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Central bearded dragons, which grow up to 60 cm in length, have a reverse system of sex chromosomes to humans. Image: Science/Alex Quinn |
CANBERRA: Sex determination in an Australian lizard depends on both genetics and temperature, say biologists who've found that heat makes genetic males develop as females.
The find challenges the traditional views of sex determination and may have implications for the affect of climate change on reptiles.
Experts had thought that the sex of reptiles was determined either by genes on sex chromosomes, which instruct the embryo to develop as male or female; or by the temperature at which embryos are incubated.
"For a long time it was thought that sex chromosomes were incompatible with temperature-determined sex," said geneticist Alex Quinn from the University of Canberra, lead author of a study revealing the find today in the U.S. journal Science.
But Quinn and his team thought there might be more to it, and looked a little deeper into sex determination in the central bearded dragon (Pogona vitticeps), a lizard with sex chromosomes that is found in much of central Australia.
Bearded dragons have a reverse system of sex chromosomes to humans. In humans, males have two different sex chromosomes, X and Y, and females have a pair of X chromosomes. The gene that determines maleness is found on the Y chromosome.
However, in bearded dragons, males have a pair of 'Z' chromosomes while the females have two different sex chromosomes, Z and 'W'.
Their experiments showed that incubating eggs at temperatures of 34°C or more, causes all genetically male dragons to develop as fully formed females, even though they retained their pair of male chromosomes. The researchers found that at lower temperatures, sex was completely determined by genetics.
"The simplest way to explain [our results] is that bearded dragons have a male-determining gene and it is carried on the Z chromosome," Quinn said. Developing bearded dragons need a high enough 'dose' from certain proteins on the Z chromosome to become male, he said. For embryos with only one Z chromosome, this dose is not reached, so it develops into a female.
The researchers speculate that temperature must somehow interfere with proteins formed from the Z chromosomes, thus lowering the dose, and allowing genetic males to develop as females.
"What we achieved was the first really conclusive demonstration that temperature can also determine sex in a species with sex chromosomes," Quinn said. "This is really exciting because it opens up more possibilities for sex determination not only in reptiles, but also in fish and amphibians."
"The more we find out about nature, the more our simple ideas… blur into much more complicated issues," commented evolutionary biologist Rick Shine from the University of Sydney. "I suspect that we'll find these multiple-factor systems to be very common, and this report is just the tip of the lizard sexual iceberg."
Scientists already new that species with temperature-dependent sex determination, like crocodiles and turtles, were vulnerable to climate change – as increasing temperatures may skew sex ratios. But this study indicates that reptiles with sex chromosomes, previously thought to be safe from this affect, could also be at risk.
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20th of April 2007, Rosslyn Beeby, The Canberra Times Australia. http:// canberra. yourguide. com. au
A four-year study of a common Australian lizard has proved hotter incubation temperatures can reverse gender in some species, turning boys into girls as the weather heats up.
The world-first discovery, made by a team of ecologists and geneticists from the University of Canberra working closely with chromosome experts from the Australian National University, challenges previous theories of sex determination and also has profound implications for climate change.
The researchers found temperatures hotter than 34 degrees during the mid-stages of incubating dragon eggs caused sex reversals, with male western bearded dragons switching gender to develop and hatch as females, but still retaining their male chromosomes.
The findings are published today in the international journal Science, and bring evolutionary biologists a step closer to discovering if there is a specific sex determination gene.
The study's lead author, University of Canberra geneticist Alex Quinn, said scientists had traditionally regarded the mechanisms of sex determination as either genetic or environmental, and therefore fundamentally different.
"We've found these two mechanisms can co-exist in one individual," he said.
Using bearded dragons caught in outback Queensland, the research team combined incubation experiments using varying temperatures with sophisticated genetic techniques that enabled them to colour-code and track the lizards' sex specific DNA.
Dr Tariq Ezaz and Professor Jennifer Marshall-Graves from the ANU's genomics group, tagged dragon DNA with green and red fluorochrome "flags" to identify and track gene sequences.
"We were surprised to also be able to pick up small sex specific differences and very distinct and small chromosomes," Dr Ezaz said.
