dam carries the slow
feathering mutation, K, she passes this
on to all her sons, and her W chromosome
to her daughters. If the sire is
pure for the wild type gene, k+, all the
daughters receive the wild type fastfeathering
gene. The male chicks are
all slow feathering and the female
12
poultry breeds and varieties in North America,
particularly the old dual-purpose or mid-level
production breeds (Box 12). As the Leghorn
chicken, Rock-Cornish cross chicken, and
broad-breasted Large White turkey became the
dominant commercial birds, commercial breeders
could see no economic benefit to maintaining
other standard breeds and varieties of poultry
recognized by the American Poultry Association
(APA 1998). Today, without fanciers, it would be
very hard to find an Ancona or Silkie chicken or
a Royal Palm turkey. The Lamona chicken breed,
developed by the USDA, is a notable American
example of a once-useful old-fashioned production
strain now fallen from favor.
In some cases, access to mid-level stocks can
help small-scale producers stay in business.
While they cannot compete with the Rock-Cornish
meat cross or Leghorn egg-layer in the
highly commercial marketplaces, they can become
financially successful by raising some of
these heirloom birds to supply specialized niche
markets (Box 12). There are many other positive
aspects to this practice: small parcels of land
can remain agriculturally productive; open space
is maintained, family farmers are aided; and
moneys go into the local economy.
Biomedical researchers are starting to become
aware of the genetic reservoir available in
the fancy breeds. They usually seek specific
standard breeds or feather patterns that can be
used in exploring biological questions (see Chapter
3) or problems related to human medical disorders,
e.g., a form of vitiligo in barred chickens
(BOWERS et al. 1994). The Silkie breed (Figure 9)
is particularly useful, with six dominant mutations:
crest (Cr), rosecomb (R), muffs-and-beard
(Mb), polydactyly (Po), ptilopody (Pt), fibromelanosis
(Fm); and one recessive mutation, hookless
(h). These mutant alleles produce: elongated
feathers on the crown of the head (Cr) and on
the face and chin (Mb), a broad, flattened comb
that is covered with small, fleshy nodules (R),
extra toes (Po), feathered legs and feet (Pt), dark
skin, bones, and viscera (Fm), and loose, exceptionally
fluffy body feathers (h). Not only have
Figure 6. White Leghorn hen (Photo courtesy
of U.K. Abbott, University of CaliforniaЦDavis).
90% of all the egg-type chickens in
North America, and probably well over
half of the commercial egg-type chickens
worldwide.
Crosses among lines of the White
Leghorn (WL) breed produce nearly all
the commercially marketed white-shell
chicken eggs in North America. The WL
lines in use today stem from the purebred
stocks sold in the 1930s and
1940s. Though there has been intercrossing
in many cases to develop new
strains, many of the currently used
stocks appear to have been selected
without intermixing for 30 years or
more. Of particular note is the common
use of the Mount Hope strain,
which is distinguishable by its large
egg size and the B-19 and B-21 major
histocompatibility complex blood types
which it carries (for an explanation of
the major histocompatibility complex
(MHC) and B-blood types, see the section
on Immunogenetics in Chapter 3).
In response to regional consumer
preferences, several commercial
brown-eggshell chicken lines have also
been developed. Typically less efficient
than the White Leghorn strains, commercial
brown-eggshell chicken lines
are usually produced by crossing
Rhode Island Red males with high production
White Leghorn females. Alternatively,
some high egg production
strains of Rhode Island Red or Barred
Plymouth Rock may be used.
Box 10. Development of egg-laying stocks
COMMERCIAL EGG-LAYING chickens (Figure
6) have shown a substantial increase
in productivity in the past 60 years.
Some of this improvement has been
due to developments in the areas of
management, nutrition, and disease
control, but the effect of genetic improvement
is clear (ARTHUR 1986). Between
1940 and 1955, the number of
eggs laid per hen in the United States
increased from 134 to 192 (USDA-NASS
1998). By 1994, eggs per hen had increased
to 254. The change in the
1940s and 1950s was primarily due to
the introduction of hybrid stock, utilizing
pure breeds which had been under
development by numerous small breeders
participating in the National Poultry
Improvement Plan (NPIP). The more recent
increase has been primarily due to
selection for increased egg numbers.
However, it should be remembered that
the work of the small breeders and the
formal testing parameters set by NPIP
shaped the foundation stocks, paving the
way for the phenomenal performance in
the modern commercial
birds.
Today, only a few large
international poultry
breeding companies produce
most of the worldТs
commercial egg-type
chickens. Just 40 years
ago, the 1958-59 summary
of US randomsample-
egg-production
tests (ARS 1960) listed
132 breeding firms. In the
most recent egg-layer test
still conducted in North
America, (NORTH CAROLINA
COOPERATIVE EXTENSION SERVICE
1996) only five breeding
companies were listed.
These were actually
owned by just three international
firms. These
three firms breed over
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the hobby breeders helped in supplying such
research birds for one-time projects, but some of
them have participated in long-term breeding
programs for researchers.
