The chicken major histocompatibility complex in disease resistance and poultry breeding

Genetic Diversity of MHC B-F/B-L Region in 21 Chicken Populations

The chicken major histocompatibility complex (MHC) on chromosome 16 is the most polymorphic region across the whole genome, and also an ideal model for genetic diversity investigation. The MHC B-F/B-L region is 92 kb in length with high GC content consisting of 18 genes and one pseudogene (Blec4), which plays important roles in immune response. To evaluate polymorphism of the Chinese indigenous chickens as well as to analyze the effect of selection to genetic diversity, we used WaferGen platform to identify sequence variants of the B-F/B-L region in 21 chicken populations, including the Red Jungle Fowl (RJF), Cornish (CS), White Leghorns (WLs), 16 Chinese domestic breeds, and two well-known inbred lines 63 and 72. A total of 3,319 single nucleotide polymorphism (SNPs) and 181 INDELs in the B-F/B-L region were identified among 21 populations, of which 2,057 SNPs (62%) and 159 INDELs (88%) were novel. Most of the variants were within the intron and the flanking regions. The average variation density was 36 SNPs and 2 INDELs per kb, indicating dramatical high diversity of this region. Furthermore, BF2 was identified as the hypervariable genes with 67 SNPs per kb. Chinese domestic populations showed higher diversity than the WLs and CS. The indigenous breeds, Nandan Yao (NY), Xishuangbanna Game (XG), Gushi (GS), and Xiayan (XY) chickens, were the top four with the highest density of SNPs and INDELs. The highly inbred lines 63 and 72 have the lowest diversity, which might be resulted from a long-term intense selection for decades. Collectively, we refined the genetic map of chicken MHC B-F/B-L region, and illustrated genetic diversity of 21 chicken populations. Abundant genetic variants were identified, which not only strikingly expanded the current Ensembl SNP database, but also provided comprehensive data for researchers to further investigate association between variants in MHC and immune traits.

RNA sequencing demonstrates large-scale temporal dysregulation of gene expression in stimulated macrophages derived from MHC-defined chicken haplotypes

Discovering genetic biomarkers associated with disease resistance and enhanced immunity is critical to developing advanced strategies for controlling viral and bacterial infections in different species. Macrophages, important cells of innate immunity, are directly involved in cellular interactions with pathogens, the release of cytokines activating other immune cells and antigen presentation to cells of the adaptive immune response. IFNγ is a potent activator of macrophages and increased production has been associated with disease resistance in several species. This study characterizes the molecular basis for dramatically different nitric oxide production and immune function between the B2 and the B19 haplotype chicken macrophages.A large-scale RNA sequencing approach was employed to sequence the RNA of purified macrophages from each haplotype group (B2 vs. B19) during differentiation and after stimulation. Our results demonstrate that a large number of genes exhibit divergent expression between B2 and B19 haplotype cells both prior and after stimulation. These differences in gene expression appear to be regulated by complex epigenetic mechanisms that need further investigation.

Dramatic differences in the response of macrophages from B2 and B19 MHC-defined haplotypes to interferon gamma and polyinosinic:polycytidylic acid stimulation

The chicken MHC has been associated with disease resistance, though the mechanisms are not understood. The functions of macrophages, critical to both innate and acquired immunity, were compared between the more infectious bronchitis virus-resistant B2 and the more infectious bronchitis virus-susceptible B19 lines. In vivo peripheral blood concentrations of monocytes were similar in B2 or B19 homozygous haplotypes. Peripheral blood-derived macrophages were stimulated with poly I:C, simulating an RNA virus, or IFNγ, a cytokine at the interface of innate and adaptive immunity. Not only did B2-derived peripheral monocytes differentiate into macrophages more readily than the B19 monocytes, but as determined by NO production, macrophages from B2 and B2 on B19 genetic background chicks were also significantly more responsive to either stimulant. In conclusion, the correlation with resistance to illness following viral infection may be directly linked to a more vigorous innate immune response.

