Role of P‐450 aromatase in sex determination of the diamondback terrapin, Malaclemys terrapin

Sex determination in the diamondback terrapin, Malaclemys terrapin, is temperature-dependent. Eggs incubated at 31 degrees C, and above, hatch in approximately 45 days as females. Eggs incubated below 27 degrees C hatch in about 60 days as males. Sex is not reversible after hatching. Nest temperatures in the wild can be as low as 20 degrees C and as high as 37 degrees C with as much as a 10 degrees C diel cycle. The shortest incubation time measured in nature was 56 days and the longest approaching 120 days. Nests in our study site produced predominantly (> 95%) male hatchlings. Treatment of developing embryos with estrogen produces females at male producing temperatures while treatment with fadrozole (a nonsteroidal aromatase inhibitor) induces partial male-like gonads. Treatment with a steroidal aromatase inhibitor (4-hydroxyandrostenedione, 4-OHA) had no effect on sex determination. Both fadrozole and 4-OHA are potent competitive inhibitors (Ki approximately 40-50 nM) for terrapin in vitro aromatase activity. These findings are consistent with aromatase expression being a key step in sex determination of terrapins. We have cloned a partial single copy P-450 aromatase from the terrapin using a cDNA library constructed from ovarian mRNA. This partial clone is highly homologous to other vertebrate aromatases.