The potential of transgenic animals for improved agricultural productivity.

The techniques involved in the transfer of foreign DNA to domestic animals have advanced to the stage where transgenic animals that express foreign genes can be reliably produced, albeit still at low efficiency. This paper reviews the current status of some of the more important areas in agriculture where this technology is being applied. Numerous attempts have been made to modify the growth performance characteristics of domestic animals by the introduction of metallothionein/growth hormone fusion genes. A summary of our work with transgenic sheep is presented. The results demonstrate that the unregulated production of growth hormone in transgenic sheep reduces carcass fat, elevates metabolic rate and heat production, causes skeletal abnormalities and impairs survival. The introduction of new metabolic pathways to domestic animals offers an attractive approach to improved animal productivity. This paper summarises recent results of research directed towards the introduction of a cysteine biosynthetic pathway and the glyoxylate cycle to transgenic sheep. So far, the genes encoding the enzymes have been isolated and expressed both in cells in culture and in transgenic mice. The results of work currently in progress demonstrate that some modification of the fusion genes is required to enhance their expression in transgenic animals.