Genetic diversity of the major histocompatibility complex region in commercial and noncommercial chicken flocks using the LEI0258 microsatellite marker

Microsatellite marker LEI0258 was used as an indicator to examine the variability of the major histocompatibility complex (MHC) region in 2 commercial layer flocks, 1 experimental layer cross, and 5 noncommercial flocks (used for free-run and free-range meat and egg production). We hypothesized that the populations from noncommercial sources may have more diversity in MHC genes than that in the commercial-source populations. Two related parameters, heterozygosity and the number of alleles harbored by a population, were used to assess the genetic variability. The different combinations of the 22 alleles created 66 genotypes in the 8 chicken populations that were studied. The noncommercial populations, except for the Silkies (SK), harbored more alleles than those in the 2 commercial populations, Lohmann Brown and Lohmann White. The observed heterozygosity of the MHC region was high in all of the populations, except for SK. Considering the 2 parameters we have examined, we can generalize that the intensively selected commercial egg-layer varieties seem to have less genetic variability in their MHC regions compared with that of the noncommercial flocks, which are less intensively selected. The LEI0258 variants can be used as markers to detect most of the MHC haplotypes, but in the different populations the same allele size may not always be associated with the same serologically defined haplotype. The information obtained from this study will be useful for genetic resource conservation and the development of breeding stocks that are suitable for free-range production.

A global analysis of molecular markers and phenotypic traits in local chicken breeds in Taiwan

Molecular and phenotypic data have been combined to characterize the genetic diversity of six local chicken breeds maintained with a long-term conservation programme. Hua-Tung, Hsin-Yi, Ju-Chi and Quemoy originated from Taiwan, Shek-Ki is from South China, and Nagoya is from Japan. Molecular tools included 24 microsatellite markers, melanocortin 1 receptor (alpha melanocyte stimulating hormone receptor) (MC1R), the LEI0258 marker located within the major histocompatibility complex (MHC), and mitochondrial DNA. Performance was recorded on the same individuals for body weight, panting rate in summer and antibody response (antigens: Newcastle disease virus and sheep red blood cells). A multivariate method previously proposed for taxonomy was used to combine the different data sets. Melanocortin 1 receptor (alpha melanocyte stimulating hormone receptor) and the MCW330 marker contributed the most to the first axis of the multiple coinertia analysis of molecular markers. Melanocortin 1 receptor (alpha melanocyte stimulating hormone receptor) showed evidence of selection, probably related to its effect on feather colour. The MHC exhibited a large diversity, with 16 alleles of the LEI0258 marker. Immune response traits contributed the most to the principal component analysis of phenotypic data. Eight mitochondrial DNA haplotypes related to clades A, B, C and E were distributed across breeds and revealed an important contribution of Indian and European breeds to Ju-Chi, Quemoy and Hsin-Yi. Phenotypic data contributed less than molecular data to the combined analysis, and two markers, LEI0258 and LEI0228, contributed the most. The combined analysis could clearly discriminate all breeds, except Ju-Chi, which was similar to Quemoy for many criteria, except immune response.

Chromosomal mapping and candidate gene discovery of chicken developmental mutants and genome-wide variation analysis of MHC congenics

The chicken has been widely used in experimental research given its importance to agriculture and its utility as a model for vertebrate biology and biomedical pursuits for over 100 years. Herein we used advanced technologies to investigate the genomic characteristics of specialized chicken congenic genetic resources developed on a highly inbred background. An Illumina 3K chicken single nucleotide polymorphism (SNP) array was utilized to study variation within and among major histocompatibility complex (MHC)-congenic lines as well as investigate line-specific genomic diversity, inbreeding coefficients, and MHC B haplotype-specific GGA 16 SNP profiles. We also investigated developmental mutant-congenic lines to map a number of single-gene mutations using both the Illumina 3K array and a recently developed Illumina 60K chicken SNP array. In addition to identifying the chromosomes and specific subregions, the mapping results affirmed prior analyses indicating recessive or dominant and autosomal or sex chromosome modes of inheritance. Priority candidate genes are described for each mutation based on association with similar phenotypes in other vertebrates. These single-gene mutations provide a means of studying amniote development and in particular serve as invaluable biomedical models for similar malformations found in human.

