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The Background and Evolutionary Advantages of Temperature-Dependent Sex Determination in Crocodilians

Daniel Thomas, Manav Padia, Sasha Wavre

ABSTRACT

Definition of Crocodilian: A large predatory semi-aquatic reptile that includes: crocodiles, alligators, caimans, and gharial. Crocodilians are distinguished by long jaws, short legs, and a powerful tail.

Along with crocodilian species, sex is dependent on temperature in the following species: sea turtles & some lizards

INTRODUCTION

For crocodilians sex determination is based on the temperature of the environment, at temperatures around 30 degrees celsius all the hatchlings will be female and temperatures around 34 degrees celsius all the hatchlings will be male. Temperatures in between will give varied ratios of males to females.

In general there are two different types of nests that crocodilians build. In the first one the eggs are laid into a holle in the ground, such as on beaches or river banks. Here the temperature is maintained relatively stable by the insolation provided by the surrounding materials.

The second type of nest is a mound that is built up using vegetation. The insulation provided by the vegetation is less than that of sand for example, but the heat is mainly generated by the rotting vegetation surrounding the eggs.

According to the charnov bull model the temperature based sex determination can enhance maternal fitness relative to genotypic sex determination.

There is a strong bias of females to males around the ratio of 10 females to 1 male, this is often found in reptiles that have temperature based sex determination.

One of the main causes of the temperature sex based determination is the enzyme aromatase.

This works on effects of temperature as enzymes are highly sensitive to temperatures. Each enzyme has an optimum temperature at which its functionality is most rapid

Other factors temperature affects includes: hatchling size, pigmentation patterns, post-hatching growth rates and thermoregulation of juvenile crocodilians

Females born at 30 degrees will have absorbed more abdominal yolk, this causes the female to grow faster and become larger than males in their early like. This leads to heavier females becoming sexually mature faster compared to lighter males and females

HISTORY OF THE DISCOVERY OF TEMPERATURE DEPENDENT SEX DETERMINATION

The first ever description of temperature dependent sex determination was done by Madeline Charnier at the university of Dakar in Senegal in 1966. She witnessed that different temperatures affected the ratios of the number of males and the number of females in a single batch of eggs of the rainbow agama lizard.

In 1974 some researchers establishes that there was an existence of genotypic sex determination among turtes, this result weakened the idea for temperature dependent sex determination in reptiles and in vertebraes. Bull and Eric Charnov proposed a model for the evolution of environmental sex determination in 1977, they suggested applying the model to plants and invertebrates and not to vertebraes. The Charnov bull model shows the conditions necessary under which the evolution of environmental sex determination occurs.

In 1979 Bull and Richard Vigt from the institute of national de pesquisas da amazonia in Brazil showed that temperature dependent sex determination existed in some reptile species. They carried out an experiment to investigate the theory on 5 species of turtles under controlled and natural field conditions. The results of this experiment found evidence of temperature dependent sex determination in four out of the five species this then confirmed that temperature dependent sex determination exists in some vertebraes. As of 2004 it is found to believe that seventy nine species of turtles show temperature dependent sex determination.

From then on scientists have worked in order to test and determine the range of temperatures in which the number of males and females are equal. Pieau and his colleagues focussed on finding the thermosensitive period or the time in which developmental changes took place of the sexual organs.

In the 1990s, David crews from the university of texas at austin discovered that some environmental pollutants causes sex altering effects in reptiles. Polychlorinated biphenyls are molecules with strictures similar to estrogen and hence this pollutants override the effects of temperature dependent sex determination.

DEVELOPMENTAL CYCLE OF A CROCODILIAN

Crocodilian eggs have an incubation period of around 65-90 days. The sex of the crocodillian is determined by the surrounding temperature. Temperatures below 30 degrees celsius produce all females, temperatures above 34 degrees celsius produce all males, and temperatures between 30 and 34 degrees will produce mixed results on the ratios of males and females. If the eggs are moved to warmer or cooler temperatures after the temperature sensitive period the sex of the crocodile will not be altered.

The temperatures sensitive period where the sex is determined is between seven and 21 days. The natural ratio of females to males is for every 1 male there are 5 females and this can be determined due to that fact that female eggs tend to weigh more than male eggs, this difference in weight may be an evolutionary advantage in that the females would reach sexual maturity faster and hence are able to breed again and reproduce more.

The basis of the temperature dependent sex determination is the aromatase enzyme.

EFFECT OF TEMPERATURE ON ENZYME ACTIVITY

Enzymes are biological catalysts that increase the rate of chemical reactions but are not used up in the reaction. Enzymes have an active site in which substrates can attach upon, one enzyme can be attached to one substrate.

Temperature affects the rate of enzyme activity, this is because at higher temperatures there are more collisions due to a higher kinetic energy, this gives the substrates a higher velocity and hence there are more successful collisions where the substrate binds onto the active site of the enzyme.

There is an optimum temperature in which enzymes work best at, some enzymes prefer higher temperatures where are some enzymes prefer cooler temperatures, these enzymes are adapted to where the organism lives, if an organism lives in the arctic the the enzymes are adapted to cooler temperatures and if the organism lives in a hotter climate then the enzyme is adapted to higher temperatures.(figure 1)

The catch is that enzymes have an optimal temperature but temperatures above this optimal temperature will slow down the rate of the reaction, this is due to the fact that above this optimal temperature enzymes start to denature and this means that the substrate can no longer fit onto the active site of the enzyme hence the enzyme is ineffective. Once an enzyme is denatured it can not go back to its original shape and form.

