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AAAAAvian genetic diversity: Domesticated speciesvian genetic diversity: Domesticated speciesvian genetic diversity: Domesticated speciesvian genetic diversity: Domesticated speciesvian genetic diversity: Domesticated species

GENETIC DIVERSITY IS CONSIDERED crucial to thecontinued survival of a species, be it wild or do-mestic. Such within-species diversity has beenthe raw material of agriculturists over millennia.In response to selective breeding and the differ-ential survival of less fit animals, preferred traitshave been accentuated and clustered to producedistinct breeds and varieties of the modern do-mesticated species (NRC 1993). In more recenttimes, researchers have deliberately isolatedvarious mutations in specialized stocks, permit-ting the systematic study of such mutations andpromoting a better understanding of the normalfunction of the affected genes.

The totality of wild and domesticated speciesform the gene pool or genetic resources basenecessary for the survival of the species. Thegenes and genotypes present in this pool repre-sent genetic resources which are accessible andcan be exploited by biologists and breeders. Inthis report, we emphasize “genetic stocks” whichhave been bred for specific traits and genes incontrast to breeds in which the individual birdshave many traits in common and can generallybe maintained with randomly breeding popula-tions. Genetic stocks are typically selected fortraits of special interest to breeders and geneti-cists. Many of them are reproductively, physi-cally, or physiologically compromised, and re-quire special care in breeding and management,even for maintenance or conservation purposes.

TTTTTarget speciesarget speciesarget speciesarget speciesarget speciesWhile the AGRTF recognizes the need for conser-vation of undomesticated avian species, this re-port primarily addresses the need for conserva-tion of specialty stocks of domesticated species,particularly chicken, turkey, and Japanesequail. A limited number of waterfowl (duck andgoose) genetic stocks and semi-domestic game-

bird stocks (ring-necked pheasant and bobwhitequail) have been developed and will be noted inthis report. Noted below are salient features ofthe most widely used domesticated species thathave the greatest need for conservation of ge-netic stocks.

Chicken

First domesticated over 6,000 years ago, thechicken (Gallus gallus or G. domesticus) presentsby far the greatest amount of genetic diversity ofthe domesticated avian species, with over 400identified genetic variations (SOMES 1988). Manyare showcased in the more-than 100 recognizedchicken breeds and commercial varieties, whichvariously integrate most of the naturally occur-ring mutations affecting size, body type, produc-tion characteristics, posture, color, featherstructure and location, comb shape, and behav-ior (see Figures 1 and 2 for wild- and domestic-type chickens). Some of the most extreme vari-ants include: the tiny, short-legged JapaneseBantam; the tall, aggressive Old English Game

Figure 1. Red Jungle Fowl rooster from UCD 001(Photo courtesy of J. Clark, University of California–Davis).

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popular with researchers or hobbyists than thechicken or the Japanese quail, at least sixbreeds are still kept for exhibition and a fewunique research stocks have been developed(Box 5), including several commercial-type long-term selected and randombred-control lines keptat Ohio State University (see survey results, Ap-pendix 2, Tables 2.1 and 2.2). Perhaps as a con-sequence of the few researchers studying theturkey, relatively few mutations (49) have beenreported in the turkey compared to the chickenand Japanese quail (SOMES 1988).

Japanese quail

Gaining in popularity as an experimental animalin both research and education, the Japanesequail (Coturnix japonica) is a small, early matur-ing, highly efficient egg and meat producer. Untilrecently, the Japanese quail was classified as a

hatch, and grow into fully functionalmales (OLSEN 1965). The existence ofthis line has given rise to the notionthat genetic imprinting does not existin birds, although this conclusion mustbe tentative in the absence of any for-mal investigation of imprinting in theunique parthenogenetic stock. How-ever, this parthenogenetic stock existsprecariously at only two research sta-tions in the world (the University ofGuelph (Ontario, Canada) and the Uni-versity of Oman).

Fowl; the light-weight Single-comb White Leghorn hen thatcan lay more than 300 eggs inher first year of production; andthe large Rock-Cornish commer-cial meat chicken, with its phe-nomenal rate of growth andwell-fleshed carcass. These areall thought to share a commonancestor in the Red Jungle Fowl(G. gallus gallus) (Figure 1)which is still found wild in partsof India and Southeast Asia(CRAWFORD 1990; FUMIHITO et al.1994), although some poultryspecialists believe that severalother jungle fowl species (G. sonnerati, G. lafay-ettei, and G. varius) also contributed to the an-cestral gene pool (CRAWFORD 1990).

