For softshell turtles, you’re male until you’re female

Softshell turtle embryos start as males, but with the right temperature and proteins can become female before they hatch, according to a sex determination study published in Philosophical Transactions of the Royal Society B in 2021.

Spiny softshell turtles (Apalone spinifera) are the largest freshwater turtle in North America. They lay their eggs in sunny sandbars or gravel banks near water, which will hatch 60–80 days later.

A team of researchers, led by Nicole Valenzuela of Iowa State University, found that the creatures determine their sex based on the amount of protein produced by their chromosomes before they hatch, instead of the amount and type of chromosomes that are present – a phenomenon called sex chromosome dosage compensation.

Not only this, but the whole process depends also on the temperature the eggs are incubated in – there could be too many boys if things are too warm.

Valenzuela explains that understanding this type of sex determination can help us learn about why and how animals, including humans, evolved to use proteins and chromosomes to determine sex, which might help in cases where there are sex chromosome abnormalities.

Sex determination is complex and differs between species, and the turtle is no exception. In humans and most mammals, biological sex is determined by the presence of specific chromosomes when an egg is fertilised, but in reptiles like turtles and crocodiles, it is more often determined well after the egg is fertilised, but before it hatches, based on how the egg is incubated.

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Read more: Turtle embryos can influence their own sex

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Each chromosome is responsible for making proteins that contribute to certain characteristics, so the type of chromosomes determines how much protein is eventually made.

Unlike humans, who have X and Y chromosomes, female softshell turtles have a ZW chromosome, and males have either one or two Z chromosomes. In humans, having only a single sex chromosome like this can have big consequences, such as Klinefelter syndrome and Turner syndrome.

If all the chromosomes are producing the same quantity of proteins, the ZZ males have double the amount of unique Z proteins compared to a ZW female or Z male – except the single Z males weren’t incurring any problems.

To understand what was happening, the team analysed tissue from embryos, hatchlings and adults to see which genes were being expressed.

During early embryo development, they found that expression of genes on the Z chromosomes doubled in female ZW females and Z males to compensate the imbalance of chromosomes, which meant they were producing about as much protein as the ZZ males.

However, during late-stage embryo development, the amount of protein made by the Z chromosome decreased, which allowed the W chromosome to take over and help the embryo become female before it hatches. Once hatched, there is no going back, though.

This meant that the ZW turtles produced the same amount of Z proteins as males in the early stages of embryo development and so appeared to be males – but this changed in the later part of embryo development and resulted in females hatching out of the egg.

Fascinatingly, this effect was more pronounced when the eggs were incubated in cooler laboratory conditions – the Z genes actually kept expressing at a higher level when it was warm, and femaleness wouldn’t get the same opportunity to develop, and they could hatch out as male-like instead.  

The authors suggest that if the climate becomes too warm, more male-like turtles could emerge at hatching.

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Fix and Release, the 2017 SCINEMA International Science Film Festival winner of Best Documentary, tells the story of the team behind a turtle trauma centre in Ontario, Canada, and the challenges that freshwater turtles face surviving in a modern world.

Watch the film here.