We just published a new paper on the evolution of mating type switching in fission yeast:
Nieuwenhuis, Bart P. S., Sergio Tusso, Pernilla Bjerling, Josefine Stångberg, Jochen B. W. Wolf, and Simone Immler. “Repeated Evolution of Self-Compatibility for Reproductive Assurance.” Nature Communications 9, no. 1 (April 24, 2018): 1639. https://doi.org/10.1038/s41467-018-04054-6.
In the paper we show experimentally that mating-type switching is highly beneficial under sexual reproduction in fission yeast. If cells cannot change mating type – due to the spatial structuring – cells that are not directly next to another cell will not be able to find a mate of the opposite mating type. Cells that can switch their mating type during asexual reproduction will during growth assure that there is always a mate present as a direct neighbor. These cells can thus mate within their clonal patch, where non-switchers can only mate at the edge of a patch.
Next to showing that there is a strong fitness advantage, we performed experimental evolution. In this experiment we mixed non-switcher strains, let them go through repeated rounds of sexual reproduction that strongly favored mating. Within 25 generations, we re-evolved mating-type switching in 9 out of 20 replicate populations.
Even though mating seems highly beneficial – which should select for maintenance of switching – in nature we see that switching is repeatedly lost. In competition experiments we showed that this is likely caused by a reduced fitness during asexual reproduction of switchers relative to non-switchers. When sexual reproduction is not required – for example when resources are plenty and long-term survival is not needed – prolonged asexual reproduction can locally select for loss of switching.