Male dragons have two Z chromosomes, and females have one Z and one W chromosome. In humans, females have two X chromosomes and males have one X and one Y chromosome.
"What we found is the dragon equivalent of a fully formed human male with no Y chromosome," MrQuinn said.
University of Canberra professor of applied ecology Arthur Georges is one of Australia leading researchers on how temperatures affect sex determination in turtles and lizards.
He said it was previously thought that reptiles had two modes of sex determination sex determined at conception by sex chromosomes (as in humans) and sex determined by temperatures experienced by embryos during incubation.
"What we've been able to do is demonstrate for the first time the co-existence of genetic and environmental influences in one species, which opens up exciting new avenues for investigating sex determination," he said.
"We've shown female development in dragons can proceed, under sex reversal, without a W chromosome and therefore sex is not determined by a gene on the W chromosome," Professor Georges said.
As yet, they don't know if gender-bending female dragons those boys who turn into girls when the heat is on will be fertile.
Mr Quinn said, "That's the big question, whether any of the sex reversal animals that have hatched will be able to breed.
"That also had big implications for climate change as hotter temperatures become the norm, rather than an occasional or temporary occurrence."
Rosslyn Beeby.
Central Florida Herp Society
Vitamin and mineral supplements are often added to many reptile
diets. However, as with many other products, too much can be as dangerous as
not enough. Preparations should contain both fat and water soluble vitamins
and minerals essential for proper nutrition. Adding any supplement to your
reptiles water may increase the decomposition of the product as well as
decrease the reptiles water consumption. Adding supplements to salads may
effect their palatability. Commercial supplements should be stored in a
cool, dark place and products without expiration dates should be avoided.
Vitamin A deficiency is rare in herbivorous reptiles. Beta carotene, the
precursor of vitamin A, is present in green leafy plants, yellow and orange
vegetables, and fruit. Yolk remaining at the time of hatching will usually
provide adequate vitamin A levels for approximately six months. Vitamin A
is stored in the liver; diets heavy in animal protein can deplete these
stores. Vitamin A deficiency is most commonly observed in young chelonians
fed improper diets. Hypovitaminosis A is the most common cause of
nonspecific, inflammatory, periocular disease in reptiles. The disease is
usually evident within the ocular, genitourinary, and respiratory systems.
Affected animals will exhibit swollen or reddened eyelids and mucous
membranes, nasal discharge, or respiratory distress (dyspnea).
Hyperkeratosis of the skin and mouth parts may also be observed. In
herbivorous reptiles, any vitamin A deficiency should be thoroughly
investigated prior to the use of supplemental vitamin A.
Vitamin A overdosage may occur secondary to excessive supplementation.
This will occur when a single, intramuscular injection, greater than
10,000U/kg is administered. Liver failure, ictrus, jaundice, depression,
and anorexia may all be observed in an animal with a vitamin A overdose.
Tissue sloughing can be observed at the injection site when injectable
vitamin A is used. The exposed tissue will then become susceptible to
bacterial infections. Glucocorticoids will prolong the overdose effect and
therefore should not be used in affected animals. There is no evidence that
vitamin A can be used to stimulate appetite in anorexic reptiles.
Thiamin (Vitamin B1) deficiency is caused by feeding items that contain the
enzyme thiaminase rather than by feeding a thiamin deficient diet.
Thiaminase breaks down the animals stored supply of thiamine. Herbivorous
reptiles may acquire this disease because thiaminase is found in ferns and
ornamental house plants, and by feeding large amounts of frozen vegetables.
Freezing decreases vitamin levels and increases thiaminase activity.
Carnivorous reptiles may acquire a deficiency because thiaminase is found in
fish. Thiamine is necessary for the proper development and function of
nervous tissues. A thiamine deficiency is characterized by nervous
disorders such as: twitching, spasms, blindness, abnormal posture, and an
inability to use specific muscle groups. In snakes, inability to accurately
strike their prey can be observed. Thiamin deficiency can also lead to
dystocia, egg-retention, and other reproductive disorders. These signs may
also be seen with other vitamin deficiencies such as vitamin E or selenium
and they may also be related to other disease processes. Treatment with a
thiamine supplement usually will correct the problem. Adding a small amount
of Brewer's yeast to any frozen food item prior to feeding will also help.