MHC haplotype and susceptibility to experimental infections (Salmonella Enteritidis, Pasteurella multocida or Ascaridia galli) in a commercial and an indigenous chicken breed

In three independent experimental infection studies, the susceptibility and course of infection of three pathogens considered of importance in most poultry production systems, Ascaridia galli, Salmonella Enteritidis and Pasteurella multocida were compared in two chicken breeds, the indigenous Vietnamese Ri and the commercial Luong Phuong. Furthermore, the association of the Major Histocompatibility Complex (MHC) with disease-related parameters was evaluated, using alleles of the LEI0258 microsatellite as markers for MHC haplotypes. The Ri chickens were found to be more resistant to A. galli and S. Enteritidis than commercial Luong Phuong chickens. In contrast, the Ri chickens were more susceptible to P. multocida, although production parameters were more affected in the Luong Phuong chickens. Furthermore, it was shown that the individual variations observed in response to the infections were influenced by the MHC. Using marker alleles of the microsatellite LEI0258, which is located within the MHC region, several MHC haplotypes were identified as being associated with infection intensity of A. galli. An association of the MHC with the specific antibody response to S. Enteritidis was also found where four MHC haplotypes were shown to be associated with high specific antibody response. Finally, one MHC haplotype was identified as being associated with pathological lesions and mortality in the P. multocida experiment. Although not statistically significant, our analysis suggested that this haplotype might be associated with resistance. These results demonstrate the presence of local genetic resources in Vietnamese chickens, which could be utilized in breeding programmes aiming at improving disease resistance.

Mapping QTL affecting resistance to Marek's disease in an F6 advanced intercross population of commercial layer chickens

BACKGROUND

Marek's disease (MD) is a T-cell lymphoma of chickens caused by the Marek's disease virus (MDV), an oncogenic avian herpesvirus. MD is a major cause of economic loss to the poultry industry and the most serious and persistent infectious disease concern. A full-sib intercross population, consisting of five independent families was generated by crossing and repeated intercrossing of two partially inbred commercial White Leghorn layer lines known to differ in genetic resistance to MD. At the F6 generation, a total of 1615 chicks were produced (98 to 248 per family) and phenotyped for MD resistance measured as survival time in days after challenge with a very virulent plus (vv+) strain of MDV.

RESULTS

QTL affecting MD resistance were identified by selective DNA pooling using a panel of 15 SNPs and 217 microsatellite markers. Since MHC blood type (BT) is known to affect MD resistance, a total of 18 independent pool pairs were constructed according to family x BT combination, with some combinations represented twice for technical reasons. Twenty-one QTL regions (QTLR) affecting post-challenge survival time were identified, distributed among 11 chromosomes (GGA1, 2, 3, 4, 5, 8, 9, 15, 18, 26 and Z), with about two-thirds of the MD resistance alleles derived from the more MD resistant parental line. Eight of the QTLR associated with MD resistance, were previously identified in a backcross (BC) mapping study with the same parental lines. Of these, 7 originated from the more resistant line, and one from the less resistant line.

CONCLUSION

There was considerable evidence suggesting that MD resistance alleles tend to be recessive. The width of the QTLR for these QTL appeared to be reduced about two-fold in the F6 as compared to that found in the previous BC study. These results provide a firm basis for high-resolution linkage disequilibrium mapping and positional cloning of the resistance genes.

Microsatellite markers associated with resistance to Marek's disease in commercial layer chickens

The objective of the current study was to identify QTL conferring resistance to Marek's disease (MD) in commercial layer chickens. To generate the resource population, 2 partially inbred lines that differed in MD-caused mortality were intermated to produce 5 backcross families. Vaccinated chicks were challenged with very virulent plus (vv+) MD virus strain 648A at 6 d and monitored for MD symptoms. A recent field isolate of the MD virus was used because the lines were resistant to commonly used older laboratory strains. Selective genotyping was employed using 81 microsatellites selected based on prior results with selective DNA pooling. Linear regression and Cox proportional hazard models were used to detect associations between marker genotypes and survival. Significance thresholds were validated by simulation. Seven and 6 markers were significant based on proportion of false positive and false discovery rate thresholds less than 0.2, respectively. Seventeen markers were associated with MD survival considering a comparison-wise error rate of 0.10, which is about twice the number expected by chance, indicating that at least some of the associations represent true effects. Thus, the present study shows that loci affecting MD resistance can be mapped in commercial layer lines. More comprehensive studies are under way to confirm and extend these results.

Genetic control of disease resistance and immunoresponsiveness

A great deal of evidence points to substantial genetic control over at least some of the immune responses, although genetic parameters for clinical disease have been less favorable. The past two decades have illustrated that single genes with a large impact on food animal health do exist and can be used to improve the health of domestic populations. The current focus on molecular genetics within food animal species will likely unveil numerous other examples of single genes with large effects, although the use of animals possessing favorable genotypes for disease resistance may represent a compromise in selection for increased production of raw product. Moreover, it is also clear that genetic control over the immune system is not limited to a few genes but is more likely influenced by many genes, each with small effects. The use of this information in animal improvement programs is not straightforward because of factors complicating the identification of superior individuals within the population. The scarcity of information dealing with phenotypic and genetic relationships between measures of disease resistance and aspects of immune response complicates the situation even further. Despite these potential hurdles, the potential for permanent improvement of disease resistance within food animal species in the future is tantalizing and merits intensified future study.

