Male Bees Don't Have Fathers: the ins and outs of sex determination.

What determines if we're male or female?

I’ll leave it to the rest of the internet to argue about the complexities of human gender. If you dare, just scroll down to the comments under any article about Caitlin Jenner, and you'll find it doing just that. I’m talking biological sex here: the bits and bobs that lurk in the abdomens and proverbial underpants of living creatures. Although the two sexes are almost universal in the animal kingdom, the mechanisms that control which individuals develop into which sex aren't. These systems have been mucked about to the extreme by evolution, and these are just some of the strange results.

Chromosomes: the birds, but not so much the bees.

However hazy and distant your last science lesson seems, you’re probably vaguely aware that human sex is (predominantly) determined by our chromosomes. We all inherit an X chromosome from our mothers, and depending on which sperm wins the race, dad donates an X or a Y. Two Xs is a recipe for a female reproductive system, and XY is a recipe for danglies galore. Now, not to dampen anyone’s proud masculinity, but the Y chromosome is tiny and a bit useless, with very few functioning genes on it. You may also remember that this leaves men more vulnerable to any dodgy recessive genes on their X chromosome. As females have two Xs, they’re more likely to inherit at least one normal version of the gene. This is why men are more prone to  problems such as red-green colour-blindness, which is caused by a faulty gene on the X chromosome.

We share very similar systems with other mammals and the odd species of insect, but birds have flipped it on its head. In the avian genome, a different pair of chromosomes wield the power of sex-determination: the Z and the W. In this system, Z is the normal-sized chromosome whereas W is its weedy counterpart, and whilst males have two Zs, females are ZW. As a result, female birds are at a greater risk of dodgy genes lurking on the Z – domestic hens are more susceptible than cockerels to a condition that makes them shake uncontrollably. However, savvy poultry breeders can use genes like these to find out the sex of their birds without paying for expensive genetic tests or waiting for them to grow up. A commonly-used gene can be found on the Z chromosome, where it causes newly-hatched chicks’ feathers to grow more quickly. When slow-feathering hens are crossed with fast-feathering males, all their daughters will inherit their father’s Z chromosome, along with his fast feather growth. Males will inherit slower-growing feathers, allowing the breeder to pick out the fuzzy females for the farm. 

Awww!
...Unless you're a male chick.

Genetic halflings

Ants, bees and wasps are generally pretty strange, and their sex determination system is no exception. Many species live in vast colonies in which only a single individual – the queen—is in charge of breeding. It’s a big responsibility; the kingdom needs an army of daughters and only a handful of sons to succeed, and so leaving her offspring’s sex to chance just isn’t an option for the queen. Thankfully, she can control whether her eggs hatch into sons or daughters, simply by choosing whether to add sperm. Although pretty celibate during their reign, queens attract lots of male attention when they leave their natal nest, and they nurture the resulting sperm for the rest of their lives. If a queen chooses to add sperm to an egg, it will become a female, with a set of chromosomes from each parent. Eggs that don’t receive sperm develop into males, inheriting just a single set of chromosomes from their mother. That’s right—male ants, bees and wasps might have plenty of half-sisters, but they don’t have fathers. True mummy’s boys indeed.

Hot girls wanted

Not all animals have their sex determined at conception. For some species, environment can also have a powerful effect on sex, during development or even in adulthood.

Some reptiles lay eggs that develop into males or females depending on their incubation temperature. This system is mostly found in crocodiles and chelonians (turtles, tortoises and terrapins), and it varies between species. In snapping turtles, females develop when eggs are either kept very cool, or very warm, whilst males hatch at intermediate temperatures. In many other species, warmer clutches hatch into females, whilst eggs in a cool nest will find themselves with a lot of brothers. This system appears to work through a temperature-sensitive enzyme called aromatase, which can convert testosterone into oestrogen under the right conditions. As different females will choose different sites to lay their eggs, the sex ratio stays fairly even. However, the temperature parameters may need to adjust as climate change takes hold, or there'll be a lot of horny female tortoises looking for mates.

"Hey babe."

One fish, two fish, girl fish, dude fish

Fish are even more flexible, with many species able to casually change sex in adulthood to cash in on breeding opportunities. This is usually a one-way system, with most or all young fish hatching as one sex, and changing permanently if the opportunity arises. For example, wrasse are mostly born female, and live in colonies led by a dominant male. The male bullies his wives to keep their stress hormones up, inhibiting sex change to ensure he has plenty of mates and no competition. However, if he dies, the females don’t rejoice and live happy lives free from bullying and oppression. The largest one simply becomes male, and begins the reign of terror all over again. The opposite is true in the more peaceful society of the clownfish, where all fish are born male, but happily become female if their mate dies. Unlike the wrasse, older clownfish are better off using their large bodies to make lots of eggs instead of fight, and so becoming female is the coveted goal. The fish don’t wait around: in some species, transforming individuals show dramatic shifts in behaviour almost immediately after the social setup changes. Alterations in colour and size follow as the old sex organs shrink and new ones awake from dormancy, and within a week, the fish is making eggs or sperm like a pro.

Take away what you will from this article. However, I think we can all agree on the following:

     Hatched in a cold nest.

At risk of genetic disease.

Daddy issues.

 

Should've become Nemo's mum.