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Imagine a lineage of multicellular organisms that had mating types and evolved their mating types into sexes. What are the possible mechanisms that might have brought this lineage to evolve sexes from mating types?

To my understanding of the definitions of sexes and mating types, the only diffenece between the two is that sexes imply gametes of different sizes. Is it correct?

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Another important reason to have mating types is to prevent self-fertilization or self-polination that produces less capable offspring. Because of this requirement, mating types may evolve also for species that produce both types of gametes, or does not differentiate them into male and female gametes. Fertilization is only possible if gametes have different mating types.

It may be more than two mating types, but if there are only two, they are probably similar to "sexes" even if gametes do not look different.

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Thanks for you answer. It is interesting so to explain the evolution of mating types. But I don't think it answers the question though in the sense that it does not address the evolution from mating types to sexes. In my question, I am mostly interested in the evolution of gamete size differentiation and of number of gametes from a lineage/species where there is different mating types but no difference in size of number of gametes. – Remi.b Mar 13 '14 at 7:38

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