Shirak, A., Lab. of Fish Immuno. and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel
Shmarina, A., Lab. of Fish Immuno. and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel
Kuperman, Y., Lab. of Fish Immuno. and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel
Avtalion, R.R., Lab. of Fish Immuno. and Genetics, Faculty of Life Sciences, Bar Ilan University, Ramat Gan 52900, Israel

Segregation of body and peritoneum pigmentation was studied in three developmental stages in F2 hybrids and progeny of F1 hybrids backcrossed with parents Oreochromis aureus (black body and peritoneum) and red O. niloticus (red body and white peritoneum). Segregation in adult F2 hybrids corroborated previous findings that red body color is dominant over black and controlled by a single gene (R). Results further showed that (a) the colors of the body and of the lower-posterior part of the peritoneum are strongly associated (p<0.001) and explained by the interaction between two separate genes; (b) the color inheritance of the upper-anterior part of the peritoneum could not be explained by a simple dominant-recessive mechanism. The analysis of color inheritance in adult hybrids resulted in a non-Mendelian segregation of the peritoneum and body pigmentations. Our data showed that this segregation was the result of selective mortality among albinos starting in 4-day embryos. Genuine segregation results were obtained only when the color analysis was performed by microscopy in embryos before the age of four days. Embryo mortality is suggested to be due to incomplete homology between chromosomes derived from the closely related species used for hybridization.