Influência da salinidade da água sobre a gametogênese da tilápia do Nilo (Oreochromis niloticus) em condições de cultivo

Abstract

Nile tilapia has considerable potential for cultivation in low salinity water. The shortage of freshwater in many countries, competition with agriculture and other urban activities have increased the pressure to develop aquaculture in brackishwater and seawater. In this sense, this study aimed to evaluate the levels of HSP70, PCNA and caspase-3 in Oreochromis niloticus gonads, correlating data with reproductive parameters including hepatic vitellogenin (Vtg) in females, and plasma concentrations of 11-ketotestosterone (11-KT), 17β-estradiol (E2), and cortisol in different salinity treatments. For this, the study was developed in two stages, using sexually mature tilapia. In the first stage, the experiment was performed with males (58.94 ± 2.64 g body weight), and the second with females (398.5 ± 128.5 g). The fish were submitted to four salinity treatments with NaCl addition in different concentrations: (S0) freshwater, (S7) 7 g/L, (S14) 14 g/L and (S21) 21 g/L. Histological, morphometric, molecular analysis and plasma levels of sexual steroids and cortisol, as well as ions and lipids were performed in samples of tissues at 1, 4 and 9 days for males and at 1, 2 and 3 weeks for females. The gonadosomatic index (GSI), hepatosomatic index (HSI) and Fulton's condition factor (K) were calculated. All males survived until the ninth day while all S21 females died on first day after the start of the experiment. All specimens were in reproductive activity. Seminiferous tubules diameters were lower in S7, S14 and S21 and the lowest GSI values occurred in S14 and S21 for males. K values showed no variation in males but were lower in S7 and S14 in females. In the morphometric analyses, spermatozoa were less numerous and histopathological changes were observed in higher salinities males: presence of empty cysts, increase of interstitial tissue and Leydig cell hyperplasia and hypertrophy. In females, no changes were observed in the proportion of ovarian follicles and in the GSI values, however vitellogenic follicles diameters were higher in S14. Inflammatory infiltrate and presence of granulocytes were also observed in S21 testis and S14 ovaries. Plasma concentrations of 11-KT in males and E2 in females did not show significant variations between treatments. Cortisol plasma levels were higher, while lipids were lower in S14 females. In the molecular analyses, HSP70 testicular levels were higher on the first day and then reduced, when increased caspase-3 and PCNA reduction at 9 days were observed in S14 and S21. PCNA labelling occurred in spermatogonia cysts whereas caspase-3 was expressed in spermatids cysts and spermatozoa. In the ovaries, PCNA was higher and caspase-3 was lower in S7 than in S0 and S14 on the third week of treatment. In females, HSI and hepatic Vtg values were lower in S14, where we observed yolk deficient oocytes. Considering that O. niloticus is the third most cultivated fish in the world, and that it has great economic importance, studies related to its reproduction are crucial for the cultures that depend on the nutrition and economy that tilapia provides. On the other hand, the impacts observed by salinity, in the present study, can be amplified for most freshwater species, since O. niloticus is an euryhaline fish with great capacity of adaptation, allowing this species to colonize several ecosystems to the detriment of native species.