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Theory of the Evolution of Sexes Tested with Algae

The varied sex lives of a type of green algae have enabled an Adelaide researcher to test a theory for why distinct males and females evolved.

“Why would a situation evolve where some individuals produce small gametes that are motile or capable of motion (the males) and others produce large, non-motile gametes (the females)?” asks Dr Jack da Silva of The University of Adelaide. “Sexual reproduction does not require males and females. All it requires is the fusion of sex cells from two different mating types.”

The Disruptive Selection Theory was proposed in the 1970s to describe how a population evolves from different mating types producing same-sized sex gametes to distinct males and females producing different-sized gametes. da Silva has provided the first test specific to this theory. “The theory is that as organisms evolve to be larger, the single-celled embryo (or zygote) is selected to be larger as well. This necessitates larger gametes to store more nutrients to give the embryo a head-start in development,” da Silva says.

“Organisms have a limited budget for producing gametes. So if there are more they will be smaller; if there are less they will be larger. Because greater numbers of gametes gives one mating type a selective advantage – more chance of successful fertilisation – the other mating type will be forced to produce larger, and therefore fewer, gametes to enable the production of the larger zygote.”

Published in Ecology and Evolution, da Silva tested this theory using data from studies of volvocine algae, whose species vary considerably in their reproductive patterns: some species have two mating types that produce motile gametes of the same size; others produce motile gametes of different sizes; while in other species one mating type produces small motile gametes and the other mating type produces large non-motile gametes (males and females).

One model of the Disruptive Selection Theory predicts that the ratio of the larger gamete to the smaller gamete must be greater than three for the evolution of males and females to remain stable. If the ratio is any less they always evolve back to the same size.

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

“This provides the first test that is specific to this theory. To date there hasn’t been a lot of strong evidence in support of any of the competing theories, but here we have confirmed that the classic theory is probably on the right track.”