Turkey

The one commercially important avian speciesoriginating in North America, the domestic tur-key of commerce, is the product of hybridizationbetween two subspecies of turkey: the domesti-cated Meleagris gallopavo gallopavo from CentralAmerica and the wild M. g. sylvestris from theeastern United States (CRAWFORD 1990). Fromthese hybrids, birds were selected for size, tame-ness, carcass yield, and rapid growth, resultingin several distinct breeds and varieties (Figure3). The modern commercial or exhibition turkeysare large, slow-maturing birds with a muchlower reproductive potential than chicken orJapanese quail (at one generation per year forthe turkey). Although this species is far less

Box 5. Parthenogenetic turkeys

PERHAPS THE MOST SPECTACULAR use of tur-keys in experimental biology was thestudy of meiosis, fertilization, and earlyembryonic development with a strainof parthenogenetic turkeys. In the1960s and 1970s, M.W. Olsen of theUnited States Department of Agricul-ture Agricultural Research Center atBeltsville developed a line of turkeysin which an embryo would form in 30to 50% of the unfertilized eggs (par-thenogenesis). Most of these embryosdie, but a small proportion of them(about 0.5%) continue to develop,

Figure 3. Flock with different turkey breeds (Photocourtesy of F.A. Bradley, University of California–Davis).

Figure 2. White Leghorn rooster from UCD 003 (Photocourtesy of J. Clark, University of California–Davis).

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subspecies of the common European quail (C.coturnix). It is now classified as a distinct speciesbecause of the nonhybridization of the two in thewild or in captivity (CHENG and KIMURA 1990).According to all available documentation, thedomestic Japanese quail strains used in meatand egg production (even in Europe) are descen-ded from C. japonica, which is still found insmall wild populations in Japan. While gainingpopularity as a food animal in the US, its smallsize has limited its use as a meat- or egg-pro-ducing animal to specialty markets. However,the Japanese quail has other qualities that makeit ideally suited for research. Usually reachingsexual maturity by six weeks of age, the femalesoften lay an egg a day for several months. Themales are aggressive breeders and maintain highfertility even when housed with four or morehens. The early maturity and short incubationinterval (16 to 17 days) permit as many as fivegenerations in a single year, in contrast to theslower-maturing chicken (one to two generationsper year) or the even slower maturing and lessproductive turkey (one generation per year). Thequail is sometimes called the mouse of the birdworld, since it has become extremely popular asa model species for biological research in severalfields, including toxicology, cell biology, nutri-tion, and selective animal breeding. Althoughmost researchers use unselected or randombredbirds, over 100 mutations are known in this spe-cies, including many affecting feather color andshape (Figures 4 and 5), and several causingembryo-lethal deformities (SOMES 1988; CHENGand KIMURA 1990). At present, most of these mu-tant strains are only maintained at the Univer-sity of British Columbia or by hobbyists. Twodrawbacks with this species are that the usual

productive life of an individual bird is quiteshort, frequently less than one year, and, unlikethe chicken, close inbreeding is not tolerated.Thus, only one moderately inbred line exists (atthe University of British Columbia).

Duck

Almost all of the 15 or so domestic duck breedsrecognized today are descended from the wildmallard duck (Anas platyrynchus platyrynchus),the exception being the Muscovy duck (Cairinamoschata) (LANCASTER 1990). In addition to thedifferent plumage patterns and colors, a varietyof body types and behavioral traits are foundamong the duck breeds, ranging from the boat-shaped, vocal Call ducks to the cane-shaped In-dian Runner ducks. Only 22 mutations havebeen described in the domestic duck, most in-volving feather color or pattern (LANCASTER1990). As such, these traits have been used indefining breed and variety standards, especiallyamong the more ornamental duck breeds, suchas the Call, Indian Runner, Crested, Cayuga,and Swedish. While such breeds are usuallyonly kept by hobbyists, a few are important incommercial meat production, particularly WhitePekins, Rouens, and, in some areas, Muscovy orMuscovy-domestic duck hybrids.

Goose

Six recognized domestic goose breeds were de-rived from the western Greylag goose of Europe(Anser anser anser). Several other breeds arethought to have descended from the smallerSwan goose of central Asia (A. cygnoides). TheAfrican 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 Co-lumbia).

Figure 5. Japanese quail porcupine mutation fromUBC PC-WB (Photo courtesy of K. Cheng, Universityof British Columbia).

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hybrid between these two species (HAWES 1990).Strict herbivores, geese have a long history ofdomestication, but their delayed maturity (twoyears) and low egg production rate make themless attractive as an experimental animal or as acommercially viable species (Box 6). However,due to the increasingly diverse consumer groupsin the US and Canada, formerly noncommercialspecies are becoming popular on a small scalefor specialty markets. One example is the de-mand from the Asian markets for a smaller, lessfatty meat goose. Until now, Europeans andNorth Americans have traditionally raised Emb-den geese for market purposes. This large-bod-ied, fatty bird is not well suited to the method ofcooking employed by Asian chefs. Therefore, wa-terfowl suppliers are now starting to grow thesmaller Chinese geese for this market.

