All-male Nile tilapia larvae production using high-temperature and low dose of MT combination treatment

Previous studies have shown that high-temperature treatment during thermosensitive periods (TSPs) induces >70% Nile tilapia to become pseudomales (XX phenotypic males) in all-XX families mainly by downregulating the 178-estradiol (E2) levels. Commercially, methyltestosterone (MT) is used to induce the sex reversal of XX individuals in Nile tilapia and produce all-male larvae. In order to reduce the dosage of methyltestosterone (MT), it is very necessary to investigate the possibility of obtaining all-male Nile tilapia larvae using combination treatment with high-temperature and low dose of MT. In this study, three families were developed by crossing XX females with XY males and were used for subsequent analysis. The results demonstrated that the male ratio in T + 0 MT (36 degrees C high-temperature treatment and 0 mu g/g MT in diet) was higher than in the C + 0 MT (28 degrees C culturing water temperature and 0 mu g/g MT in diet) or C + 2 MT (28 degrees C culturing water temperature and 2 mu g/g MT in diet) group and lower than in C + 10 MT. The lowest MT concentration achieving a 100% male ratio was 10 mu g/g in the high-temperature-treated group and 20 mu g/g in the control group. High-temperature and MT combination treatment further downregulated E2 levels compared to the control and various MT-treated groups. High-temperature treatment significantly increased 11-KT levels in XX Nile tilapia trunks, and 11-KT levels in T + 0 MT were significantly higher than in C + 0 MT or C + 2 MT and were not significantly different from that in XY. Interestingly, 11-KT levels in the XX Nile tilapia trunk in the C + 10 MT, C + 20 MT, T + 10 MT and T + 20 MT groups were significantly higher than in the XY group. High-temperature treatment significantly downregulated mRNA and protein expression levels of Cyp19a1a in 21 dpf (days post fertilization) XX Nile tilapia gonads (T + 0 MT). High-temperature and MT combination treatment further downregulated mRNA and protein expression levels of Cyp19a1a in 21 dpf XX Nile tilapia gonads (T + 10 MT) to levels similar to those in the XY group. In contrast to Cyp19a1a, high-temperature and MT combined treatment significantly upregulated expression levels of Dmrt1 in 21 dpf XX Nile tilapia gonads (T + 10 MT), and expression levels of Dmrt1 in T + 10 MT were higher than in XY. Furthermore, we found that high-temperature and low dose of MT combined treatments synergistically promote the growth of Nile tilapia. These results illustrated the potential applicability of high-temperature and low dose of MT combined treatment for all-male production of Nile tilapia larvae used in aquaculture industry.