Our results demonstrate that high temperature (37C) treatment can induce permanent sterility in female Nile tilapia (O.
niloticus, Chitralada) through a complete loss of germ cells. The
induced sterility was irreversible and tested in four age groups
(juvenile through adult stage) of the female Nile tilapia. To confirm
the irreversibility of sterility in the treated fish, the 37C-treated
fish were transferred to water at control conditions for additional
three to six months until the control fish were at the stage of final
sexual maturation. The 37C treatment induced sterile fish had
very small, thread-like gonads without germ cells and extremely
low levels of steroid hormones. The 9 dah-stage juvenile fish cultured at 35C for 45 days had gonads with degenerating germ
cells. The germ cell degeneration was stopped and proliferation
resumed when the exposed fish were transferred to the water at
control temperature and maintained for additional six months. In
contrast, all the fish that were reared at 37C permanently lost
germ cells and germ cells did not recover even after the extended
period of rearing in the water at control temperature. TheO. niloticus(Chitralada strain) seemed to have lost all germline stem cells
at 37C in the initial 30–45 days of exposure, whereas the 35C
seemed not to affect germline stem cells as a result germ cells
resumed proliferation when maintained at control temperature.
Present observations also suggest that although the exposure to
35C suppressed germ cell development, the effective temperature
to induce sterility in female Nile tilapia (O. niloticus, Chitralada
strain) is 37C. The exposure duration needed to induce permanent sterility seems to depend on the age of fish, with a shorter
duration appropriate for early stage juveniles and a longer duration
for young and adults.