Gamebirds

Several species of game birds are commonlybred commercially or by hobbyists, includingmany subspecies of the Ring-necked pheasant(Phasianus colchicus) and the Bobwhite quail(Colinus virginianus). Nine color mutations havebeen identified in the pheasant, along with sev-eral affecting skin color and feather structure,and 11 that produce biochemical polymorphisms(SOMES 1990). For most populations, very littleselective breeding or inbreeding is deliberatelypracticed, and the development of gamebirdstocks for genetic research is unusual. Excep-tions include the now-extinct inbred pheasantlines developed at the University of California–Davis (WOODARD et al. 1983) and the Bobwhite

and pheasant blood-type variants currently keptat Northern Illinois University (JARVI et al. 1996).

TTTTTypes of genetic stocksypes of genetic stocksypes of genetic stocksypes of genetic stocksypes of genetic stocksFor the purposes of this report, genetic stocksare classified into four categories that reflect thegenetic composition and type and the breedingsystem used to maintain them.

• Randombred

• Highly inbred

• Long-term selected

• Mutant (including cytogenetic variantsand transgenics)

We are primarily concerned in this reportwith conservation of genetic stocks developed forresearch purposes, which include all of thesecategories. Conservation of genetic stocks in thedifferent categories present different challengesfor successful conservation, including: high em-bryonic mortality, low viability, poor reproduc-tive traits, pronounced susceptibility to one ormore diseases, large and deleterious geneticload, poor response to specific environmentalstressors, poor recovery of cryopreserved semen,and need for a very large gene pool (more than100 birds per generation).

Randombred lines are maintained as relativelylarge populations of birds (usually over 100) inwhich little, if any, selection of breeding stock isdone by the curator. Quite simply, the numberof progeny from each male or female depends on

the reproductive success of thatbird at the time the eggs are col-lected to reproduce the popula-tion. Such randombred stocksare generally kept as closedflocks, although new bloodlinesmay be introduced to the popu-lation to improve the vigor of theflock, particularly if inbreedingdepression is observed. Thebirds may be reared and bred ina single large enclosure, with allmales having access to all fe-males. This is a common forpheasants, ducks, geese, somechickens, and naturally breed-ing turkey stocks. Alternatively,the birds may be randomly seg-regated into smaller floor pensor randomly paired or groupedin cages, as is common with

and improving egg production withcontrolled lighting and trapnesting.Early studies with Pilgrim geese (theonly goose breed that shows strongsexual dimorphism) included a demon-stration of increased egg productionwith selection (MERRITT 1962), and useof light control to increase egg produc-tion. More recently, artificial insemina-tion techniques have been improved(GRUNDER and PAWLUCZUK 1991) andgeese have been shown to lack endog-enous viruses (i.e., viral DNA integratedinto the host bird chromosomes) of theavian leukosis type (GRUNDER et al.1993). Unfortunately, with the loss offunding from the Canadian govern-ment in April of 1997, these stockswere either eliminated or dispersed.

Box 6. Research with goose breeds in Canada

WHILE GEESE ARE NOT COMMONLY used forexperimental purposes, a relativelylarge experimental population of geesewas maintained at the Center for FoodAnimal Research (CFAR) in Ottawa. Sev-eral distinct stocks of Chinese, Emb-den, and Chinese X Pilgrim hybridgeese were developed in Ottawa tostudy production traits and DNA fin-gerprinting patterns (GRUNDER et al.1994). The stocks included standardbreed control strains, stocks selectedfor multiple traits, and the unselectedreference strains (SOMES 1988). As withchicken and turkey research in the earlypart of this century, most studies withthe relatively undeveloped pure andcrossed goose varieties have been re-lated to agricultural objectives, includ-ing methods of rearing broiler geese

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Japanese quail. A minimum of 25 pairs, usuallymore than 150 birds, is needed to keep inbreed-ing at a minimum. These stocks are often keptas a source of “normal” control birds, and alsofunction as a resource stock, from which inbredor selected stocks can be derived or qualitativemutations isolated.

