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Andy Edge
Hades Dragons
Janauray 2008
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 "homogametic"
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, humans 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 absence 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 genetic and physiological males.
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. TDSR females can start to appear from around
93f.
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 non-viability, 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.
4. There have been reports of so-called "hot" female Bearded
Dragons. This is a well-known phenomenon in some female Leopard Geckos which
have been incubated at a high temperature (intended to produce males). I have
written another Article looking at this:
Temperature-dependent Sex Determination in Leopard Geckos. The
Leopard Gecko mechanism predicts an increased sensitivity to Testosterone caused
by increased egg incubation temperature. If a similar effect was present in
Bearded Dragons, this would explain aggressive, stereotypical "male" behaviour
by "hot" females.
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. Additionally, many of the TDSR
"females" produced may have behaviour problems that would make them entirely
unsuitable for breeding/living with other females. This pretty much defeats the
point of attempting to produce more females in the first place.
** 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. **
As this is fairly
recent research (2007), at the time of writing there has not been sufficient
time for the team to grow on the TDSR "female" babies, to see whether they are
able to produce viable eggs, and whether all of those eggs would turn out to be
genetic males as predicted.
References:
"The dragon lizard P.vitticeps has ZZ/ZW micro-sex chromosomes" - T.Azaz,
A.Quinn et al. 2005 - Chromosome Research.
"Temperature sex reversal implies sex gene dosage in a reptile" - A.Quinn et
al. 2007 - Science vol. 316.
"Effect of hormonal manipulation on Sociosexual Behaviours
in adult female E.macularius" - Flores and Crews 1995, Texas University -
Hormones and Behaviour 29.
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