Bistability of endosymbiont evolution of genome size and host sex control

Eukaryotic organisms often harbor several genetic factors in their cytoplasm. These cytoplasmic genetic elements (CGEs) include both eukaryotic organelles (mitochondria, chloroplasts) and bacterial endosymbionts, which have evolved from free-living bacteria. A common feature of CGEs is their cytoplasmic inheritance from mother to offspring. A striking difference is that some CGEs have evolved a short genome size (e.g., animal mitochondria), while others cause a sex ratio distortion (SRD) in their hosts (e.g., Wolbachia). In this study, we sought to resolve the evolution of these endosymbiont properties using a population genetics approach. Our model divides the endosymbiont genome into a functional part and a part that can cause SRD, and our results indicate that the cytoplasmic inheritance system at the initiation of symbiosis plays a key role in determining the evolutionary trajectory of CGEs. We show that in endosymbiotic evolution, two states can be bistable, depending on the parameters. The evolution of the cytoplasmic inheritance system from biparental to uniparental can result in hysteresis in the evolution of cytoplasmic symbionts.