The mitochondnal genome was screened for genetic variation associated with production traits in White Leghorns. Analysis was done on an unselected randomly mated population of White Leghorn chickens. An AsnI restriction fragment length polymorphism (RFLP) was detected in the control region of the mitochondrial genome, a region responsible for transcription and replication. The RFLP segregated at a frequency of 0.76 for the AsnI (+) allele (presence of an AsnI restriction site). The AsnI RFLP was in linkage disequilibrium with a previously detected MspI RFLP in the NADH sub-unit IV of the mitochondrial genome for the same population of White Leghorns. Genotypes were denoted as follows: The first sign represented individuals that were positive (+) or negative (-) for the MspI restriction site, where as the second denoted whether individuals were positive or negative for the AsnI restriction site. Comparison of the three genotypic classes (+,+), (-,+) and (-,-) revealed that (-,-) individuals had a significantly higher body weight (5.5%) than individuals of the other genotypes. Dot blot analysis revealed no difference in the mitochondrial DNA copy number in peripheral blood cells between AsnI(-) or AsnI(+) individuals.

The incidences of endogenous viral genes (ev-genes) and myb-gene polymorphisms were determined in substrains of strains R, M and G which had been divergently selected or susceptibility to tumour formation induced by Rous Sarcoma virus (RSV) of type A and B. Frequencies of myb gene polymorphisms were also determined in two replicates of strains selected for high and low multiple immune response to challenge with Pasteurella multocida and Mycoplasma gallisepticum. Strains R, M and G were found to contain different sets of ev-genes reflecting their distinct genetic origins. Among eleven ev-genes identified, two showed a significantly increased frequency in the susceptible substrains. One was ev-6, which expresses the viral envelope protein of the endogenous avian leucosis, while the other was a new endogenous viral gene New-E, whose phenotype is unknown. A significant increased incidence of ev-4, reported to be a silent ev-gene, was observed in resistant rather than susceptible substrain. Myb gene polymorphisms were assessed using a cDNA probe and a genomic probe yielding 2 and 3 restriction fragment length polymorphisms (RFLPs) respectively. In strains M and G, only one polymorphism (PM5$ sp+$) observed at an Msp I site located downstream of the last of the c-myb exons was found to be significantly co-selected for susceptibility. Analysis of RFLPs of myb-gene in strains selected for high or low multiple immune response did not reveal any significant response to selection. Rather, polymorphisms seemed to reflect a founder effect as revealed by opposite frequencies obtained in the two replicates. DNA methylation, a possible epigenetic mechanism regulating gene expression, was also investigated in the myb-gene. DNA from semen, blood, spleen, liver and thymus was extracted from organs obtained from chickens at different ages.

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