^BSperm fertilising egg.^b Coloured scanning electron micrograph (SEM) of two sperm (orange) attempting to fertilise a human egg (blue). The rounded head of each sperm is seen and their long tail. Women usually produce one large egg (ovum) for fertili- sation, whereas men release some 300 million much smaller sperm (spermatozoa) which travel through the uterus to reach the egg. A few hundred sperm survive this journey and encounter the egg in the fallopian tube. However, only one sperm succeeds in penetrating the egg's wall and fertilises the egg by fusing with its nucleus. When this occurs, the egg membrane forms a barrier to other sperm. Magnification: unknown.

In a new study, researchers at Northwestern Engineering found that competition and natural selection led to this strange size difference. Using mathematical modeling, the researchers considered a very early period in evolution when primitive species reproduced using external breeding. In the model, larger reproductive cells [gametes] showed a competitive advantage because they could retain more nutrients for a potential zygote. Smaller gametes, however, required fewer resources to create, which was less stressful for the parent.  Organisms either needed to produce larger gametes with more supplies or smaller gametes with fewer resources, and this size difference is almost inevitable in sexual intercourse, and natural selection.

The group model begins with isogamy, a primordial state in which all gametes were about the same size and there were no different sexes yet. The team developed and applied a simple mathematical model to show how isogamy turned into anisogamy, a condition in which gametes either became too small or large enough [sperm and egg precursors] to be associated with biological sex today. Anisogamy arose from competition to survive in a resource-constrained environment. Gametes were more likely to survive if they had a size advantage over their neighbors, leading to an “armaments race” that favored growing gametes. But, organisms could not produce many sex cells without needing more and more resources themselves. They could, however, save their resources by producing many tiny gametes.

In the beginning, when sexual reproduction appeared, the gametes were symmetrical, but this symmetry is broken and we end up with some organisms that specialize in large gametes and others that specialize in small gametes.

It remains a mystery why some isogamous species still exist today [types of algae and fungi]. For example, they reproduce either asexually or with symmetrical types of mating because we can only study species that exist today.

SOURCE: Northwestern University Journal of Theoretical Biology Οct. 2021