Although the majority of exhibition and midlevel
production poultry breeds have continued
to exist under the rather informal stewardship of
the hobby breeders and the different breed organizations,
a number of problems are associated
with their conservation: 1) most of the amateur
conservators often only keep their stocks for a
short period of time (typically just five years);
2) small-scale hobby breeders who get breeding
stock from a central clearing house of poultry
Box 11. Development of meat-producing stocks
IN 1950, A COMMERCIAL BROILER took 84
days to grow to 1800 grams; by 1970,
this was cut to 59 days, and by 1988,
it was down to 43 days (from HUNTON
1990). As with the egg-type chickens,
a large proportion of the improved performance
of meat birds can be attributed
to developments in the areas of
management, nutrition, and disease
control. But choice of foundation
breeding stock and early use of breed
crosses were also important in the development
of the broiler industry.
More so than the egg market, the
broiler market is strongly consumerdriven
(POLLOCK 1999). Early consumer
input (chicken of tomorrow competitions
between 1946 and 1948) gave
the broiler-breeders and growers a
good picture of consumer preferences:
compact, well-fleshed carcasses at affordable
prices. In other words, the
scrawny, angular cockerels (Figure 7)
available in large numbers from eggselected
Single-comb White Leghorn
lines did not even approach the consumer
ideal. The broiler-breeders were
fortunate to have available the Cornish
breed (derived from fighting stock imported
from India), which had many of
the desired carcass characteristics. The
breeders also found that the production
characteristics (body type, rate of gain,
feed conversion) improved rapidly in response
to selection. Unfortunately, improvement
in these areas had a strong
negative effect on the already poor reproduction
characteristics of the Cornish
lines (low egg production, low fertility,
poor hatchability, reduced chick viability),
and seriously impaired disease resistance
(see section on immunogenetics,
Chapter 3). The early breeders found
that crossing the Cornish roosters with
hens from improved dual purpose breeds
solved many of these problems. These
"female" line breeds, including the Plymouth
Rock and New Hampshire, have
better body type than Single-comb White
Leghorns, yet lay eggs at a relatively high
rate compared to the Cornish "male"
lines. Today, the commercial sire is often
a cross between two
predominantly Cornish
strains, and the commercial
dam is a cross between
two strains descended
from one or more
of the dual-purpose breeds. The outbred
or crossbred parents have better
reproductive traits and general vigor
than parents from the pure-lines, and
their offspring, a three- or four-way
cross, exhibit even more hybrid vigor.
Unfortunately, despite careful evaluation
of the breeding stock, some serious
structural and physiological problems
have surfaced that appear to be
the result of the intense selection for
desirable production characteristics.
These include: leg weakness, cardiopulmonary
insufficiency, breast blisters,
increased fat deposition, and
muscle anomalies.
While the turkey industry is considerably
smaller than the broiler chicken
industry, many of the same breeding
methods have been used, and many
of the same problems have been encountered
(HUNTON 1990). With a
smaller genetic base, and a much
larger bird to start with (Figure 8), the
structural and physiological problems
found in chickens are often magnified
in turkeys. Considering the small number
of primary breeders (three) and the
scarcity of exhibition or research turkey
breeding stock, it is imperative to
safeguard the remaining genetic diversity
of this domestic species.
Figure 8. Commercial Large White turkey tom (Photo courtesy
of R.A. Ernst, University of CaliforniaЦDavis).
Figure 7. Traditional broiler-type chicken
carcass of the 1940s (Photo courtesy of F.A.
Bradley, University of CaliforniaЦDavis).
14
stocks may never know their egg source or the
degree of relationship of their foundation stock;
3) breeding populations are often very small,
particularly for the rarer breeds, and pedigree
information is frequently limited or not available;
4) some hobbyists deliberately inter-cross different
breeds or varieties in attempts to improve or
modify exhibition traits; 5) selection for production
characteristics (e.g., fertility, viability, egg
production, or disease resistance) may be largely
ignored in these small-scale breeding programs,
although de facto natural selection will tend to
eliminate the infertile, disease-susceptible, or
least-viable individuals; and 6) backyard breeders
tend to have problems in controlling diseases
and may have serious endemic diseases. If there
were a formal conservation program
for avian genetic resources,
it would be logical for it to
provide technical services to
these hobbyists who are a very
important component of avian
genetic resources conservation.
Figure 9. Silkie rooster from UCD Silkie (Photo courtesy
of J. Clark, University of CaliforniaЦDavis).
Box 12. Small renaissance of old-style chicken breeds
WITH THE INCREASING CULTURAL diversity
of our population, the white-feathered,
highly selected meat- or egg-producing
bird no longer meets the needs of
all consumers. In response to a great
demand by ethnic markets and the
many upscale restaurants searching for
the "chicken of yesterday", more and
more small producers are starting to
raise "old fashioned" mid-level production
or dual purpose breeds (those that
are reasonably efficient at producing
both meat and eggs). These producers
are getting their stocks from the
few people who still maintain populations
of true Rhode Island Reds, Speckled
Sussex, New Hampshires, and so
on. Those supplying the specialty egg
markets are also looking for different
breeds to produce a colored egg that
will be distinctive (brown, tan, green,
or blue), such as Orpington, Rhode Island
Red, and Ameraucana. Unfortunately,
most of these so-called midlevel
production breeds have all but
disappeared from American farms

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