The mechanism of the denaturation is that enzymes are made up of a sequence of amino acids folded into a specific three dimensional structure, upon heating of the enzyme the molecules start to vibrate a lot and after a certain point of vibration is reached then the enzyme changes form and the confirmation of the active site changes. synthax:

Cooling an enzyme reduces the rate of the chemical reaction as there are less successful collisions per unit time hence only a few substrates attach onto the active sites per unit time. Freezing the enzymes will reduce the rate significantly, because enzymes are not rigid and the active site is able to mould to the substrate to a certain degree hence freezing will make the active site rigid and the substrate will not be able to attach to the active site.

WHAT IS AROMATASE AND WHAT DOES IT DO

This particular enzyme belongs to the much larger family of enzymes called Cytochrome P450. This general family of enzymes are monooxygenases which catalyse a wide variety of reactions. The one we will be talking about is the specific enzyme called Aromatase.

The Aromatase enzyme is found in various tissues around the body, these include; the gonads, brain and adipose tissue. It has other names which include oestrogen synthetase or oestrogen synthase. As the name suggests the enzyme is responsible for an important step in the synthesis of the hormone oestrogen. The function of the enzyme is to convert androgens into oestrogen. Androgens are a group of sex hormones which are produced during puberty and include testosterone which is the main one. Aromatase enzyme converts testosterone and G19 steroids into estradiol (a form of oestrogen).

Oestrogen is an essential hormone used to regulate sex differentation, the maturation of sexual organs and the sexual characteristics of a female organism. Its other significant role is within the menstrual cycle and enables pregnancies to occur. It carries out its role by binding to receptors on the surface of the cells. Oestrogen doesnt only affect sexual organs but has been linked to having effects on various other tissues and even the nervous system.

SEXUAL MATURATION OF MALES VS. FEMALES

The expression of the aromatase enzyme through transcription and translation of the specific gene called CYP19A1 will determine the relative concentrations of the enzyme present within certain tissue types as mentioned above. The expression will have effects not only on neuroendocrine events and reproduction but also synaptic plasticity and cell survival.

Theory papers that have been written about temperature dependent sex determination are majority based on a common assumption that crocodilian populations are female biased with regard to the adult population.

As stated previously the main advantage to using temperature dependent sex determination is that the females reach sexual maturity faster than their male counterparts. This in turn means that the females are able to reproduce sooner and allow survival of the species.

OTHER EFFECTS OF TEMPERATURE ON THE DEVELOPMENT OF CROCODILIANS

The effect of temperature on a hatching egg will affect aspects pre hatching and post hatching. Higher temperatures of incubation will lead to increased growth and development rates in the embryo. As a result of this the hatching time will also be decreased compared to eggs which have been incubated at lower temperatures. In general the males growth rate is much higher than the females’ if the two were incubated at the same temperature.

The mass of hatchlings is also affected by the temperature, higher temperatures will lead to lower birth weights compared to lower temperatures, along with this lower temperatures will have a much larger abdominal yolk present.

Pigmentation of the hatchlings is also heavily dependent on the temperature of incubation. Higher incubation (around 33 degrees) will cause more darker stripes to be formed compared to the cooler incubation (around 30 degrees). The patterns of males and females are also affected, in males the number of white stripes is increased along the entire body compared to the females.

Lastly the temperature preferences of the hatchlings is also determined by the temperature at which they were incubated at. Males which are produced at higher incubation temperatures often preferred the warmer areas

GLOBAL WARMING AND ITS POTENTIAL EFFECT ON SEX DETERMINATION

Since sex of crocodillians is not based on chromosomes but on temperature during incubation. The eggs incubation temperature is very sensitive, that even a 2 degree change in temperature can drastically change the sex ratio

Since warmer temperatures skew the sex ratio towards males, it will become increasingly harder to find mates. Hence, global warming proposes a major threat on the extinction of the species

TSD species would have to change their geographic regions or even transition to genetic sex determination (GSD) in order to prevail global warming,

However, there is also data that suggests that these species may be incapable of evolving fast enough to counteract the negative consequences of global warming effects. They however can serve as ideal indicators of its biological impact.

ADVANTANGES OF TEMPERATURE DEPENDENT SEX DETERMINATION

The main evolutionary significance of temperature dependent sex determination (TSD) is still unknown. However, certain advantages to species that posses it is sexual dimorphism. TSD Maximizes fitness in the individual, since the embryo develops into the sex-best suited to theincubation conditions at the time.

Eggs withholding females crocodilians typically tend to be heavier since they absorb more abdominal yolk. This extra energy preserved helps females to grow larger than males in their early years of life.

Female crocodillians also reach sexual maturity faster than males. Which benefits the species as they can start to reproduce sooner.

CONCLUSION

In conclusion temperature has a major role in the sex determination of crocodilians and the ratio of males vs females that are hatched, if the temperature is above 34 degrees celsius all males will be produced and if its below 30 degrees celsius all females will be produced. The main cause of this is the enzymatic effect of aromatase which is responsible for the production of the sex hormone oestrogen. The effect of global warming can potentially change the ratio of males and females hatched. The evolutionary advantage is that females reach sexual maturation faster and hence are able to reproduce.

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