How is it possible that in some animal species, such as stick insects and crocodiles, the sex is still determined in the egg and can even depend on the incubation temperature? I keep stick insects myself and for some species an incubation temperature above 30°C can result in a higher percentage of males.
This is of course strange because you would say that the given genotype has already determined what the sex is, namely xx for females and xy for males. Or do they have 3 sex chromosomes, for example xxy, and can one of the 2 (x or y) be activated later, for example on the basis of environmental factors such as temperature?
Answer
Best,
Indeed, in birds and mammals, sex is determined at fertilization based on the presence of a sex chromosome. In some reptiles, such as certain crocodiles, the sex is determined at a later stage, i.e. during embryonic development. This means that the genetic information for both sexes is present in all animals: there is no real sex chromosome. The newly fertilized egg (and the newly laid egg) therefore does not yet have a sex.
The question is therefore how these genes are activated, because once the genitals are sufficiently developed, there is apparently no way back and the gender remains fixed. This is in contrast to certain fish that can still change sex as an adult under the influence of the social situation of the population.
This system has already been better studied in certain crocodiles. There is a short ‘critical’ period in the development of the embryo in which a small difference in temperature activates or not the sex-determining genes. Once the sexual organs have taken the female or male path, they continue to develop in the same direction. Halfway through the incubation period, the process is clearly closed. In the crocodiles studied, 31-32°C is the temperature for males. Temperatures above or below yield females. In alligators, a temperature below 31°C produces females and a temperature above 32°C produces males.
This temperature of the eggs is somewhat influenced by the females, who determine the composition of their offspring to some extent through the choice of the nest site, but also through their brood care.
The success of this method naturally depends on the ambient temperature, which is always an essential factor in poikilothermic animals. The risk that the conditions for an entire population (and this for a series of successive clutches) would always yield the same sex is apparently extremely small: crocodiles already existed in the dinosaur era, while only the birds of the dinosaurs have survived. Also with crocodiles, the choice of the nest site is an activity that requires a lot of energy.
On the other hand, they do not run the risk, as with chromosome-linked sex determination, that one of the sexes (with XY for males in mammals and ZW for females in birds) would be more susceptible to genetic defects. Which may be an evolutionary advantage for the species, because in the heterogametic situation these bad alleles (even if they are recessive) are continuously eliminated.
It is also striking that (e.g. in crocodiles) relatively fewer males eventually emerge, at least in comparison with what one would expect with a chromosome-determined sex determination (in principle, an equal number of male and female sperm cells could arise in mammals).
It goes without saying that sex determination via temperature is not an interesting option for homeothermic animals (such as birds and mammals)…
Answered by
Dr. Danny Meirte
Herpetology Reptiles Snakes Lizards
Leuvensesteenweg 13 3080 Tervuren
http://www.africamuseum.be
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