Sex reversal of the newt Triturus cristatus reared at extreme temperatures

Crested newt larvae were reared at defined temperatures, either from unclea ved eggs or from early feeding larvae, until metamorphosis when sexual diff erentiation had occurred. Trials at 18-24 degreesC showed a 1:1 sex ratio. A higher temperature trial produced more males than females, including some XX neomales, Lower temperatures resulted in a significant excess of female s, including XY neofemales. Sex reversal only occurred in about half the po ssible cases on average. Extreme temperatures must perturb the normal XX/XY system of sex determination, to reveal either an ancestral ZZ/ZW system or a still more primitive environmental control. It is suggested that neofema les (but not neomales) could occur in nature.