Impact of genetics on disease resistance

The genetics of a bird or flock has a profound impact on its ability to resist disease, because genetics define the maximum achievable performance level. Careful attention should be paid to genetics as an important component of a comprehensive disease management program including high-level biosecurity, sanitation, and appropriate vaccination programs. Some specific genes (e.g., the MHC) are known to play a role in disease resistance, but resistance is generally a polygenic phenomenon. Future research directions will expand knowledge of the impact of genetics on disease resistance by identifying non-MHC genetic control of resistance and by further elucidating mechanisms regulating expression of genes related to immune response.

Major histocompatibility complex class II DNA polymorphisms in chicken strains selected for Marek's disease resistance and egg production or for egg production alone

The objective of this study was to investigate frequencies of major histocompatibility complex (MHC) class II restriction fragments in two groups of White Leghorn strains. Each group consisted of an unselected control, a strain selected for egg production traits, and a strain selected for egg production traits and Marek's disease (MD) resistance. PvuII-digested genomic DNA was hybridized with a chicken genomic MHC class II probe. The MHC class II DNA fragment frequencies in the selected strains differed from those in the related unselected control and in the strain selected using the same criteria from a different base population. Based on the sizes of the breeding populations, particularly those in the control strain and in the strain selected for egg production, it was considered unlikely that the observed changes of the MHC class II fragment frequencies were due to random genetic drift. The data suggested that some MHC class II bands are associated with production traits or with MD resistance, and that these associations tend to be unique to each genetic background. Hence, MHC class II genes are likely candidates for the investigation of quantitative trait loci in egg production and disease resistance traits such as those for which the studied strains were selected.

Gordon Memorial Lecture. Stress, strains and resistance

1. Stress describes the bird's defence mechanisms and a stressor is the situation that elicits the defence response. 2. As the environment can be viewed as a composite of interacting stressors, the bird's success in coping with its environment depends on the severity of the stressors and the physiological ability to respond properly and thus maintain homeostasis. 3. The neural, endocrine and more recently immune systems are considered to be integrators of the stress response. Although stress responses may be necessary for survival in wild bird populations, they are often detrimental to efficient growth, skeletal integrity and disease resistance in domesticated fowl. 4. Stress responses are modified by the genetic components.

Poultry immunogenetics: which way do we go?

A major goal in poultry immunogenetics is the enhancement of innate immunoresponsiveness and resistance to disease. This may be pursued by studying either single genes or polygenic traits. The MHC is perhaps the best-characterized family of host genes that modulates response to a variety of antigens and pathogenic challenges. The association of different MHC alleles with disease resistance has been known for decades. But only recently has analysis at the DNA level opened new avenues of understanding and new opportunities for application of genetic variation in the MHC with immunoresponsiveness. An alternate approach to molecular analysis is selection for a desired phenotype controlled by polygenes. Several studies have illustrated the successful alteration of immunoresponsiveness by genetic selection for antibody production. Recently, a selection program based upon multiple traits of immune response was conducted. Results of this project demonstrated that selection on multiple immune-response traits altered immunophysiology, MHC allelic frequencies, and disease resistance. Several areas for future pursuits in poultry immunogenetics research are proposed.

Genetic parameters for yield and reproductive traits of Holstein and Jersey cattle in Florida

Records of yield and reproduction from 4293 Holstein and 2143 Jersey first lactation cows from eight Holstein and six Jersey herds were utilized to evaluate genetic parameters for Florida, a subtropical environment. Statistical analyses were by derivative-free REML with the animal model. Genetic variances were based on variation in estimated breeding values of individual cows. Heritabilities were .27 to .43 for yields (6 estimates), .38 to .51 for constituent percentages (4 estimates), and .025 to .056 for reproduction (6 estimates), which were similar to estimates for temperature areas from similar procedures. Also, correlations of breeding values between yields were high and between yields and reproduction were low and generally antagonistic. Correlated responses in calving interval from selection for yield, with selection intensities of 1.0 to 1.5, would be expected to lead to increases of 1.0 to 5.2 d per generation (12 estimates). Thus, estimates of genetic parameters and correlated responses in this subtropical environment did not differ appreciably from those that occur in temperate dairy areas.

