A mechanistic model of temperature‐dependent sex determination in a chelonian: the European pond turtle

Abstract

Summary 1. In temperature-dependent sex determination (TSD) species, offspring sex-ratio is dependent on the environmental temperature. For oviparous sauropsid species, temperature within the nest influences gonadal sexual differentiation during a small window of embryogenesis called the thermosensitive period (TSP). 2. The absence of sexual dimorphic characteristic in juveniles of TSD species coupled with the lack of a non-invasive method to determine sex is a great obstacle to studies estimating sex-ratios under natural conditions. Some authors have proposed proxies of sex-ratio obtained through correlative approaches. They commonly extrapolate the empirical profile of sex-ratios as a function of constant incubation temperature established for several species in the laboratory to a field context. However, most of these proxies have been refuted by studies realized under field conditions, and consequently they cannot be used to predict sex-ratio under natural conditions. 3. Here, we propose a new thermal model of TSD using a mechanistic approach. We built this model from a collection of published data of physiological processes (i.e. the growth of the embryo, the growth of gonads and the activity of the enzyme aromatase) underlying the TSD mechanism, for the European pond turtle ( Emys orbicularis ). This new approach provides integration of incubation temperature fluctuations, as well as the cumulative and differential effect of high and low temperatures on sexual differentiation to embryo sex determination. 4. The significant consistency obtained between observed and predicted sex-ratios both at diverse constant and fluctuating incubation temperatures provides hope to develop an efficient method to predict sex-ratio under natural conditions. The reliable validity of this new model could have wideranging implications for the understanding of the TSD mechanism, as well as its evolutionary and ecological consequences in natural populations.