The birds and the bees for plants
In humans, as previously discussed, males get two different sex chromosomes, X and Y, while females get two Xs.
BUT other critters have very different ideas. Birds, for example, also have sex chromosomes – but the females have two different ones, and the males have two the same. To distinguish them from the mammal-style way of doing it, this is called the ZW system (males are ZZ, females are ZW); and it’s not confined to birds.*
But no one** cares about animals. What about plants?
Most flowering plant species are hermaphroditic – both male and female gametes are produced by the same individual. Sometimes individual flowers are both male and female (called bisexual or perfect). Sometimes they have separate-sex flowers – such as hazel, where the male catkins are conspicuous in the spring, but the female flowers are small and inconspicuous.
The upshot of this is that most plants can
have sex with themselves self-fertilise.
The advantages of self-fertilisation should be obvious – you don’t have to find a partner to make as many little seedlings as you can manage (think like a plant here!). However, selfing has a major drawback too: loss of genetic variation. Heterozygosity – a measure of genetic variation that means ‘the number of genes that you have two different copies of’ – is halved with each generation of strict self-fertilisation.
Loss of genetic variation is bad – it limits your ability to adapt (as a population) to changing environments, and increases the rates of deleterious gene variants. So plants often have mechanisms to limit selfing, such as male and female bits flowering at different times. The most extreme of these is known as dioecy – each individual only producing either male or female gametes.
(Wait, that sounds familiar…)
So why am I leading you down this particular garden path? Well, dioecy can lead to sex chromosomes. Sex chromosomes, you’ll remember, evolve due to differing selection pressures on males and females, in collusion with a sex-determination gene and suppressed recombination. Clearly, hermaphrodites have little use for sex chromosomes.
And why are plant sex chromosomes cool? Because they’re new. Compared to the human versions, which have been around for at least 300 million years, plant sex chromosomes are just opening their little kitten eyes and peering out at the strange, bright world.
* There are other, more exotic (to take the anthrocentric view of things) ways of dividing up your sexes. In bees, for instance, if an egg is fertilised it hatches female, and if not it hatches male. In turtles, the temperature of incubation determines sex. And the platypus has not just one, but FIVE pairs of sex chromosomes; males are XYXYXYXYXY, and females are XXXXXXXXXX. Apparently, the chromosomes form chains during meiosis (where normal cells divide into eggs or sperm) so they don’t get all jumbled up, but it’s not yet clear how this works.
** At least no one worth talking about.