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

## TL;DR

This paper uses mathematical modeling to show that increased fitness variation, specifically mortality differences, reduces the number of mating types in evolving isogamous species, explaining observed diversity.

## Contribution

It introduces a quantitative analysis linking fitness variation to the number of mating types, providing a theoretical explanation for empirical diversity patterns.

## Key findings

- Number of mating types decreases with higher mortality variance.
- Mathematical model predicts fewer mating types in populations with greater fitness differences.
- Results align with observed diversity, where species with high fitness variation have fewer mating types.

## Abstract

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 ten. In this paper, we present a mathematical analysis to quantify the role of fitness variation - characterised by different mortality rates - in determining the number mating types emerging in simple evolutionary models. We predict that the number of mating types decreases as the variance of mortality increases.