What is it about?
Why did male and female gametes evolve to be so different? Evolutionary theories propose that such differences can arise when investing resources in future offspring benefits the offspring but comes at a cost to the gametes themselves. However, direct experimental evidence for how this process might begin has been scarce. We studied fission yeast, whose two mating partners produce gametes that look alike. Despite this similarity, we found that the two gametes do not contribute equally to their future offspring. One gamete preferentially produces the protein Rec8 before fertilization. This early investment helps the resulting zygote correctly segregate its chromosomes during meiosis and produce viable offspring. But there is a catch: if fertilization fails, producing Rec8 reduces the gamete's own survival. Combining experiments with evolutionary modelling, we show that this conflict between helping the offspring and preserving the gamete can favor a division of labor in which one partner makes a larger investment than the other.
Featured Image
Photo by masakazu sasaki on Unsplash
Why is it important?
The origin of different male and female gametes is a major transition in the evolution of sexual reproduction. Classical evolutionary models have proposed that these differences could originate from a trade-off between investment in offspring and gamete survival or proliferation, but testing this idea experimentally has been difficult. Our work identifies such a trade-off at the molecular level in an organism whose gametes are still morphologically indistinguishable. Remarkably, the asymmetry does not require one gamete to be larger than the other: it begins as a difference in gene expression and in how much each partner invests in the zygote. Our experimentally informed evolutionary model shows that the measured costs and benefits of this investment can favor asymmetric contributions by the two gametes. The results therefore provide a possible molecular route by which initially similar mating partners can begin to specialize, potentially representing an early step toward the evolution of more pronounced gamete differences.
Perspectives
This work opens the possibility of studying the evolution of gamete differences at its earliest stages, before those differences become visible as distinct cell sizes or morphologies. Fission yeast provides an experimentally tractable system in which we can now ask how initially small asymmetries in signalling and gene regulation become established, how selection acts on them, and whether they can drive progressively greater specialization between mating partners. An exciting next step will be to determine how general these principles are. Rec8 is unlikely to be the only cellular investment that benefits the future zygote while imposing a cost on the gamete that produces it. Identifying other such factors, and examining related species with different reproductive strategies, could reveal whether conflicts between gamete survival and zygotic success represent a general route toward asymmetric parental investment. More broadly, this provides an opportunity to connect molecular and cellular mechanisms with the evolution of mating types, gamete dimorphism and, ultimately, the biological sexes.
Aleksandar Vjestica
Universite de Lausanne
Read the Original
This page is a summary of: Meiotic cohesin Rec8 imposes fitness costs on fission yeast gametes favoring the evolution of parental bias in gene expression, Proceedings of the National Academy of Sciences, July 2026, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2524968123.
You can read the full text:
Resources
Contributors
The following have contributed to this page
