It’s a good thing human sex isn’t determined the same way a parasitoid wasp’s is, because “sugar and spice and everything nice” is much easier to rhyme than “sperm and moderate temperatures.” But that’s what little wasp girls are made of. A mother wasp can choose the sex of each egg she lays by deciding whether or not to fertilize it. Depending on the temperature of her environment, though, she may not get her way.
Whether you are biologically male or female isn’t really determined by spices, snips or snails, of course, but by the genes you receive from your parents. Outside of the mammal world, things get more interesting. In reptiles such as turtles or alligators, the sex of a developing egg may depend on the temperature of its nest. In certain insects, every fertilized egg becomes a female, while every unfertilized egg becomes a male (who has half the number of chromosomes as his sisters).*
This system of fertilized females and DNA-poor males shows up in bees, ants, and wasps, among other insects. But temperature could still matter—maybe one type of embryo is less likely to survive a cold snap, for example. Joffrey Moiroux, a biologist at the University of Montreal, wanted to tease apart these two different means of sex determination in a wasp species. It would require him scrutinizing every wiggle of a female wasp’s abdomen as she laid her eggs.
The species Moiroux and his coauthors chose to study was Trichogramma euproctidis, a tiny parasitoid wasp that lays its own eggs inside the eggs of butterflies and moths. (Things don’t end well for the butterfly or moth embryos already inhabiting those eggs.)
The researchers kept 90 recently mated female wasps at either 14, 24, or 34 degrees Celsius for an hour. (That’s 57, 75, or a toasty 93 degrees Fahrenheit.) Then they presented each female with 20 moth eggs, laid out in a neat grid like a jumbo egg carton.
By studying video recordings of each female wasp laying her eggs inside the unfortunate moth eggs, Moiroux could tell which offspring the mother intended to be female. Partway through laying these eggs, she’d pause for a few seconds—this is when she added sperm, which she’d stored inside her body after mating, to the egg. The eggs that she popped out with no pause, and no sperm, would become the unfertilized males.
For each batch of eggs, then, Moiroux knew the ratio of male to female offspring that the mother meant to create. Later on, when those eggs hatched, he counted the young to see whether their mother’s plan had worked.
At the highest temperature, mothers laid more (no-pause) male eggs than they did at medium or low temperatures. This is likely something a mother does to increase the fitness of her offspring, the researchers say. Higher temperatures make insects grow smaller; perhaps the mother lays more male eggs because they’ll be just as fertile at a smaller size, while her female young won’t.
When the eggs hatched, the sex ratios at medium and warm temperatures were similar to what the mothers intended. But the coldest temperature skewed the sex ratio toward males. This wasn’t because female embryos died more often, the scientists found. It might have been because cold temperatures damage or impair sperm—and when a mother thinks she’s fertilizing an egg, it may not work.
So extreme temperatures produce extra male wasps, whether it’s the mother’s choice (in hot temperatures) or outside of her control (in cold). This means climate change could lead to extra males in this species, says Moiroux. Extreme weather events can mean both heat waves and cold snaps, and both of these will result in more male wasp babies.
However, Moiroux adds, behavior can evolve very quickly. Since mother wasps already adjust their behavior at high temperatures, opting to lay more male eggs, it’s possible that they could adapt to a changing climate by tweaking the sex ratio again. At cold temperatures, though, things will still be outside of their control; that may be the time to try puppy dog tails.
Image: Trichogramma kaykai wasps (the same genus as the wasps in the story, but not the same species) laying eggs inside butterfly eggs. Duotone Wageningen, The Netherlands.
Moiroux, J., Brodeur, J., & Boivin, G. (2014). Sex ratio variations with temperature in an egg parasitoid: behavioural adjustment and physiological constraint Animal Behaviour, 91, 61-66 DOI: 10.1016/j.anbehav.2014.02.021
*This results in sisters who share about 75% of their genes, rather than the usual 50%, and may help explain why these insects don’t mind slaving away over a queen’s young—they’re more closely related than a standard family.