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the wxyz of frogs: sex changes in frogs

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Dr Arthur White, President of the NSW Frog and Tadpole Study Group (FATS) looks at the factors that influence the sex determination of frogs, with some surprising information.

Humans have 46 chromosomes in their cells, 44 of which are uniquely paired (autosomes) and the other two are the sex chromosomes designated as the X and Y chromosomes. Two paired X chromosomes make a female human and one X and one Y chromosome (unpaired) make a male one. These sex chromosomes trigger a sex gene (that is not located on the sex chromosomes) that in turn fires off all the secondary sex hormones which lead to the development of adult human sexual structures that are generally irreversible from conception.

Elsewhere in the animal kingdom sex reversals as adults are much more common Mammals and some birds who rarely change sex are the exceptions in the animal kingdom. Sex reversal was probably the ancestral condition for all vertebrates, there being considerable advantages to being able to change sex ratios in certain conditions. The ovaries and testes of amphibians (and all vertebrate animals) develop from the same primary cell mass. Most amphibians, regardless of sex, retain remnants of the other sex organs inside the body, although these vestigial organs are usually inactive (unless activated). Other animals, like rodents, have plasticity in their gender determination up until the time they are born but once determined that is also irreversible. Fish are perhaps the best known animals that can change sex: many fish species start life as one sex and and then later change sex with age. Some fish species change gender according to how much food is available or whether there are enough fish of the opposite sex to mate with.

Frogs have two sex chromosomes like humans but they are not always the same pairs. They can have X and Y pairs but also Z and W pairs (ZZ, ZW). The X and Y chromosomes and the Z and W chromosomes are species specific although no one knows why or how this evolved in frogs. Frogs with X and Y chromosomes cannot mate with species that have Z and W chromosomes. Indeed, there are genera within some of those families that have X and Y chromosomes and others that have Z and X chromosomes. Frogs can be genetically coded to be a male and still come out as female by simply inactivating the female chromosomes, which means that adult frogs can change sex if the need arises.

The African Reed Frog Hyperolius viridiflavus is probably the most diverse species of vertebrate on the planet, with 60 known sub-species. It is widely distributed over much of central and southern Africa so it is extremely successful. If a population of these frogs has a lot of females and hardly any males some of those females will opt to become males to create more optimal breeding pairs. The reverse situation is also true but it is not known how they decide which frog is going to change and what is the chemical trigger.

The African Reed Frog carries both male and female sex organs but only one sex organ is activated at a particular time, the other remaining dormant. The change happens over a couple of months. It is only recently that this knowledge has come to light and people have started field observations and controlled laboratory trials to see how the trigger mechanism works.

There has been work done on fish because there are commercial reasons for doing so and there has also been a large research program looking at reptiles, in particular at the temperature dependence of sex ratios. When crocodiles and alligators lay eggs and incubate them in a temperature range of 24-27° then nearly all males will hatch. Outside of that range (higher or lower) the eggs will nearly all hatch as females. For other reptiles the sex determination pattern is that low temperatures produce all females and rising temperatures eventually produce all males.

Evolution, then, is dependent a lot of the time on circumstances as well as genetics, who survives and who doesn’t and how many different niches there are for very different biological outcomes. Frogs, amongst vertebrate animals, have retained the ability to change sex when required, for humans sex change requires surgical intervention.

Dr Arthur White was interviewed by Ruby Vincent for A Question of Balance, Summary text by Victor Barry, March 2016.

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