Differences in major histocompatibility complex frequencies after multitrait, divergent selection for immunocompetence

White Leghorn chickens from lines selected for four immune-response traits (IR lines) were serotyped for B system alloantigens characterizing the haplotypes and genotypes to examine the effect of divergent selection for multitrait immunocompetence on MHC haplotype and genotype frequencies. The selected lines were derived from the Ottawa Strain 7. The selection index included four immunocompetence traits: antibody production against Mycoplasma gallisepticum (MG) and Pasteurella multocida, inflammatory response to phytohemagglutinin, and reticuloendothelial carbon clearance. The four lines include two replicates of high and low multitrait-immunocompetence lines. After four cycles of selection, significant differences (P < .05) in several B system haplotype frequencies were observed, both among IR lines and between the IR lines and the Ottawa Strain 7. The B2 haplotype frequency was greater in all IR lines than in the Ottawa Strain 7. The B21 frequency was less in both high lines than in the Ottawa Strain 7. In comparisons among lines, frequencies of B21 were greater in both replicates of the low lines and the B12 and B19 frequencies were significantly greater (P < .05) in the high lines. A gene substitution model showed effects (P < .10) of specific haplotypes on MG and on the index. The B2 haplotype had a positive effect associated with MG. Haplotype B21 was positively associated with the multitrait index. Haplotype B13 had a negative effect on both MG and the index. Significant differences (P < .01) in genotype frequencies were also noted among the IR lines. Associations between specific MHC haplotypes or genotypes and immune-response traits may offer insight into MHC-mediated mechanisms of disease resistance.

Designation by restriction fragment length polymorphism of major histocompatibility complex class IV haplotypes in meat-type chickens

Major histocompatibility complex (MHC) class IV haplotypes were identified in a population of meat-type chickens by restriction fragment length polymorphism (RFLP) analysis. Fourteen different haplotypes were designated on the basis of restriction patterns obtained from Southern blots of PvuII- or BglII-digested DNA, hybridized with the MHC class IV cDNA probe bg32.1. Digestion with each restriction enzyme yielded the same level of polymorphism among individuals. For each haplotype, 4-10 restriction fragments ranging from 0.8 to 8 kb were observed. Such a designation of meat-type chicken MHC class IV haplotypes enables a rapid recognition of previously defined haplotypes, is readily adjustable to additional, newly found restriction patterns and could prove useful in practical breeding programmes.

The turkey major histocompatibility complex: characterization by mixed lymphocyte, graft-versus-host splenomegaly, and skin graft reactions

A turkey subline at the Ohio Agricultural Research and Development Center was developed by DNA typing of the MHC using a chicken MHC Class II probe, and it segregated for specific MHC genotypes. Histocompatibility was examined between turkeys of known MHC genotype using skin graft procedures, mixed lymphocyte reactions (MLR), and graft-versus-host reactions (GVHR). Skin grafts were exchanged among 3-wk-old turkeys and it was found that when birds shared DNA patterns (genotypes), the skin grafts were usually accepted. In contrast, skin grafts were always rejected when birds did not share the identical DNA pattern. Similarly, MLR only occurred when the lymphocytes were derived from birds that did not share the same DNA pattern. Lastly, GVHR were examined in embryos injected with either sire or dam blood. The GVHR in embryos was dependent on the parental MHC genotype. Four MHC haplotypes were identified in the turkey subline. The turkey MHC has been designated MhcMega-B, and each of the haplotypes, Mega-B(1) through Mega-B(4).

The Turkey Major Histocompatibility Complex: Identification of Class II Genotypes by Restriction Fragment Length Polymorphism Analysis of Deoxyribonucleic Acid

Using a chicken Class II MHC clone in Northern blot analysis, tissue-specific expression of turkey Class II MHC genes was observed in the embryonic bursa of Fabricius as well as in the adult spleen. In contrast, there was no detectable expression in the embryonic liver, brain, or spleen. Southern blot analysis of BamHI-digested turkey DNA revealed two restriction fragment length polymorphism (RFLP) patterns that did not deviate significantly from single-gene Mendelian inheritance. Further analysis of PvuII-digested DNA from 325 turkeys showed four distinct RFLP patterns that segregated within the turkey lines studied. Because the chicken Class II MHC clone hybridized specifically to mRNA in immune-associated tissues, and because it identified polymorphisms among turkeys, the chicken clone is suggested to identify four turkey Class II MHC genotypes. The current study provides good evidence that RFLP analysis of DNA can be used as a means for molecular genotyping at the MHC in turkeys.

