How did sperm and egg evolve?

Sex comes at a large cost: a sexual female is only half as fertile as an asexual individual, because the sexual female has to divide her offspring into males and females. So when an asexual organism can produce 50 offspring capable of reproducing, the sexual one can only produce 25, since the males and females have to pair. This is called the double cost of sex.

It’s a situation that’s made more complicated by anisogamy: different-sized gametes, like sperm and egg. This results in the two sexes investing different amounts of time and energy into reproduction, with the females usually bearing the brunt of it (e.g. gestation period), meaning that besides the double cost of sex, they also have this additional cost to pay in reproduction.

These two factors lie behind a lot of very interesting evolutionary biology. In this post, we’ll look at gamete sizes and how anisogamy evolves.

The state before anisogamy, when gametes are of equal size, is called isogamy. Isogamous species include those in the multicellular colonial green alga Volvox. Species here form colonies of different sizes, and there is one trend that’s very noticeable. In those species that produce small colonies, the gametes are of equal size. In species with large colonies, the gametes are separated into two types: one small, mobile gamete, and a large, non-moving gamete (Knowlton, 1974) – just like sperm and egg. In other words, anisogamy is correlated with colony size.

A generalised explanation for the evolution of such a trend is provided by Bulmer & Parker (2002). The key thing to realise is that the ultimate role of gametes is to provide enough nutrients and support for the embryo that they will become. Isogametes vary slightly in size – and whenever you have variation, you can have evolution wedging through. A larger gamete means an offspring that’s better provided for, leading to selection for larger gametes. However, this selection leads indirectly to selection on smaller gametes, since those can move faster and reach other gametes more quickly. Overall, this leads to divergent selection, with the extremes being selected for:

  • Large gametes are slow, therefore can’t fertilise each other; but they provide much better for the offspring.
  • Small gametes are quick and can get to the large gametes earlier; but they provide very little provisioning for the offspring.

The middle ground disappears, and we are left with this stark dimorphism, as seen with sperm and egg.

References:

Bulmer MG & Parker GA. 2002. The evolution of anisogamy: a game-theoretic approach. Proc. R. Soc. B 269, 2381-2388.

Knowlton N. 1974. A note on the evolution of gamete dimorphism. Journal of Theoretical Biology 46, 283-285.

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