There’s much more to sex than the birds and the bees

Sex is a peculiar and mysterious feature of life on this planet. Almost all multicellular organisms and some unicellular ones have two sexes. That is, there is a differentiation between males and females. Despite this apparent commonality, the mechanism of sex determination varies enormously from species to species, so much so that the birds and the bees aren't really that useful for explaining how things work in our own species.

In mammals, sex determination is controlled by the X and Y chromosomes (the "sex chromosomes"). If you inherit an X from both your mother and your father, then you are female, and if you inherit an X from your mother and your father's Y, then you are male.

Birds also have sex chromosomes, called Z and W. These are completely unrelated to our X and Y. Additionally, whereas in mammals the males have one of each kind of sex chromosome, in birds this is reversed, and females are ZW and males ZZ.

In both cases the sex is determined by the presence of particular chromosomes, but in bees it is entirely different. The eggs laid by the queen are either fertilised, in which case they develop as females, or unfertilised, in which case they develop as males. So, whereas the females have two sets of chromosomes, one from each parent, the males only have one parent and only one set of chromosomes.

In other animals, sex is dependent on the environment. In turtles and some reptiles, the incubation temperature of the eggs acts as a trigger to initiate either male or female development. For any species there is an optimal ratio of males to females to maintain a healthy population. Global climate change has the potential to wipe out species with temperature-dependent sex determination by skewing the sex ratio too far in one direction. If these species cannot adapt quickly enough, they could experience a catastrophic reduction in the number of mating pairs.

Sex changes

Sex (as distinct from gender) is defined biologically through the type of sex cells (that is, eggs or sperm; or ova or pollen) known as “gametes” produced. One commonality in the sex-determining mechanisms described above is that, once a given individual is male or female, he or she stays that way. A bee or a turtle or a chicken or a squirrel that produces eggs is female, and cannot suddenly start producing sperm.

However, some species can change sex during their lifetime. All clownfish are born male and become female later (this is why I'm hoping that they make a sequel to Finding Nemo). Similarly, male goby fish and wrasses usually start out as female and become male once they are large enough to defend their nesting territory.

So, why bother with all this mess? Why have so many complicated and convoluted ways to arrive at two different sexes? There is a simpler way, which is just to avoid sex altogether. This is what strawberry plants are doing when they send out runners along the ground: starting new plants that are genetic clones of the original. Similarly, aphids, such as greenflies, are capable of asexual reproduction, whereby a female can produce many clones of herself that are genetically identical.

Asexual reproduction also has the advantage of efficiency, at least in the short term. If you measure reproductive success as the passing-on of your genes, then asexual reproduction passes on all of the genes all the time. By contrast, with sexual reproduction, you are spending all your resources on offspring that only carry half of your genes (along with the half from the other parent).

The reasons for the evolution of sex aren’t completely clear, but can be roughly summarised as the good, the bad, and the ugly. Sexual reproduction is the mixing-up of the genes of two parents. This means that the good stuff from each parent can potentially be combined into a new individual. The bad stuff (and we all have bad stuff, genetically speaking) can be potentially avoided rather than constantly added to. And finally, sexual reproduction is probably an important part of our defence against genomic parasites (the ugly).

The genetic mixing that is sexual reproduction is considered to be essential for the evolution of complex life. So all of this adds to the natural beauty and wonder of the world around us, and should also make that birds and bees talk a bit more entertaining.

Aoife McLysaght is a professor in genetics in TCD, where she leads a research group focusing on identifying and interpreting the evolutionary patterns in animal genomes