Temperature-dependent Sex Determination - Types

Types

Within the mechanism, two distinct patterns have been discovered and named Pattern I and Pattern II, with Pattern I further divided into IA and IB. Pattern IA has a single transition zone, where eggs predominantly hatch males if incubated below this temperature zone, and predominantly hatch females if incubated above it. Pattern IB also has a single transition zone, but females are produced below it and males above it. Pattern II has two transition zones, with males dominating at intermediate temperatures and females dominating at both extremes. Very near or at the pivotal temperature of sex determination, mixed sex ratios and, more rarely, intersex individuals are produced.

In turtles with TSD, males are generally produced at lower incubation temperatures than females (TSD IA), with this change occurring over a range of temperatures as little as 1-2 °C. At cooler temperatures ranging between 22.5 and 27 degrees Celsius mostly male turtles arise, and at warmer temperatures around 30 degrees Celsius only female turtles arise. In lizards and crocodilians, this pattern is reversed (TSD IB).

It has been proposed that essentially all modes of TSD are actually type II and those that deviate from the expected female-male-female pattern are simply never exposed to extreme temperature ranges on either end.