Shirak, A., Institute of Animal Science, Agricultural Research Organization, P. O. Box 6, Bet-Dagan 50250, Israel
Palti, Y., National Center for Cool and Cold Water Aquaculture, ARS-USDA, 11861 Leetown Rd., Kearneysville, WV 25430, United States
Bern, O., Laboratory of Fish Immunology and Genetics, The Everard and Mina Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel
Kocher, T.D., Department of Biology, University of Maryland, College Park, MD, United States
Gootwine, E., Institute of Animal Science, Agricultural Research Organization, P. O. Box 6, Bet-Dagan 50250, Israel
Seroussi, E., Institute of Animal Science, Agricultural Research Organization, P. O. Box 6, Bet-Dagan 50250, 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, The Everard and Mina Goodman Faculty of Life Sciences, Bar-Ilan University, Ramat-Gan 52900, Israel

Offspring of a highly inbred gynogenetic line of Oreochromis aureus displayed 12-fold increase in twinning rate compared to the outbred population. Asymmetric conjoined twins, which consist of a normal embryo attached to a malformed-atrophic twin, were frequently encountered in both gynogenetic (90·7%) and outbred (38·2%) embryos. The monozygotic origin of these twins was determined using five microsatellite markers. Progeny of heterozygous parents for the microsatellite UNH159 were separated into sub-sets of twins and normal full-sibs. Consistent with previous reports, the normal embryo sub-set exhibited elimination of both types of homozygotes for the UNH159 genetic marker at 2-8 days after fertilization. Unexpectedly, this elimination was less frequent in twins. The UNH159 marker as well as RNA-binding motif protein, X-linked (rbmx), SRY-box containing gene 3 (sox3) and alpha-thalassemia/mental retardation syndrome X-linked (atrx) genes were mapped to linkage group 2. These gene orthologues are all located on the mammalian X chromosome and atrx is necessary for the X-chromosome inactivation. © 2012 The Authors. Journal of Fish Biology © 2012 The Fisheries Society of the British Isles.