Relations between egg size, reproductive success and growth performance of progeny in isogenic Oreochromis niloticus lines

The present study followed the reproductive performance within four genetic groups of Oreochromis niloticus females belonging to two homozygous isogenic lines (C1, C2) developed by induced gynogenesis, a heterozygous isogenic line (C1 × C2) resulting from crossbreeding C1 and C2, and a heterogenic control group. Each genetic group consisted of 12 females and observations continued for 60 days. Digital photographs were taken of the egg batches and hatching larvae. With the help of image analysing software, the two-dimensional surface of individual eggs was measured to ascertain egg size. Larval size was also recorded. To follow their particular growth progresses, isogenic larvae belonging to C1 and C2 were reared individually from day 6 until day 80 of life under standardised environmental conditions. Their homozygous isogenic status was later proven by gene marker studies. In comparison to the heterogenic controls, the coefficient of variation of egg size was similarly higher both in C1 and C2, and lower in C1 × C2. Egg size variation observed within the egg batches from the homozygous isogenic females indicated the influence of environmental and/or epigenetic effects during the earliest developmental phases. Compared to the control, high variation in egg size and hatching rates as well as low mean egg numbers and developmental rates was observed within the two homozygous isogenic groups. The significant differences in mean egg size between C1 (5.7 mm2) and C2 (4.4 mm2) were not reflected in the measurements taken during the later growth phase. In both the homozygous isogenic lines the coefficient of variation for body length increased from day 6 (mean: 11.7%) until day 80 of life (mean: 19.0%). As shown by comparably low coefficients of correlation between measurements the size ranking of individual larvae and fingerlings within C1 and C2 changed during growth.