Inbred lines are produced by breeding togetherclose relatives for many generations, resulting inincreasingly homozygous and homogeneousprogenies. Different types of mating schemes areused, depending on how rapidly the researcheris attempting to approach complete homozygos-ity. Disregarding parthenogenesis, the mostrapid inbreeding is produced by father-daughter,mother-son, or brother-sister (full-sib) matings.These breeding schemes can also be used to ex-pose deleterious recessive traits or to fix pre-ferred or beneficial single-gene traits in a popu-lation. Unfortunately, even in the absence of ma-jor genetic defects, the fertility and viability ofthe inbred offspring are almost always lowerthan the more outbred parent strain, a charac-teristic called inbreeding depression. If selectionand breeding strategies do not compensate forthis decline, inbreeding depression can result inthe extinction of the line within a few genera-tions. This is a particular concern in lines propa-gated by full-sib matings which also have largegenetic loads (many deleterious alleles). How-ever, once the lethal and sub-vital alleles havebeen purged from the inbred strain, it can theo-retically be bred to essentially complete homozy-gosity while maintaining reasonable reproductiveperformance traits (fertility, egg hatchability, eggproduction rates, viability, etc.). Such geneticallyuniform stocks can then be used as a standardgenetic background in the study of individualgenes and gene complexes. Par-ticularly useful inbred lines arethose which have been bred forcontrasting phenotypes due toallelic differences at single loci.These lines, having the samegenetic background for practi-cally all loci except for the alle-les of interest, are calledcongenic lines. They are used tostudy single-gene effects on pro-ductivity, for molecular charac-terization of genes affecting de-velopmental traits or diseaseresistance, and many other usesin basic biological and biomedi-cal research (ABPLANALP 1992).

Highly inbred genetic stocks are invaluable ina wide range of research fields, particularlygenomics (gene mapping) and immunogenetics(Box 7). A good example of the usefulness of in-bred strains in genomics is the mapping of clas-sical mutations. While over 80 classically identi-fied genetic mutations have been assigned to thechicken linkage map, only a few have been lo-cated on the molecular map. This is due to thelack of genetic characterization of the exhibitionbreeds and lines in which most of these muta-tions are found. Such nonuniform genetic back-grounds make them difficult to use in matingsdesigned to integrate the maps. In contrast,congenic lines, mentioned above, are uniquelyuseful for such genetic mapping. The integrationof genes in exhibition breeds into defined inbredlines would provide the necessary uniformity formolecular mapping of these traits.

Long-term selected stocks are the result ofmany generations of testing and selective breed-ing for traits governed by multiple genes (the so-called quantitative or polygenic traits). Many val-ued heritable characteristics in the poultrybreeds belong in this category. These include eggproduction rate, egg size, feed efficiency, fertility,hatchability, viability, disease resistance, bodysize and shape, and behavioral characteristics.To change the population mean for one or moreof these quantitative traits requires rigoroustesting and ranking of the individuals and familygroups for the traits-of-interest each generation,followed by selective breeding of the higher-ranked individuals and families to produce thenext generation. Many factors can affect the rateof improvement in response to selection includ-ing 1) degree of heritability of the trait or traitsinvolved, 2) selection stringency, 3) level of in-

Box 7. Highly inbred stocks in immunogenetics

BASIC INFORMATION ABOUT FACTORS control-ling disease resistance in the chickenhas been gathered largely from stud-ies with congenic strains of chickens(birds with identical, highly inbredbackgrounds but different major his-tocompatibility complex (MHC)haplotypes; ABPLANALP 1992). A num-ber of these congenic strains have beendeveloped at the University of Califor-nia–Davis, the USDA Avian Disease andOncology Laboratory in East Lansing,MI, the University of New Hampshire,and Iowa State University. Researchershave shown how each MHC-haplotypecould directly affect the resistance ofa bird to a variety of different diseases,

including coccidiosis, Newcastle’s dis-ease, and the tumor-inducing virusesthat cause Marek’s disease and lym-phoid leukosis. The congenic MHCstrains, most requiring at least ten gen-erations of back-crossing and blood-testing to develop, are key resourcesrequired for furthering our understand-ing of the way the MHC genes func-tion. Studies with these stocks have al-ready given the primary poultry breed-ers vital information to use in deter-mining the best of several alternativebreeding strategies to enhance diseaseresis-tance potential of their produc-tion stocks.

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breeding, and 4) genetic variation in the originalsource population. Selected stocks usually re-quire several generations to develop, tend to re-vert towards the original stock values if the se-lection pressure is lifted (i.e., if random or non-selected pedigree reproduction is used), andusually need to be reproduced in large numbers(several hundred birds) each generation for thebest selection differential with minimized in-breeding.