When fish is used as a food item, make sure it is either fresh or
fresh-frozen. Boiling fish prior to its being fed will denature the
thiaminase.
The other B complex vitamins are synthesized by the bacteria and protozoa
with the intestinal tract of normal reptiles. Raw egg whites contain
avidin, which prevents biotin from being processed within the animal. A
deficiency may occur in feeding egg-eating reptiles a diet of exclusively
whole raw eggs. Egg-eating reptiles in nature rarely acquire a biotin
deficiency because most eggs eaten are fertile and embryonic tissue contains
biotin. Occasionally, especially following any antimicrobial therapy, the
normal flora will die off, allowing a deficiency to occur. By using vitamin
B complex supplementation as well as intestinal culture inoculation, the
situation is easily correctable.
Vitamin C (ascorbic acid) is produced within the kidney and
gastrointestinal tract of reptiles. A herbivorous diet, rich in green,
leafy vegetables and citrus fruits is sufficient to prevent this deficiency
from occurring. Affected reptiles will exhibit signs of bruising, bleeding
gums, separation and tearing of the skin, and occasionally diarrhea.
Administration of Vitamin C, is recommended as part of the medical
treatment of infectious stomatitis.
Vitamin D levels will have a direct role o n the reptiles calcium level.
In carnivorous reptiles, this will occur when the animals are fed skeletal
muscle and viscera without the bones. In herbivorous reptiles, this will
occur when the animals are fed diets containing oxalates or from low or no
exposure to ultraviolet stimulation either from direct, unfiltered sunlight
or from full-spectrum reptile lights. Vitamin D deficiency will result in a
decrease in the calcium absorption from the large intestine and a subsequent
decrease in the quality of bone growth and development. These changes will
be more obvious in young, rapidly growing reptiles.
Vitamin D overdosage will cause an excess amount of calcium to be absorbed,
resulting in calcium deposits within soft tissues, including the heart.
This may occur secondary to oversupplementation or following ingestion of
rodentacides containing cholecalciferol.
Vitamin E deficiency is usually related to appalling husbandry practice
pertaining to the reptiles diet, such as feeding rancid or spoiled food.
Carnivorous reptiles may develop a deficiency because fish is high in
saturated fats, resulting in this condition. If fish is fed to a reptile,
you need to make sure it is either fresh or fresh-frozen and avoid fatty
fish such as goldfish. Signs of vitamin E deficiency include anorexia,
steatitis, and swollen nodules under the skin. These nodules are actually
abnormal fat deposits, the skin covering these areas is usually discolored
yellow or orange. Occasionally, cardiac muscle abnormalities similar to
"white muscle disease" in mammals, have been diagnosed in reptiles with a
vitamin E deficiency. This myopathy is caused by a decrease in the membrane
integrity of the muscle cells, due to a decrease in the antioxidant effect
of vitamin E.
Vitamin K is synthesized by the normal intestinal flora of reptiles.
Vitamin K deficiency may occur following long-term use of oral antibiotics,
or following consumption of animals poisoned with warfarin, strychnine, or
other coumadin derivatives. Feeding fresh yogurt, Lactobacillus, or fecal
cultures will help replenish the reptiles normal intestinal flora.
When treating sick reptiles, administration of vitamins should be performed
with some caution. Most vitamins work as enzymes with other nutrients and
this process requires fuel to occur. Administering vitamins to anorexic
animals without also administering food, may result in an overdose of the
vitamin. There are varied doses of most vitamins for reptiles. Because of
this, specific doses should be calculated for each patient prior to their
use.
References
1.) Mader, Douglas: Reptile Medicine and Surgery, W.B. Saunders Company,
1996.
2.) Frye, Fredric: Reptile Care; An Atlas of Diseases and Treatments, Volume
1. T.F.H. Publications. New Jersey, 1991.
3.) Frye, Fredric: A Practical Guide for Feeding Captive Reptiles, Krieger
Publishing Company, 1993.
