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Exposing yeast mating practices

A closer look at the reproductive behavior of yeast reveals how genetic parasites spread

19 January 2022

During S. pombe reproduction, a yeast cell gives rise to four spores enclosed in a sac-like structure called an ascus (dotted outlines). In this image, the outcomes of different reproductive behaviors are shown. The filled arrowheads point to an ascus of spores generated by inbreeding (all purple or all blue), while the open arrowhead points to spores generated by outcrossing (a mix of purple and blue). Scale bar represents 10 μm. Image courtesy of Zanders Lab.

In a recent scientific article published in eLife, Predoctoral Researcher José Fabricio Hernández López from the Graduate School of the Stowers Institute (GSSIMR) and coauthors describe their latest research to identify rules that govern the spread of certain “selfish” genes, which, as their name indicates, are selfish in their behavior and stack the odds of their own transmission to the next generation in their favor. Selfish genes are considered parasites of the genome because they do nothing to promote the overall fitness of the organism and sometimes even cause problems such as infertility.

When organisms make reproductive cells like eggs or sperm, normally only one of two gene copies at a given site is distributed to each reproductive cell. This process, known as Mendel’s law of segregation, serves to maintain genome size in their offspring, who get half of their genes from each parent. But sometimes laws are broken. And some of the violators of the segregation law are selfish genes, which are found in many living organisms, from single-cell yeast to humans.

In the lab of Associate Investigator and GSSIMR Vice Dean Sarah Zanders, PhD, Hernández López and collaborators studied aspects of selfish gene transmission in the yeast S. pombe. Although seemingly simple, yeast cells can exhibit different mating behaviors. Instead of eggs and sperm, their reproductive cells are spores. The researchers examined how patterns of outcrossing, or mating between individuals of distinct lineages, and inbreeding affected the spread of selfish genes.

José Fabricio Hernández López

What led to the study was an intriguing paradox: Selfish genes gain their transmission advantage in the outcrossing scenario. The yeast S. pombe has many selfish genes but has been reported to outcross only rarely, which leads to the question of why the selfish genes are present.

The research team found that S. pombe mating behavior depends on cell density and availability of mating partners. The researchers also found that different isolates of yeast, while genetically very similar, can exhibit natural variation in mating behavior.

After studying the impact of these parameters, the researchers detected the spread of selfish genes in S. pombe populations in just a few weeks in an inbreeding population. This finding is important because it shows that selfish genes can persist and spread in S. pombe even when outcrossing is infrequent.

Overall, this work revealed aspects of S. pombe and its environment that are important for understanding mating behavior and evolution of this species. This study also helps explain the success of selfish genes in S. pombe and potentially the general rules for how selfish genes spread in other organisms including humans.

Additional contributors to the study were Rachel Helston, PhD, Blake Billmyre, PhD, Samantha Schaffner, and Michael Eickbush from the Zanders Lab, and Jeffrey Lange, PhD, and Scott McCroskey from Stowers Institute Technology Centers.

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