Major Histocompatibility Complex Class I Restriction Fragment Length Polymorphism Analysis in Highly Inbred Chicken Lines and Lines Selected for Major Histocompatibility Complex and Immunoglobulin Production

Selected chicken populations were analyzed by restriction fragment length polymorphism (RFLP) with a chicken MHC Class I (B-F) cDNA probe. The 13 highly inbred chicken lines differed in genetic origin and in MHC (B) haplotype, as distinguished by using hemagglutination with antisera against B-G and B-F antigens. The S1 sublines differed for B haplotype and antibody response to a synthetic polypeptide, GAT. In the highly inbred lines, band-sharing between lines from different origins was less than that between lines from same origin, showing the influence of the genetic background on chicken MHC Class I gene RFLP. In the S1 line, use of three restriction endonucleases (BglII, PvuII, and TaqI) produced MHC Class I RFLP patterns that were associated with B haplotype, but not with immune response to GAT (IrGAT). A previous study in the authors' laboratory also demonstrated an association of MHC Class II beta RFLP patterns with B haplotype, but not IrGAT, in the same line, suggesting that IrGAT is not controlled by MHC Class I or Class II beta genes.

Immunogenetics and the major histocompatibility complex

The poultry immune system is a complex system involving many different cell types and soluble factors that must act in concert to give rise to an effective response to pathogenic challenge. The complexity of the immune system allows the opportunity for genetic regulation at many different levels. Cellular communication in the immune response, the production of soluble factors, and the rate of development of immune competency are all subject to genetic influences. The genes of the major histocompatibility complex (MHC) encode proteins which have a crucial role in the functioning of the immune system. The MHC antigens of chickens are cell surface glycoproteins of three different classes: Class I (B-F), Class II (B-L) and Class IV (B-G). The MHC antigens serve as essential elements in the regulation of cell-cell interactions. The MHC has been shown to influence immune response and resistance to autoimmune, viral, bacterial and parasitic disease in chickens. The MHC has been the primary set of genes identified with genetic control of immune response and disease resistance, but there are many lesser-characterized genes outside of the MHC that also regulate immunoresponsiveness. Polygenic control has been identified in selection experiments that have produced lines of chickens differing in antibody levels or kinetics of antibody production. These lines also differ in immunoresponsiveness and resistance to a variety of diseases. Understanding the genetic bases for differences in immunoresponsiveness allows the opportunity selectively to breed birds which are more resistant to disease. Indirect markers that can be used for this selection can include the MHC genes and immune response traits that have been associated with specific or general resistance to disease.

Disease resistance in farm animals

Genetic variations in disease resistance of farm animals can be observed at all levels of defence against infectious agents. In most cases susceptibility to infections has polygenic origins. In domestic animals only a few instances of a single genetic locus responsible for disease resistance are known. A well-examined example is the Mx1 gene product of certain mice strains conferring selective resistance to influenza virus infections. Attempts to improve disease resistance by gene transfer of different gene constructs into farm animals include the use of monoclonal antibody gene constructs, transgenes consisting of antisense RNA genes directed against viruses and Mx1 cDNA containing transgenes.

Selection for disease resistance

Approaches to disease control are prioritized. Genetic improvement could reduce need for treatment and culling but would not reduce the need for proper management and sanitation. Results of several studies indicate that disease incidence and cost increases with selection for milk yield. The large array of disease resistance mechanisms in animals suggests a large number of loci are involved in disease resistance. A few loci, e.g., the major histocompatibility complex, may account for a major portion of genetic variance in disease. Rate of genetic gain from selection for a major locus alone or in combination with performance is discussed. Four criteria for including traits in a breeding program are outlined, and each is discussed with respect to disease. In spite of low heritabilities for disease traits, genetic variation for disease incidence is economically important and justifies including disease in breeding programs. An industry-wide standard for recording and accumulating field data for disease is lacking. Institutional relationships among segments of the animal breeding and animal health industries are needed to facilitate genetic improvement for disease resistance.