Why is a human egg 10 million times the volume of a sperm cell

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The size of eggs and sperm has evolved from competition. Limited resources have caused gametes to differ considerably in size and number.

In most living animals, the eggs are much larger than the sperm. In humans, for example, a single egg is 10 million times the volume of a sperm.

In a new study, researchers at Northwestern Engineering found that competition and natural selection were behind this curious size difference.

Using mathematical modeling, the researchers considered a very early period in evolution when primordial species reproduced by external fertilization. In the model, larger reproductive cells, or gametes, had a competitive advantage because they could hold more nutrients for a potential zygote. The smaller gametes, however, required fewer resources to manufacture, which put less stress on the parent.

“Organisms had to either produce the largest gametes with the most supplies or the smallest gametes to use the fewest resources,” said Daniel Abrams, professor of engineering and applied mathematics at the McCormick School of Engineering and lead author of the study. “We believe this size difference is almost inevitable, based on plausible assumptions about how sexual reproduction works and how natural selection works.”

The research was published online in the Journal of Theoretical Biology. Joseph Johnson, a doctoral student in Abrams’ lab, is the article’s first author. Nathan White and Alain Kangabire, undergraduates at Abrams Lab, co-authored the article.

The Northwestern team’s model begins with isogamy, a primordial state in which all gametes were roughly the same size and distinct sexes did not yet exist. The team then developed and applied a simple mathematical model to show how isogamy shifted to anisogamy, a condition in which gametes grew very small or large enough – precursors to sperm and ova associated with biological sexes today. ‘hui.

In the model, anisogamy emerged from the competition to survive in a resource-limited environment. Gametes were more likely to survive if they had a sizeable advantage over their neighbors, leading to an “arms race” in favor of larger and larger gametes. But organisms could not produce many sex cells without needing more and more resources themselves. They could, however, save their resources by producing a lot of small gametes.

“Early in evolution, when sexual reproduction emerged, the gametes were symmetrical. But that’s where that symmetry breaks down, ”Abrams said. “We end up with some organizations that specialize in large gametes and others that specialize in small gametes. “

Abrams said a remaining mystery is why some isogamous species still exist today. Some types of algae and fungi, for example, reproduce asexually or with symmetrical mating types.

“There have been different theories as to how anisogamy emerged, going all the way back to Charles Darwin,” Abrams said. “Evolutionary biology problems are very difficult to test because we can only study the species that exist today. We can’t see what they looked like billions of years ago. The use of mathematical models can provide new insight and understanding.

Reference: “A dynamical model for the origin of anisogamy” by Joseph D. Johnson, Nathan L. White, Alain Kangabire and Daniel M. Abrams, March 18, 2021, Journal of Theoretical Biology.
DOI: 10.1016 / j.jtbi.2021.110669

The research, “A dynamic model for the origin of anisogamy,” was supported by the National Science Foundation (grant number 1547394).


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