Major Genes Affect Reproduction and Early Growth

associated with low weight gain, higher rectal temperature and increased incidence of lung lesions, but correlations among these variables were low (ranging from-.59 to .03). Some pigs replicated the virus at high rates and showed all the clinical symptoms of PRRS. They grew slowly, had high body temperature , and had lung lesions indicating interstitial pneumonia. Other pigs with similar levels of viremia showed few symptoms of PRRS. They gained weight at normal rates, had normal or only slightly elevated body temperature, and had few lung lesions. Similarly, there were pigs in this sample with relatively low levels of viremia that showed typical symptoms of PRRS, whereas others showed few clinical effects of the virus. Line differences and line by challenge interactions across days are evidence of genetic mechanisms involved in immune responses to PRRSV. The nature of these genetic differences or whether it is possible to select for greater resistance cannot be determined from the data collected so far. The next step in this research will be to investigate differences in expression of specific genes in the resistant/suscep-tible classes of pigs. The focus will be on genes expressed in macro-phage cells in the lung, but genes expressed in other tissues involved in immune responses (e.g., lymph and spleen) could also be important. Because of the difficulty in applying quantitative methods to select for PRSSV resistance, experiments to identify the genes involved are critical as it unlikely that genetic change can occur until selection directly for these genes in the absence of PRRSV can be applied. The Nebraska Index Line is reproductively superior to its contemporary control, producing approximately four pigs more per litter. However, the genes or quantitative trait loci (QTL) that cause these differences are unknown. A previous study with an F 2 resource population created by crossing the NE Index and Control lines identified one QTL affecting ovulation rate, one QTL affecting number of fully formed fetuses, one QTL affecting number of pigs born alive, two QTL affecting number of stillborn pigs, five QTL affecting nipple number, and six QTL affecting age at puberty. However, individual birth weight and weaning weight had not been included in the analyses. In addition, improved statistical models with greater power to identify QTL and test for additional kinds of gene action have been developed. The objective of this experiment was to apply these more powerful models to the data from the …