Temperature-Dependent Sex Determination In Turtles

Introduction

            Temperature-dependent sex determination (TSD) is a significant biological experience recorded for both invertebrates and vertebrates. Turtles are recognized as lacking of heteromorphic sex chromosomes, it could be the ZW for female heterogamety or XY for male heterogamety; thus, their means and method used for sex determination is very much distinct from human beings. Among the family of turtles, only a small number of turtles do not adapt temperature-dependant sex determination. The eggs’ incubation temperature at a delicate and susceptible stage of growth and progress during the mid-trimester activates the gonadal development which leads to the sex of the hatchling (see “Temperature-dependent sex determination (TSD)”. ETI-Turtles of the World). Moreover, the temperature does not affect the sex-determination of a soft-shelled turtle but it restrains sex differentiation instead of causing a differential mortality of sexes (see Bull, J.J. and Vogt, R.C. “Temperature-dependent sex determination in turtles”. Vol. 206, issue 4423). But apparently, temperature influences the sex of hatchling Chelonia mydas, green turtles, coming from eggs that are hatched in the beach hatchery in various temperature regimes. Cool and cola nests have a temperature lower than 28 degrees and do not give more females. Some inter-sex hatchling were coming from low temperatures (see Morreale, S.J., Ruiz, G.J and et. Al. “Temperature-dependent sex determination: current practices threaten conservation of sea turtles).

* TSD Patterns in turtles.

            There are two TSD patterns that been found out in turtles. In pattern I, it has one transformation zone of temperature below in which the incubation produces about 100 percent males and above produces only females. Pattern I of TSD can be found in selected Bataguridae and the Cheloniidae, Emydidae, Carettochelyidae, Dermochelyidae and Testudinidae. While in pattern II, identified from the Kinosternidae, Macroclemys temminckii, Pelomedusidae and a number of Bataguridae has two transformation zones having males dominate at intermediary temperatures while females into extreme. Pattern I happens mainly in turtles where the adult females are bigger compared to adult males while pattern II is located primarily in smaller female turtles compared to male turtles. The tinier sex is normally yielded in the coldest incubation temperatures. In addition, incubation temperature appears that there is no great effect on sex ratios in the Chelidae’s family. The left picture below shows the pseudemydura umbrina-western swamp turtle, photo was taken by Gerald Kuchling, while the picture on the right side shows the clemmys insculpta-wood turtle, photo was taken by James H. Harding (see “Temperature-dependent sex determination (TSD). ETI-Turtles of the World).

*Sex Determination

            In several organisms, an offspring’s sex will be only identified through its sex chromosomes or a group of genes on the chromosomes no matter in spite of the environmental adaptation. This kind of condition is recognized as “genotypic sex determination”. Nonetheless, in various organisms, the immediate surrounding identifies the gender of the offspring whether it is a male or female in which the condition is denoted as “environmental sex determination”. Nevertheless, environmental sex determination is influenced too by genes (see Madge, David. “Temperature and Sex Determination in Reptiles with Reference to Chelonians. Vol. 2, Number 3, 1985).

*Genotypic Sex Determination

            It is a fact that several animals have a pair of sex chromosomes which identifies whether the offspring is male or female. In every mammal and majority of the insects, the male gamete always have both X and Y chromosomes which a half of its sperm carries the Y chromosome and a half of it carries the X chromosome. The male is known as heterogametic. On the other hand, the female gamete only has the X chromosomes where the unfertilized eggs transmit an X chromosome. The female is known as homogametic. The offspring will become a male when Y sperm fertilizes the X chromosome which is the egg; but if the X sperm fertilizes the X chromosome (egg), the offspring will be a female (see Madge, David. “Temperature and Sex Determination in Reptiles with Reference to Chelonians. Vol. 2, Number 3, 1985). 

References

1. “Temperature-dependent sex determination (TSD). ETI-Turtles of the World http://www.eti.uva.nl/Turtles/Turtles2a.html
2.  Bull, J.J. and Vogt, R.C. “Temperature-dependent sex determination in turtles”. Vol. 206, issue 4423. http://www.sciencemag.org/cgi/content/abstract/206/4423/1186
3. Morreale, S.J., Ruiz, G.J and ET. Al. “Temperature-dependent sex determination: current practices threaten conservation of sea turtles. Vol. 216, issue 4551. http://www.sciencemag.org/cgi/content/abstract/216/4551/1245

4. Madge, David. “Temperature and Sex Determination in Reptiles with Reference to Chelonians. Vol. 2, Number 3, 1985.        http://www.deancloseprep.gloucs.sch.uk/chelonia/testudo/articles/v2n3sex.html