Fitness differences suppress the number of mating types in evolving isogamous species

Yvonne Krumbeck, George W.A. Constable, Tim Rogers

Research output: Contribution to journalArticlepeer-review


Sexual reproduction is not always synonymous with the existence of two morphologically different sexes; isogamous species produce sex cells of equal size, typically falling into multiple distinct self-incompatible classes, termed mating types. A long-standing open question in evolutionary biology is: what governs the number of these mating types across species? Simple theoretical arguments imply an advantage to rare types, suggesting the number of types should grow consistently; however, empirical observations are very different. While some isogamous species exhibit thousands of mating types, such species are exceedingly rare, and most have fewer than 10. In this paper, we present a mathematical analysis to quantify the role of fitness variation—characterized by different mortality rates—in determining the number of mating types emerging in simple evolutionary models. We predict that the number of mating types decreases as the variance of mortality increases.

Original languageEnglish
Article number192126
Number of pages19
JournalRoyal Society Open Science
Issue number2
Publication statusPublished - 26 Feb 2020

Bibliographical note

© 2020 The Authors.


  • Balancing selection
  • Isogamy
  • Mating types
  • Negative frequency-dependent selection
  • Self-incompatibility

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