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PostPosted: Mon Mar 17, 2008 8:58 pm 
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I've been flicking through some of the research undertaken by Alex Quinn et al. at the University of Canberra, Australia, and thought I'd put some thoughts on it up on here. It will later form part of an article for my website, but for now I don't have the time to write the full article. It looks at the effects of incubation temperature on the sex of the babies in Bearded Dragons.

Most people are aware that in species such as Leopard Geckos, there is a strong effect of incubation temperature on the sex of the hatchlings, but most people seem pretty unsure about what effects it may or may not have on Bearded Dragons. Frequently, there are dragons up for sale in various Reptile Classifieds sections, that are "incubated for female" - but when asked, the breeder doesn't actually know how to "incubate for female", and there is no consistent temperature given between various breeders who claim to incubate for female.

Apologies if some of this is a little bit too "sciency" - there will be a couple of "concluding" paragraphs at the end, for anyone who just wants the information, and not the background stuff etc.


As a bit of background:

In mammals (and humans) there are two different types "sex chromosomes" - X and Y. You get one from your mother, one from your father. If you get XX you are a girl (XX is known as the "homogemetic" combination, as the sex chromosomes are of the same type). If you get XY, you are a boy ("heterogametic").

Many reptiles and birds do it differently. In Bearded Dragons, the chromosomes are called Z and W, and it is the opposite way round. ZZ (homogametic) gives a "genetic" male, and ZW (heterogametic) gives a "genetic" female. The reasons for putting "genetic" in inverted commas will become apparent later.


The Canberra research team first of all identified the sex chromosomes - until 2005, it was known that Bearded Dragons had genetically-determined gender, but no-one had managed to identify the sex chromosomes, because they were too small. Once they could be identified, and the Z and W could be distinguished, it was possible to check whether a Bearded Dragon had a "physical" gender (hemipenes etc) that matches up to its "genetic" gender.


Their latest findings, published in April last year, show that there is a temperature-dependent element to gender:

Basically, the Z chromosome carries a gene which is crucial for male development. It probably encodes an enzyme, which catalyses a reaction to produce something that causes "maleness". This is a similar situation to humans - by default, we develop as females, and require a gene from the Y chromosome - "sry" to develop testes, which then release hormones causing development into a male. In the abscence of this gene, we follow a pathway of female embryonic development.

The reason that the Z-chromosome gene probably encodes an enzyme is that it appears to be temperature-sensitive, with an optimal temperature range, believed to be around 21c (70f) - 34c (93f). Within this range, it functions as normal.

In order to become "male", the embryo needs a high dose of whatever the end-product of the enzyme reaction is (I will refer to is as "sex-determining factor" - SDF - from now on). There is a threshold value for this - above threshold, and you get a male; below threshold and you get a female as that is the default developmental pathway. A ZZ male has two copies of the gene; a ZW female has only a single copy. This means that the dosage to the male will be doubled. A genetic female will never receive enough of the product from the single Z chromosome to develop as a physical male. However, it is possible for a genetic male to not receive enough of the SDF to develop as a male - in which case, the embryo will follow the default pathway, and become female.

Genetic Z-chromosome defects aside - which are often lethal anyway - the only way for a genetic male to not receive enough SDF is if the enzyme that catalyses its production is working suboptimally - i.e. outside of its optimal temperature range. This means that the amount of SDF produced is below threshold, so although genetically the embryo is male, it will develop as a female, with female reproductive organs etc.

Eggs incubated above 93f (the top-end of the enzyme's optimal activity range) will occasionally not receive sufficient SDF to develop as males. Above 36c (97f), the research team found that 100% of hatchlings were females, with a gradual change of %females in between. Obviously there was a high amount of mortality at that temperature, due to the incubation temperature being so extreme. They tried analysing the results assuming that all mortalities were "genetic, physical males", and still found that significantly more than 50% of eggs would have contained females. Of the "females" that hatched, 51% were found to be genetically male. They had proved temperature-dependent sex reversal (TDSR) was possible in Bearded Dragons, for high temperatures. As the enzyme's optimal range is believed to have 21c as a low-end of the range, they have not been able to prove any form of TDSR for low temperatures - no eggs incubated below 22c hatched, so there were no babies to analyse.


This of course throws up some interesting problems:

If you have a genetic male trying to produce eggs, there may be meiotic failures due to chromosomes not lining up properly, which would cause infertility. This may not be the case, as ZZ chromosomes can line up during sperm production.

If viable eggs were to be produced, they would all be genetically male - both parents are ZZ, so all babies would be as well. Unless the eggs were incubated at very high temperatures to ensure TDSR, all babies would be males.

At this point I'd like to say well done to anyone still reading, who hasn't just skipped straight to the conclusion - during my re-read I was very tempted to miss out all of the middle section...


Just to conclude, for anyone who didn't fancy reading all the stuff above:

1. It is possible to "incubate for female", but at much higher temperatures that those commonly used for incubation. I have never found anyone who would incubate BD eggs above 97f (indeed, I've never met anyone incubating above 90f, due to the risk of birth defects...). At 97f, there are very high mortality rates, and birth defects are not uncommon, due to other, non-sex-determining enzymes being affected by the high temperatures.

2. The key temperature-sensitive phase of incubation is the middle third of egg development.

3. Any "male" eggs which produce females due to incubation temperature should theoretically have reproductive problems - either egg inviability, or 100% male offspring. As the research is less than a year old, I don't think they have had chance to test these theoretical ideas - it may be that more papers are published over the next couple of years, when the team gets the chance to grow some TDSR "females" to breeding size, and breed them with normal males, to see what offspring (if any) are produced.


This means that whilst it is possible to "incubate for female" in a lab setting, a lot of the eggs will fail, and a lot of the hatchlings will have problems associated with too high an incubation temperature, so it is a dangerous and (given the theoretical fertility/100% male offspring problems) ill-advised strategy.

Babies advertised as "incubated for female" will almost certainly have the usual 50:50 chance of being male or female, unless the breeder risks high mortality and birth defects by incubating at an extreme temperature.

I hope that some people find it interesting and helpful (and apologies for the length - I've just re-read it all and it took a while...)

Andy


A couple of references I used to write this:

"The dragon lizard P.vitticeps has ZZ/ZW micro-sex chromosomes" - T.Azaz, A.Quinn et al. 2005 - Chromosome Research 13:763-776

"Temperature sex reversal implies sex gene dosage in a reptile" - A.Quinn et al. 2007 - Science vol. 316.

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PostPosted: Sat Jun 28, 2008 7:56 pm 
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I thought this was amazing!
Now i can impress my parents lol

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PostPosted: Fri Jan 25, 2013 7:02 pm 
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Fascinating read. Thank you

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