Sex determination

When we think about sex determination, we think about males and females. What controls whether you are a boy or a girl? Your genes of course. Males have a Y chromosome and females don't. Well not all animals determine sex the same way as we humans do. Here we are going to look at some of them and focus on some of the stranger ways of sex determination.

Why are there sexes?
Why do we have sexes in the first place? The short answer is that it increases genetic diversity and allows the species to better adapt to changing conditions.

Imagine there's a population of cats. Each cat has 2 copies of every gene. Let's pretend that a long-haired male cat meets a short-haired female cat and they decided to mate. When a cat makes a kitten, she only pass on half of her genes to the kitten. The other half comes from the father. They can't choose which half of their genes to pass. It's completely random. And so the kitten gets half of his genes from the mother and half from the father. Each kitten is different because it gets a different half of genes from each parent. So the kittens will have a variety of hair lengths, some have long hair and some short.

Say these kittens are suddenly shipped to Siberia where it is really cold. The ones with long hair are going to survive better than the ones with short hair and so at least some of the cats will do well. If for example, the short hair mother passed all her genes to her kittens instead of just half, then all the kittens are going to have short hair and perish in the cold Siberia winter. This is obviously a highly artificial example. No one ships cats to Siberia and leaves them out in the cold but you can see how increasing the variety in the population is beneficial to changes in the environment.

How do different animals determine sexes?
Most people are familiar with human sex determination using the X and Y chromosomes. The special sex chromosome, the Y chromosome, contains male-determining genes. Therefore people with XY are male and those with XX are female. This is true for all mammals.

Other animals do it quite differently. Birds for example also use a sex chromosome system. But for them, the females get the special chromosome. We call them Z and W to distinguish them from our system. Males are ZZ and females are ZW. The W chromosome carries female-specific genes.

Then there are animals that are even more different from us. Social insects like bees and ants don't have sex chromosomes. Instead, their sex determination depends on how many copy of every gene they have. As I mentioned before, we humans (and cats) have 2 sets of every gene, one from the father and one from the mother. Male ants have only 1 copy of every gene while females have 2 copies. All of the male ant's genes comes from his mother.

There are animals which don't use genes to determine sex at all. Reptiles like turtles and alligators determine sex based on the temperature at which the eggs develop. Sea turtles lay their eggs on the beach. The temperature at which the eggs are at in the middle third of the incubation determines whether the baby turtles will be male or female. At low temperatures, the embryo will develop into a male and at high temperatures, it becomes a female. The temperature threshold is only a few degrees apart and it could be the difference between being in the sun or shade.

Why are there different ways of sex determination? We don't really know but that's how evolution worked. There are advantages to using genes and ones for using temperature as the sex determination trigger. Systems using sex chromosomes are more stable. The sex ratio is always 50-50. Temperature dependent systems are more variable and sex ratios can change quite a bit based on the climate. This can be advantageous if for example, females and males mature at different rates. Eggs laid early in the season are of one sex while those later in the season are a different sex. This can make up for the difference in maturation time between the sexes.

Here's a really cool video that explains the above mentioned sex systems and more.

Animals that use both genes and environment to determine sex
Using sex chromosomes and using temperature seem very different. How did animals evolve such different systems? Was one of them the ancestral system and the other a younger system? How did the switch happen?

There are animals that use both genes and the environment to determine sex. One such animal is an Australian lizard with a really cool name, the bearded dragon (Pogona vitticeps). Like birds, its sex chromosomes are Z and W, where females have the special sex chromosome W. ZZ individuals are males but scientists recently discover females who are also ZZ. What's going on here?

Bearded dragon (Pogona vitticeps)
Image from Wikipedia Commons

Turns out that the dragons are also sensitive to the temperature at which the eggs are incubated. At low temperatures, all ZZ babies develop as males. But when the incubation temperature increases, some ZZ babies develop as females instead of males. The ZZ "sex-reversed" females are indistinguishable from the ZW females until you look at their chromosomes. The temperature effect is so strong that at the highest viable incubation temperature (37C), almost all the ZZs are females.

How does that work? Well, we are not sure. One model suggests that instead of thinking of a "female-determining factor" on the W chromosome, there is a "male-determining factor" on the Z chromosome. At normal, low temperatures, this factor is stable and ZZ babies are males. At higher temperatures, the male factor is degraded faster. With less of this male factor, the ZZ babies become female.

In the wild, there are ZW females and ZZ "sex-reversed" females. Both types of females are fertile and survive fine. But scientists have found that over the past 15 years, the ratio of ZZ females to ZW has been increasing. This may be due to climate change where the temperatures at which these animals live in increases. What will happen to the bearded dragon in the future as global temperature rises further is anyone's guess. Will the W chromosome be lost in the population? Will the ratio of males to females change? Is the bearded dragon more vulnerable or resilient than other animals in the face of climate change?

The bearded dragon is an example of a species that uses both temperature and genes to determine sex. It shows that the 2 sex-determining systems need not be mutually exclusive. Both have their advantages and disadvantages. They could have evolved independently and not one from another. As for whether there are other animals using both the environment and sex chromosomes for sex determination, we'll have to wait for scientists to find that out.