Temperature-dependent sex determination
Sex determination is a complex biological system that decides the type of sexual characteristics to develop on each organism. Many people are conversant with chromosomal sex determination which is actually genetic in nature and mostly leads to the development of two sexes in sexual organisms. Genetic sex determination involves different genes and alleles for males and females where by in some cases these are located in different chromosomes. These genes determine the resultant sexual morphology; and a good example is human beings. This is not always the case. In some organisms, sex is determined by social cues or environmental cues like temperature while some sex determination systems have not been clearly understood. These are called non-genetic sex determination and are common in fish and many species of reptiles including turtle, tuatara, alligators, and some snails. Nevertheless the fully understanding of the mechanisms that determine the sex of each individual has remained complicated and a challenge to evolutionary biology.
Temperature-dependent sex determination (TSD) is one type of non genetic sex determination system where by the temperature in which the eggs are incubated establish the sex of the resulting organism. This type of sex determination is most prevalent in amniote vertebrates which fall under class reptilian, but also exists in some species of birds, and fish. Eggs are affected by temperature changes during a very critical period of incubation known as thermo sensitive period, which is the middle third of the development of the embryo. This period has been carefully studied and identified in gonads of turtles which experience temperature-dependent sex determination.
It has been widely accepted that, genotypic sex determination and temperature dependent sex determination are incompatible and mutually exclusive mechanisms and that the sex of the organisms is never in any way under the influence of either environmental temperatures or sex chromosomes. This theory points out that there is no genetic predisposition in the embryos of the temperature sensitive organisms to grow as either a female or a male organism. This can simply translate in to the fact that, the embryos of these organisms do not have defined sex until they enter into the thermo sensitive period of their development. This model has however been challenged recently with evidence emerging to support the fact that, there are indeed chromosomes determining sex and temperature influence involved in this sex determination system.
Organisms in which both sex chromosomes and incubation temperatures act together to determine the sex of the embryo represent an evolutionary state between the two extreme points; that is, complete TSD and complete GSD. There are possibilities that some species of reptiles exhibit even more complicated scenarios where incubation in either high or low temperature can lead to sex reversal.
Since Temperature-dependent sex determination was discovered, the number of organisms employing this mode of development has increased considerably regardless of the fact that most reptiles are yet to be investigated. This is due to the improved technology of research tools for determination of TSD which is considered difficult than the determination of sex chromosomes. TSD has been revealed to exist in all species of crocodiles, common in many species of turtles and also exists in some lizards.
Research in crocodiles has revealed that, temperature also determines other traits like pigmentation, post hatching growth rates, hatching size, and also thermoregulation by the hatched crocodiles. It has been hypothesized that there exists some initial sex differentiation mechanism which engages a quantum period of time. After this period, a thresh hold is achieved for the required dose of male determining factor. The conditions governing the induction of male crocodiles are precise, but they vary greatly within given individuals even within the same population. The development of females seems to be default with the hypothalamus playing an important role in the later determination sex differentiation mechanism.
Two different types of TSD have been identified and classified. These are pattern 1 and pattern 2. Pattern 1 is further sub divided into 1A and 1B and a transition region in pattern 1a where the eggs develop into male embryos when incubated below this temperature region and into female embryos when incubated at temperatures above this temperature region. Pattern 1B's transition zone has females being produced below it and males being produced above it. On the other hand, pattern 2 has an intermediate zone where males are produced and the females in this pattern are produced in both extremes. In all scenarios, mixed sex ratios or intersex organisms are produced very near the temperature of determination.
The transition zones are usually defined by the range of temperatures whereby 1-2 degrees Celsius are most prevalent and there is the possibility of production of 50% of each sex or phenotype. These transitional zones also known as thresholds vary from organism to organism intraspecifically and also from species to species. The threshold temperature has however been conserved in related or close species. For example; in the turtle Chelydra serpentina, when the eggs of this turtle were incubated at 260 Celsius, 88 per cent of the offspring were males. The three subsequent broods that followed however yielded 100 per cent males (Janzen & Paukstis, 1991). It is quite important to note that heteromorphy sex chromosomes do not necessarily trigger either TSD or GSD systems of sex determination. In some cases, it has been observed that TSD may occur due to confusion between sex differentiation and sex determination. Keen study of these scenarios is always necessary as confusions have been made before regarding reabsorption and differential embryo mortality and even temperature stimulated sex reversals.
Although in these organisms sex determination and the development of gonads in either ovaries or testes is entirely temperature dependent during the thermosensitive period, different treatments before or after incubation have clearly indicated the involvement of estrogens in the differentiation of the sexes. When eggs are treated with exogenous estrogens at a male producing temperature, they have been shown to develop ovaries where as treatment of eggs with aromatase inhibitors or antiestrogens trigger the development of testicles in at a female development temperature.
These studies opened a new field of research involving the synergism between hormones and temperature. Studies indicate a very close pathway between hoe hormones and how temperature work. The administration of less hormones have been observed to result to sexual shift which is witnessed when the incubation of the eggs near the pivotal temperature. Scientists have hypothesized that there may be genes, which temperature acts on, coding for steroidogenic enzymes and test have indicated that there may be a genetic start point. This just proved the complexities involved in this system and how it over laps with the traditionally known Genetic Sexual determination pathway thus proving our poor understanding of the mechanisms which control male and female commitment.
Research has however provided evidence that in organisms exhibiting TSD, the gonads are usually targeted by the temperature which triggers sexual differentiation. It has also been proven that, hormone aromatase and estrogen also targets these gonads. a theory put forward in 1970s tried to explain that, TSD was more advantageous than GSD in that, natural selection would favor TSD over GSD. This theory was later validated but its credibility in reptiles remained unclear. Scientist is also doing further research to gather evidence of sex manipulation through parental care. There has however been no break through regarding this hypothesis. Much research is still needed to uncover all the truths of Temperature-dependent Sex Determination.
Tay, . E. B. K. Fink, S. H. McKnight, S. Yarlagadda, and J. W. Gillespie Jr. Accelerated Curing of Adhesives in Bonded Joints by Induction Heating
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