Mutant stocks incorporate one or more of themany single-gene mutations that have a majoreffect on specific morphological or physiologicaltraits. These include variants (alleles) that affecteggshell color, feather color or shape, skin color,comb shape, metabolic function, major histo-compatibility complex (MHC) haplotype identity,and pattern formation in the developing embryo.The wide array of mutations affecting feathercolor and shape are important for distinguishingbetween breeds and varieties within breeds. Inthe poultry industry, eggshell color, skin color,feather color, and feathering rate mutations,and, more recently, MHC types, have all playedimportant roles in the development of commer-cial strains and varieties. Of particular interestto biomedical researchers are those mutationsthat cause disease conditions that mimic humangenetic disorders, including muscular dystrophy,scoliosis, scleroderma, and a variety of develop-mental mutations (usually lethal) that affect thedevelopment of the face, limbs, integument, andinternal organs.

Cytogenetic variants are birds that have chro-mosomal abnormalities, such as aneuploidy,polyploidy, translocations, and large insertionsor deletions. A small number have been estab-lished in the chicken, and these have provideduseful model systems for the study of meiosis,inheritance, recombination, linkage, transcrip-tional regulation, and gene dos-age effects. Such stocks in-clude: aneuploidy for the chro-mosome encoding the MHC andnucleolar organizer region(NOR), complete triploidy (threecopies, instead of two, of allchromosomes), large deletions(the mPNU line, in which thereis segregation of an MHC/NORchromosome with a deletedNOR), and various stocks carry-ing translocations betweenmacrochromosomes (Box 8).

Transgenic stocks areformed by inserting foreign

DNA, usually containing a gene of interest, intoone of the chromosomes of germline or somaticcells. While some transgenic chickens have beenproduced in the past few years (SALTER et al.1986; 1987; SALTER and CRITTENDEN 1989), thecreation of transgenics is still very much experi-mental in chickens and other avian species.However, a number of research groups continueto develop and refine transgenic methodologies,and report promising advances in the productionof transgenic birds (SALTER et al. 1987; LOVE etal. 1994; THORAVAL et al. 1995; MARUYAMA et al.1998).

RRRRResearch genetic stocksesearch genetic stocksesearch genetic stocksesearch genetic stocksesearch genetic stocksGenetic stocks are used in three areas of re-search: agricultural, biomedical, and basic orfundamental biological research.

Agriculturally important avian genetic stocksprimarily include those selected for various pro-duction-related characteristics (egg production,body shape, feed-use efficiency, leg strength, dis-ease resistance). Another use for such stocks isto provide a flexible, rapidly responding modelsystem for testing breeding techniques and sys-tems that might also be useful with large live-stock species (e.g., pigs, sheep, and cattle). Thesestocks are particularly vulnerable to fundingcuts due to the long development period neededfor most selected stocks, and the relatively largenumbers that must be produced and monitoredannually to produce the selected population.

Biomedical research specifically uses animalmodels for the study of various human diseases.Avian models, mostly in the chicken, exist forthe autoimmune forms of vitiligo, scleroderma,and thyroiditis, as well as for various develop-mental defects, such as polydactyly, scoliosis,and cleft palate. Genetic stocks are also used inavian health research for studying the nature of

FROM THE MID-1960S TO THE 1980s, ani-mal genetics laboratories at Ohio StateUniversity, the University of Minnesota,and New Mexico State University de-veloped about 40 different chromo-some rearrangement strains in thechicken (ZARTMAN 1971; WOOSTER et al.1977; WANG et al. 1982). A number ofstudies by these laboratories made im-portant contributions to our under-standing of chromosome behavior inavian species (including recombination,chromosome segregation, identifica-tion of pseudoautosomal regions on

Box 8. Chicken chromosome rearrangement stocks

the sex chromosomes, and sources ofaneuploids). Unfortunately, with thelack of support by various agenciesover the last ten years, over thirty ofthese unique genetic resources wereirretrievably lost. The seven still in ex-istence, along with a recently isolatedspontaneous translocation, are cur-rently being maintained at the Univer-sity of Wisconsin. However, with de-partmental reorganizations and bud-getary difficulties, these stocks are alsothreatened.

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mercial importance, such as the sex-linked genecontroling the rate of feather growth that hasbeen heavily utilized by modern chicken breed-ers (Box 9).

In marked contrast to the general perceptionthat commercial poultry stocks all have a rela-tively small and diminishing genetic base, someresearchers have reported the opposite. Specifi-cally, DUNNINGTON et al. (1994) used DNA finger-printing to measure variability among commer-cial chicken breeding populations and concludedthat a considerable reservoir of genetic diversityyet remained. IRAQI et al. (1991) reported a greatdegree of polymorphism for endogenous viral (ev)genes in five egg-type populations maintained byan Israeli commercial breeder. AARTS et al. (1991)also found variation for ev genes among andwithin six WL and four medium-heavy browneggshell lines. While none of these methods spe-cifically reflects the variation remaining in genesassociated with economically important traits,the recent substantial progress in the develop-ment of the genetic map of the chicken (CHENG etal. 1995; CHENG 1997) should soon lead to morethorough and realistic assessment of the amountof economic trait variability remaining in com-mercial poultry populations.

