Strategies for the Production of Transgenic Chickens

Insertion of foreign DNA into the germline requires access to the chromosomes of cells that give rise to sperm or eggs. From a theoretical point of view, the newly fertilized zygote is the most appropriate cell for genetic alteration because any modification of the genome will be inherited by every cell that subsequently develops within the organism. In mammals, random insertion of genes via injection of DNA into the pronucleus is the most cost-effective route for modification of the genome (see Chapter 31 ). If specific, targeted modifications to the genome are desired, however, an elaborate culture system is required that simultaneously supports proliferation of pluripotential embryonic stem cells and facilitates selection of cells that bear the specific genetic change (see Chapter 31 ). In the chicken, injection into the pronuclei of the newly fertilized egg is not feasible for two reasons. Firstly, the oocyte is located on the surface of the large and fragile yolk and techniques to identify, manipulate, and inject the female pronucleus have not yet been developed. Secondly, fertilization in avian species is associated with the entry of several sperm into the egg (1 ) and it is not yet possible to distinguish individual male pronuclei or to determine which male pronucleus will unite with the female pronucleus. Strategies for gaining access to the germline of chickens, therefore, have arisen through an improved understanding of the morphological and physiological events that occur between fertilization and differentiation of the primordial germ cell lineage.