Mitochondria, mating types and sexes.
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The existence of binary mating systems in the vast majority of sexual
organisms has long been an evolutionary puzzle. Why mating partners
need to be of different type or sex is unclear. A possible explanation
relates to mitochondrial inheritance. Uniparental inheritance (UPI) of
mitochondria is widely spread in many taxa. Several studies have
proposed that two sexes evolved as a respond to the need for UPI so
that one sex passes on its mitochondria whereas the other does not. In
this work we re-assess these postulations by developing a novel
mathematical model which explicitly considers mitochondrial evolution
and the parallel evolution of genes imposing uniparental inheritance.
Our results suggest that UPI of mitochondria would spread in a
biparental population but never to fixation without pre-existing
mating types. This is because the fitness benefits ‘leak’ through the
population as a result of successive uniparental and biparental
matings. Only when binary mating types already exist can uniparental
mutants spread to fixation. We conclude that the requirement for UPI
probably did not drive the evolution of two mating types, but is
readily fixed afterwards, giving rise to the patterns actually seen in
protists.
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