FFFFFancy breeds and mid-levelancy breeds and mid-levelancy breeds and mid-levelancy breeds and mid-levelancy breeds and mid-levelproduction stocksproduction stocksproduction stocksproduction stocksproduction stocks

For at least 50 years, poultry fanciers have beenthe main conservators of the majority of the

disease resistance and effects of specific geneson productivity under disease stresses.

Most of the genetic stocks are of value forstudying questions in basic biology that maylead to more applied biomedical or agriculturalresearch, or by simply contributing to the knowl-edge of how different biological systems functionin a wide variety of studies in the life sciences.

Some of the specialized stocks have beenderived directly from commercial chicken, tur-key, or Japanese quail lines, while others weredeveloped from special breeds, landraces, orwild-types.

Commercial stocksCommercial stocksCommercial stocksCommercial stocksCommercial stocksThe commercial poultry stocks have made re-markable genetic progress in the last 50 years(Boxes 9, 10, and 11). At this time, selectedstocks used in commercial egg or meat produc-tion must fit very specialized production criteria.To develop these criteria, each breeding companyhas identified particular commercial goals (eggproduction, weight gain, feed conversion, carcasscharacteristics, etc.) and seeks to meet them inthe shortest possible time (EMSLEY 1993). In thisway, the fundamental difference between basicand applied research is highlighted. While a re-searcher may have a preferred outcome for anexperiment, any result can provide useful infor-mation to that researcher or others in the re-search community. For the commercial breeder,the only outcome that is acceptable is one thatimproves the commercial product for the con-sumer, and increases final prof-itability for the producer(HUNTON 1990).

From a commercial produc-tion point of view, the loss ofunique avian germplasm has anumber of negative repercus-sions. To start with, productionobjectives and economic stan-dards are constantly changing,particularly for meat productionbirds. This means that agro-nomic industries will continueto need access to genetic diver-sity to meet future market de-mands, to adapt to adverse en-vironmental conditions, to fightnew diseases, and to meet thedemands for different nutri-tional values. Thus, an effortmust be made to identify andconserve all useful genetic re-sources that could have com-

chicks are all fast feathering. Suchchicks can be easily sexed at hatch timeby the relative feather growth by any-one with a minimum of training. Previ-ously chicks were sexed using the ventsexing method, whereby rudimentarycopulatory organs were examined todetermine sex. This was a costly pro-cedure, and at $0.03 per chick wouldcost a hatchery $3,000 for every100,000 chicks hatched. Over 600 mil-lion egg-type chicks are hatched an-nually in the US. If only half of theseare sexed by feather sexing, thechicken industry saves over $9 millionper year. Broiler breeders are incorpo-rating this gene also, as sex-separaterearing becomes more prevalent. Withover 9 billion broilers hatched in theUS each year, this also will have an eco-nomic advantage to the industry.

Box 9. Economics of sex-linked genes and chicken genetics

IN 1908, SPILLMAN REPORTED that the fe-male was the heterogametic sex inchickens (now described as ZW, as com-pared to mammals where the male isheterogametic, XY). This was based onthe finding that the barring gene wasinherited as a sex-linked gene, beingpassed from the dam to her sons. Thisearly finding has played an importantrole in commercial poultry breeding,as many lines are now sexed at hatchtime using the sex-linked rate-of-feath-ering gene. This gene influences de-velopment of the early wing feathersin the chick. If the dam carries the slowfeathering mutation, K, she passes thison to all her sons, and her W chromo-some to her daughters. If the sire ispure for the wild type gene, k+, all thedaughters receive the wild type fast-feathering gene. The male chicks areall slow feathering and the female

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poultry breeds and varieties in North America,particularly the old dual-purpose or mid-levelproduction breeds (Box 12). As the Leghornchicken, Rock-Cornish cross chicken, andbroad-breasted Large White turkey became thedominant commercial birds, commercial breed-ers could see no economic benefit to maintainingother standard breeds and varieties of poultryrecognized by the American Poultry Association(APA 1998). Today, without fanciers, it would bevery hard to find an Ancona or Silkie chicken ora Royal Palm turkey. The Lamona chicken breed,developed by the USDA, is a notable Americanexample of a once-useful old-fashioned produc-tion strain now fallen from favor.

