TBT-INDUCED IMPOSEX IN MARINE NEOGASTROPODS IS MEDIATED BY AN INCREASING ANDROGEN LEVEL

Tributyltin (TBT) exposure at different concentrations (5, 50, and 100 ng TBT as Sn/l) induces a concentration- and time-dependent imposer ( = pseudohermaphroditism) development in female Nucella lapillus and Hi nia reticulata. In both species the average imposer stage, termed as v as deferens sequence (VDS) index, and the average female penis length increases with increasing TBT concentration and duration of TBT exposu re. Testosterone added at a concentration of 500 ng/l induces a faster and more intensive imposer development compared to that induced by th e TBT concentrations used in the present experiments. Radioimmunologic al determination of endogenous steroid content reveals increasing test osterone titres in female gastropods exposed to TBT which correlate wi th the TBT concentration used and the duration of the experiment. The most marked and highest increase of the endogenous testosterone level is exhibited by females of both species exposed to testosterone. Simul taneous exposure to TBT and to the antiandrogen cyproterone acetate wh ich suppresses imposer development completely in N. lapillus and reduc es imposer development strongly in H. reticulata proves that the impos er-inducing effects of TBT are mediated by an increasing androgen leve l and are not caused directly by the organotin compound itself. Furthe rmore, TBT-induced imposer development can be suppressed in both snail s by adding estrogens to the aqueous medium. These observations sugges t that TBT causes an inhibition of the cytochrome P-450 dependent arom atase system which catalyses the aromatization of androgens to estroge ns. The increase of the androgen content or the shift of the androgen- estrogen balance in favour of androgens induces the development of pse udohermaphroditism in marine prosobranchs. Artificial inhibition of th e cytochrome P-450 dependent aromatase system using SH 489 (1-methyl-1 ,4-androstadiene-3,17-dione) as a steroidal aromatase inhibitor and fl avone as a nonsteroidal aromatase inhibitor induces imposer developmen t in N. lapillus as well as in H. reticulata.