Inherited differences in susceptibility to disease exist in all species and for most, if not all, forms of disease. This is certainly true in poultry where variation in resistance has been described for all the major diseases. The structure of the poultry industry, in which very large numbers of production birds (tens of millions) are derived from a limited number (hundreds) of grand parents small elite flocks, lends itself to exploitation of this resistance. However, a number of factors, particularly the need for continuous selection for productivity, have made other means of disease control more attractive.

There have been two principal exceptions to this, in both cases where single genes have been identified with disease resistance: these are the major histocompatibility complex (MHC) associated resistance to Marek’s disease, and the genes coding for receptors to the various subgroups of lymphoid leukosis virus.

The MHC haplotypes of birds can be easily identified by red blood cell agglutination using specific antisera, and particular haplotypes, notably the B21 haplotype, show a consistent association with resistance to Marek’s disease. In many ways this is an ideal situation since identification of resistant birds is quick, cheap and does not involve exposure to the pathogen once initial experiments to determine the relative resistance of haplotypes have been completed.

In the case of the receptor genes for lymphoid leukosis, resistance, (the absence of the receptor), is recessive. It is therefore difficult to find resistant birds in flocks which are fairly susceptible and the problems of identifying heterozygotes by progeny testing are considerable. Very recently the gene coding for the receptor for the A subgroup of the virus has been identified and this should allow much more certain identification of resistant animals.

Recent improvements in protein detection and particularly DNA probes provide better means of identifying resistant birds and the development of genetic maps of the chicken genome have resolutionised the possibilities for identifying resistance genes. To an even greater extent the development of direct gene transfer has transformed the prospects for genetic resistance by offering the possibility of creating totally novel resistance mechanisms.

In mammals, especially mice, gene transfer by direct injection of DNA into the pronucleus during in vitro fertilisation is now a well-established technology. Unfortunately it is difficult to translate this technique to poultry because of the much greater size of the yolk and the stringent requirements of packaging the egg. Fortunately, as alternative means of introducing genetic material exists, the use of retroviral vectors.

Retroviruses contain single stranded RNA. After infection of the host cell a viral polymerase produces a double stranded DNA copy of the viral genome which enters the nucleus and inserts into the chromosomal DNA of the host cell, this then serves as a template for further virus production. Normally this process takes place in somatic cells, but also, very rarely, in germ line cells, in which case the viral genes are transmitted to progeny in the normal Mendelian manner.

If useful genes can be introduced into the viral genome the normal process of infection will thus efficiently introduce the genes into the host cell DNA.

Genomes of both types of retroviruses of chicken (avian leukosis viruses and reticuloendotheliosis viruses) have been cloned and exist in conventional plasmid form; the introduction of DNA into these clones and their transfection into cells in vitro is fairly straightforward. Once the viral DNA has been introduced into suitable cells production of the modified virus occurs in the normal manner.

Potential drawbacks of this system are the limited size of the virion, which restricts the size of additional DNA to around 10 kilobases (1–5 genes) there may also be problems in getting the virus to infect the germ line cells, or of controlling the subsequent infectivity of the introduced virus. However, these problems have already been largely overcome. The size requirement can be avoided by multiple infections if necessary. Groups in the USA have been successful in obtaining germ line integration either by injecting virus near the blastodisk of newly laid eggs, or by introducing virus within the follicle of the developing ova. To prevent the uncontrolled spread of the introduced virus it is possible to produce virions capable of infection but not subsequent replication. In collaboration with Reading University we have improved this methodology by isolating the primordial germ cells, infecting them and returning them to recipient embryo.

To take advantage of this new methodology we still require resistance (or production) genes, defined at the molecular level. There are various possible sources for such genes; conventional resistance genes from the chicken are potentially a promising source, but even the best defined such as the B21 MHC genes, present a huge and complex region for analysis at the molecular level. A second source of resistance genes is the pathogens themselves, either by providing blocking proteins or interfering RNA species. The use of envelope protein to block cellular receptors and prevent infection has been demonstrated for leukosis virus, and complementary antisense RNA has been shown to inhibit Rous sarcoma infection. In principal any viral genome, once cloned, could be used as a source of complementary DNA.

A third possible source of material are immunological genes. Only the genes coding for the immunoglobulin have been identified in chickens, but the homology of these genes with their mammalian counterparts suggests other genes such as the T cell receptor genes, interleukin and interferon genes may not be difficult to identify using mammalian probes. Some of these genes could be used specifically e.g. tailored pathogen-specific immunoglobulins, other to generally enhance the immune system.

Perhaps the greatest problem in utilizing any of these genes lies in controlling their pattern and level of expression. However, this problem is common to many aspects of molecular biology and regulatory elements are becoming increasingly well defined in other applications.

Further Reading

Bumstead, N. and Palyga, J. (1992) A preliminary linkage map of the chicken genome. Genomics, 13: 690–697.

Freeman, B. M. and Bumstead, N. (1987) Breeding for disease resistance – the prospective role of genetic manipulation. Avian Pathology, 16: 353–363.

Burt, D., Bumstead, N., Bitgood, J. J., Ponce de Leon, F. A. and Crittenden, L. B. (1995) Chicken genome mapping: a new era in avian molecular genetics. Trends in Genetics, 11: 190–194.