In some cases, access to mid-level stocks canhelp small-scale producers stay in business.While they cannot compete with the Rock-Cor-nish meat cross or Leghorn egg-layer in thehighly commercial marketplaces, they can be-come financially successful by raising some ofthese heirloom birds to supply specialized nichemarkets (Box 12). There are many other positive

aspects to this practice: small parcels of landcan remain agriculturally productive; open spaceis maintained, family farmers are aided; andmoneys go into the local economy.

Biomedical researchers are starting to be-come aware of the genetic reservoir available inthe fancy breeds. They usually seek specificstandard breeds or feather patterns that can beused in exploring biological questions (see Chap-ter 3) or problems related to human medical dis-orders, e.g., a form of vitiligo in barred chickens(BOWERS et al. 1994). The Silkie breed (Figure 9)is particularly useful, with six dominant muta-tions: crest (Cr), rosecomb (R), muffs-and-beard(Mb), polydactyly (Po), ptilopody (Pt), fibromel-anosis (Fm); and one recessive mutation, hook-less (h). These mutant alleles produce: elongatedfeathers on the crown of the head (Cr) and onthe face and chin (Mb), a broad, flattened combthat is covered with small, fleshy nodules (R),extra toes (Po), feathered legs and feet (Pt), darkskin, bones, and viscera (Fm), and loose, excep-tionally fluffy body feathers (h). Not only have

Figure 6. White Leghorn hen (Photo courtesyof U.K. Abbott, University of California–Davis).

90% of all the egg-type chickens inNorth America, and probably well overhalf of the commercial egg-type chick-ens worldwide.

Crosses among lines of the WhiteLeghorn (WL) breed produce nearly allthe commercially marketed white-shellchicken eggs in North America. The WLlines in use today stem from the pure-bred stocks sold in the 1930s and1940s. Though there has been inter-crossing in many cases to develop newstrains, many of the currently usedstocks appear to have been selectedwithout intermixing for 30 years ormore. Of particular note is the com-mon use of the Mount Hope strain,which is distinguishable by its largeegg size and the B-19 and B-21 majorhistocompatibility complex blood typeswhich it carries (for an explanation ofthe major histocompatibility complex(MHC) and B-blood types, see the sec-tion on Immunogenetics in Chapter 3).

In response to regional consumerpreferences, several commercialbrown-eggshell chicken lines have alsobeen developed. Typically less efficientthan the White Leghorn strains, com-mercial brown-eggshell chicken linesare usually produced by crossingRhode Island Red males with high pro-duction White Leghorn females. Alter-natively, some high egg productionstrains of Rhode Island Red or BarredPlymouth Rock may be used.

Box 10. Development of egg-laying stocks

COMMERCIAL EGG-LAYING chickens (Figure6) have shown a substantial increasein productivity in the past 60 years.Some of this improvement has beendue to developments in the areas ofmanagement, nutrition, and diseasecontrol, but the effect of genetic im-provement is clear (ARTHUR 1986). Be-tween 1940 and 1955, the number ofeggs laid per hen in the United Statesincreased from 134 to 192 (USDA-NASS1998). By 1994, eggs per hen had in-creased to 254. The change in the

1940s and 1950s was primarily due tothe introduction of hybrid stock, utiliz-ing pure breeds which had been underdevelopment by numerous small breed-ers participating in the National PoultryImprovement Plan (NPIP). The more re-cent increase has been primarily due toselection for increased egg numbers.However, it should be remembered thatthe work of the small breeders and theformal testing parameters set by NPIPshaped the foundation stocks, paving theway for the phenomenal performance in

the modern commercialbirds.

Today, only a few largeinternational poultrybreeding companies pro-duce most of the world’scommercial egg-typechickens. Just 40 yearsago, the 1958-59 sum-mary of US random-sample-egg-productiontests (ARS 1960) listed132 breeding firms. In themost recent egg-layer teststill conducted in NorthAmerica, (NORTH CAROLINA

COOPERATIVE EXTENSION SER-VICE 1996) only five breed-ing companies were listed.These were actuallyowned by just three inter-national firms. Thesethree firms breed over

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the hobby breeders helped in supplying suchresearch birds for one-time projects, but some ofthem have participated in long-term breedingprograms for researchers.

Although the majority of exhibition and mid-level production poultry breeds have continuedto exist under the rather informal stewardship of

the hobby breeders and the different breed orga-nizations, a number of problems are associatedwith their conservation: 1) most of the amateurconservators often only keep their stocks for ashort period of time (typically just five years);2) small-scale hobby breeders who get breedingstock from a central clearing house of poultry

Box 11. Development of meat-producing stocks

IN 1950, A COMMERCIAL BROILER took 84days 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 per-formance of meat birds can be attrib-uted to developments in the areas ofmanagement, nutrition, and diseasecontrol. But choice of foundationbreeding stock and early use of breedcrosses were also important in the de-velopment of the broiler industry.

