TSD Basics

 

Temperature dependent sex-determination (abbreviated as TSD) is an important phenomenon that first came to light in the scientific community during the 1960’s and 70’s. One of the earliest investigations of temperature effects on sex ratio was reported by Charnier in the West African lizard Agama agama (Valenzuela et al, 2003). Investigation in depth with regard to TSD began in earnest with the publication of J.J Bull’s article “Sex Determination in Reptiles” (Bull, 1980). In his paper, Bull set forth a number of hypotheses regarding the evolution, patterns, and advantages of TSD, many of which are still being researched today. It is important to clarify exactly what is meant by terms such as genotypic sex-determination (GSD) and environmental sex-determination ESD). For the purposes of this site, GSD is taken to mean a system in which the sex of the offspring is irreversibly determined by its genotype. Conversely, ESD is a system in which the sex of the offspring is determined by the environment, or in the case of TSD, by temperature. There are in fact other types of ESD such as moisture dependent sex-determination, but these are not nearly as well researched as TSD (Janzen, 1991).

Since TSD was first discovered, the number of reptilian species shown to employ TSD has grown considerably, but still the vast majority of reptiles have not been investigated (Janzen, 1991). Oftentimes, determining the presence of TSD is more difficult than determining the presence of sex chromosomes. Of the species karyotyped thus far, it appears the sex chromosomes are absent in crocodilians and most turtles and present in most lizards and snakes. TSD has been shown to exist in all crocodiles and the majority of turtles along with a few species of lizards (Janzen, 1991). Among turtles, only two species in the mud turtle family of Kinosternon have been found to have sex chromosomes (Bull, 1980).

Hatchling Kinosternon subrubrum. Photo by J.D. Willson, used with permission by Michael Dorcas.

TSD has been documented to occur in two distinct patterns in turtles:

  • Pattern 1- Males produced at low temperatures and females produced at high temperature with a single transition zone
  • Pattern 2- Females produced at both low and high temperatures with males produced at intermediate temperatures; two transition zones observed

Transition zones are typically defined as the range of temperatures, typically 1-2 degrees Celsius, in which 50% of each offspring phenotype are produced (Pieau et al, 1995). These transitions zones, or threshold temperatures vary from species to species and may even vary intraspecifically. In one study of the turtle Chelydra serpentina, 88% percent males were produced at 26 degrees Celsius, but three subsequent broods at the same temperature yielded nearly 100% males (Janzen and Paukstis, 1991). However, it appears that threshold temperature is conserved among closely related species (Bull, 1980).

Chelydra Serpentina Photo by Michael Dorcas, used with permission from Michael Dorcas.

Table 1.

Sex-determining mechanism

Turtle Species (Family, genera, or species)

TSD- Pattern 1

Bataguridae (some) Carettochelyidae, Cheloniidae, Dermocheylidae, Emydidae, Testudinidae

TSD- Pattern 2

Pelomedusidae, Kinosternidae, Macroclemys temminckii, Bataguridae (some)

GSD

Platemys, Staurotypus, Siebenrockiella, Kachuga smithii, Chelidae

Table 1 depicts the variety of sex-determining mechanisms present in the different families, genera, and species of turtles. (Information compiled from ETI- Turtles of the World <http://www.eti.uva.nl/turtles/Turtles2a.html>).

Fig. 1

Figure 1 adapted from: Valenzuela, Nicole, Dean C. Adams, and Frederic J. Janzen. “Pattern Does Not Equal Process: Exactly When is Sex Environmentally Determined?” The American Naturalist 161.4 (2003). NCLIVE. InfoTrac OneFile. Davidson Coll. Lib., Davidson, NC. 15 Sept. 2004.

It is important to note that the absence of heteromorphic sex chromosomes does not necessarily imply that a species exhibits TSD. Also, a number of cases of TSD may in fact be the result of confusion between sex determination and sex differentiation. Other circumstances such as differential embryo mortality and reabsorption or temperature induced sex reversals may be mistaken for TSD (Valenzuela et al, 2003).

 

 

Questions? Email me kykinsell@davidson.edu