Nucleolar variation in response to nutritional condition in juvenile pejerrey Odontesthes bonariensis (Valenciennes)

The variations of the nucleolar characteristics size and number were studied in the gill and liver of Odontesthes bonariensis under different feeding regimes in order to test the hypothesis that functional genomes at the nucleolus, which determine variation of size (heterogeneity) in juvenile fishes, are modulated by environmental factors such as food availability. The programme that controls genetic changes of the nucleolar characteristics of the diploid cells of O. bonariensis was different from that described in the diploid cells of cyprinid fishes. No variations of the mean nucleolar number per nucleus were observed in liver or gill cells in relation to the feeding regime. Conversely, significant reduction of the mean nucleolar volume per nucleus ( V _MN) and change in the shape of the frequency distribution of the total nucleolar volume per cell was evident in both cell types in response to starvation or food restriction. Tissue specificity was observed in relation to the absolute values of the nucleolar characteristics; however, the relative behaviour was comparable in digestive and non-digestive tissues. The time-course reduction of V _MN in gill and liver cells of restrictively fed fish followed a negative exponential. A good correlation between V _MN and the condition index of the fish ( K ) was observed after long-term food restriction; however, a faster response of V _MN was observed after short-term fasting events. Results supported the stated hypothesis and showed nucleolar activity as a direct, rapid and sensitive indicator of the nutritional status of the fish.