Vectors and genes for improvement of animal strains.

Strain improvement of agriculturally important animals will require efficient techniques for gene delivery, the ability to regulate the expression of the newly introduced genes and, most important, the identification of genes whose appropriate expression could cause improvement of the animal. We have developed a series of avian retroviral vectors that can be used to introduce new genetic information into the germ line of chickens, for which transgenics cannot be created by direct microinjection of DNA into fertilized eggs. We have identified a 220-bp segment of the chicken skeletal muscle alpha-actin gene that can cause other genes to be expressed specifically in striated muscle. This chicken promoter shows correct tissue specificity in transgenic mice and presumably could be used in other mammalian species. The skeletal muscle alpha-actin promoter has been inserted into the avian retroviral vectors and the promoter is functional in cultured cells infected by these retroviral vectors. The tissue specificity of the expression of the skeletal muscle alpha-actin promoter carried by the retroviral vectors will soon be tested in vivo. We are studying two types of genes that might be useful in strain improvement; genes that could produce dominant resistance to infection by pathogenic viruses, and genes that could play critical roles in muscle development. Expression of the envelope glycoprotein of retroviruses can specifically block the cellular receptor that viruses use to infect a susceptible cell. Expression of the avian leukosis virus subgroup A envelope in transgenic chickens prevents infection by pathogenic viruses of the same subgroup. We are attempting to block reticuloendotheliosis virus infection by expressing the reticuloendotheliosis envelope glycoprotein. We have shown that we can block infection in cultured cells, and we are now creating retroviral vectors for experiments in vivo. We have also begun to study the cellular homologue of the ski oncogene, which has been shown to stimulate the differentiation of quail myoblasts in vitro. Biologically active cDNAs have been isolated; we have now begun to analyse the effects of expressing the c-ski proteins in the whole animal.