Shirak, A., Laboratory of Fish Immunology and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel, Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Palti, Y., Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel, NCCCWA-USDA/ARS, 11876 Leetown Rd., Kearneysville, WV 25430, United States
Cnaani, A., Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Korol, A., Institute of Evolution, University of Haifa 31905, Israel
Hulata, G., Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Ron, M., Institute of Animal Science, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
Avtalion, R.R., Laboratory of Fish Immunology and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel

Three microsatellite markers (UNH159, UNH231, and UNH216) were examined for association with both deleterious genes and sex-ratio distortions in a full-sib family of 222 progeny from the fourth generation of a meiogynogenetic tilapia line (Oreochromis aureus). The three markers were mapped previously to different linkage groups and were shown to be associated with genes with deleterious alleles in this line. A restricted maximum likelihood model was used for analysis of major effects and their interactions on sex ratio and viability. This model was based on selective mortality of genders, ignoring effects of possible sex-determining genes. The results showed that deleterious genes linked to UNH216 and UNH231 exert higher lethality in females than in males (P < .0005 and P < .05, respectively). UNH159 was not associated directly with sex ratio distortion, but acts strongly as a modifier of sex ratio in combination with UNH216 and UNH231. Each of the three loci was found to have a significant effect on viability (P < .05) in the maximum likelihood analysis. The deleterious single-locus effects act strongly against females, while most of the epistatic interactions exert higher lethality in males. This contradiction results in a close to 1:1 sex ratio at maturity. The genetic mechanism and significance of such a balance between genders are still unknown. A detailed analysis of sex-specific lethality may be applied by screening in appropriate series of matings and fine mapping with additional markers. Our data suggest that UNH216 and UNH231 are linked to sex ratio distortion genes and that UNH159 may be linked to a modifier of these genes.