When it comes to sex, size does count

You probably haven’t stopped to think about it, but why is the living world so often divided into sexes?

As Jack da Silva, from the University of Adelaide’s School of Biological Sciences points out, “Sexual reproduction does not require males and females, all it requires is the fusion of sex cells from two different mating types”. 

Da Silva explores this idea in a paper published in the journal Ecology and Evolution.

The sexes aren’t a necessary featured of life at all. So how did they come to be?

Well, the sex lives of green algae might be able to help us answer that question. 

One of the distinctive features of the sexes is the variation in size of sex cells known as gametes: sperm and eggs. Sperm are small and move around (a phenomenon known as “motility”), while eggs are large and nonmotile. Biologists refer to this as ‘oogamy’, and it is characteristic of all plants and animals.

The evolutionary explanation for oogamy has centred on a form of natural selection called ‘disruptive selection’, first described by the famous evolutionary biologist Maynard Smith in the 1970s. If you imagine a characteristic within a population as a bell curve, disruptive selection favours the edges of graph, the outliers, rather than the middle. In this case, the outliers concern extremes of gamete size. This form of selection is responsible for the rare phenomenon of ‘sympatric speciation’, where new species evolve from a common ancestor in the same geographical range.

“As body size increases there is selection for a larger zygote to store nutrients in early development, which in turn selects for larger gametes,” writes da Silva. 

“This may provide the opportunity for one mating type to produce more numerous, smaller gametes, thereby increasing its fertility via competition with other gametes of the same mating type, and thus forcing the other mating type to produce fewer, larger gametes.”

In 2002, a paper by statisticians MG Bulmer and GA Parker put forward a mathematical model of disruptive selection based on game theory.

Da Silva uses this model to generate the prediction that evolution of males and females will only be stable if “the ratio of the size of the larger gamete to the smaller gamete has to be greater than three.” 

Any less, disruptive selection will collapse, and the gametes will return to equal sizes. This prediction is the first, it turns out, that is specific to the disruptive selection theory of oogamy.

To test it, da Silva turned to volvocine algae, a fresh water species. 

“The algae were perfect for testing this theory: they showed the whole range of variation of gamete size differences,” he reports. “And the theory held. Wherever there were gametes of different sizes, the larger ones were always at least three times bigger.”

This will aid scientists to understand sex difference more clearly, he adds, because “differences between males and females stem from this original evolution of the size and motility of the sex cells.”