Atsushi Yamauchi
(Center for Ecological Research, Kyoto University )

09/1/30, 13:30 at Room 3521 (5th floor of building 3 of the Faculty of Sciences)

Eukaryotic organisms often harbor several genetic factors in their cytoplasm. These cytoplasmic genetic elements (CGEs) include both eukaryotic organelles (mitochondria, chloroplasts) and bacterial endosymbionts (Wolbachia, Buchnera, etc.), 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 (i.e., animal mitochondria), while others cause a sex ratio distortion (SRD) in their hosts (i.e., Wolbachia). For example, both Wolbachia and mitochondria are endosymbionts originating from the same group of bacteria, but both now coexist in eukaryotic cells using these different strategies. The present study was performed to resolve this issue using a population genetic approach. In our model, the endosymbiont genome is divided 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 small genome size tends to evolve under biparental inheritance, whereas SRD evolves under uniparental inheritance. These 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 evolution of cytoplasmic symbionts. We suggest that endosymbiotic hysteresis can explain both the evolution toward small genome respective SRD and the coexistence of CGEs that differ in these strategies.

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