More so than the egg market, thebroiler market is strongly consumer-driven (POLLOCK 1999). Early consumerinput (chicken of tomorrow competi-tions between 1946 and 1948) gavethe broiler-breeders and growers agood picture of consumer preferences:compact, well-fleshed carcasses at af-fordable prices. In other words, thescrawny, angular cockerels (Figure 7)available in large numbers from egg-selected Single-comb White Leghornlines did not even approach the con-

sumer ideal. The broiler-breeders werefortunate to have available the Cornishbreed (derived from fighting stock im-ported from India), which had many ofthe desired carcass characteristics. Thebreeders also found that the productioncharacteristics (body type, rate of gain,feed conversion) improved rapidly in re-sponse to selection. Unfortunately, im-provement in these areas had a strongnegative effect on the already poor re-production characteristics of the Cornishlines (low egg production, low fertility,poor hatchability, reduced chick viabil-ity), and seriously impaired disease re-sistance (see section on immunogenet-ics, Chapter 3). The early breeders foundthat crossing the Cornish roosters withhens from improved dual purpose breedssolved many of these problems. These“female” line breeds, including the Ply-mouth Rock and New Hampshire, havebetter body type than Single-comb WhiteLeghorns, yet lay eggs at a relatively highrate compared to the Cornish “male”lines. Today, the commercial sire is of-

ten a cross between twopredominantly Cornishstrains, and the commer-cial dam is a cross be-tween two strains de-scended from one or more

of the dual-purpose breeds. The out-bred or crossbred parents have betterreproductive traits and general vigorthan parents from the pure-lines, andtheir offspring, a three- or four-waycross, exhibit even more hybrid vigor.Unfortunately, despite careful evalua-tion of the breeding stock, some seri-ous structural and physiological prob-lems have surfaced that appear to bethe result of the intense selection fordesirable production characteristics.These include: leg weakness, cardio-pulmonary insufficiency, breast blis-ters, increased fat deposition, andmuscle anomalies.

While the turkey industry is consid-erably smaller than the broiler chickenindustry, many of the same breedingmethods have been used, and manyof the same problems have been en-countered (HUNTON 1990). With asmaller genetic base, and a muchlarger bird to start with (Figure 8), thestructural and physiological problemsfound in chickens are often magnifiedin turkeys. Considering the small num-ber of primary breeders (three) and thescarcity of exhibition or research tur-key breeding stock, it is imperative tosafeguard the remaining genetic diver-sity of this domestic species.

Figure 8. Commercial Large White turkey tom (Photo cour-tesy of R.A. Ernst, University of California–Davis).

Figure 7. Traditional broiler-type chickencarcass of the 1940s (Photo courtesy of F.A.Bradley, University of California–Davis).

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stocks may never know their egg source or thedegree of relationship of their foundation stock;3) breeding populations are often very small,particularly for the rarer breeds, and pedigreeinformation is frequently limited or not available;4) some hobbyists deliberately inter-cross differ-ent breeds or varieties in attempts to improve ormodify exhibition traits; 5) selection for produc-tion characteristics (e.g., fertility, viability, eggproduction, or disease resistance) may be largelyignored in these small-scale breeding programs,although de facto natural selection will tend toeliminate the infertile, disease-susceptible, orleast-viable individuals; and 6) backyard breed-ers tend to have problems in controlling diseasesand may have serious endemic diseases. If therewere a formal conservation pro-gram for avian genetic resour-ces, it would be logical for it toprovide technical services tothese hobbyists who are a veryimportant component of aviangenetic resources conservation.

Figure 9. Silkie rooster from UCD Silkie (Photo cour-tesy of J. Clark, University of California–Davis).

Box 12. Small renaissance of old-style chicken breeds

WITH THE INCREASING CULTURAL diversityof our population, the white-feathered,highly selected meat- or egg-produc-ing bird no longer meets the needs ofall consumers. In response to a greatdemand by ethnic markets and themany upscale restaurants searching forthe “chicken of yesterday”, more andmore small producers are starting toraise “old fashioned” mid-level produc-tion or dual purpose breeds (those thatare reasonably efficient at producingboth meat and eggs). These produc-

ers are getting their stocks from thefew people who still maintain popula-tions of true Rhode Island Reds, Speck-led Sussex, New Hampshires, and soon. Those supplying the specialty eggmarkets are also looking for differentbreeds to produce a colored egg thatwill be distinctive (brown, tan, green,or blue), such as Orpington, Rhode Is-land Red, and Ameraucana. Unfortu-nately, most of these so-called mid-level production breeds have all butdisappeared from American farms.