What is most ancestral: isogamy or anisogamy? Sexual reproduction can be feasible with anisogamy (gametes of different sizes i.e. genders) or isogamy (gametes of same size i.e. mating types) or with undifferentiated gametes (i.e. true random mating; although I have to confess that I am not aware of such example). De novo evolution of sexual reproduction happened several times independently. **What seem to be the most ancestral states of gamete differentiation (isogamy or anisogamy)?**. Does it differ between lineages?

As stated here the first sexual reproduction seems to be isogamy, which's purpose was likely to repair the DNA, as a damaged cell could repair its' (DNA) strands via recombination with a "healthy" one.

As for lineages, the only thing I could find was also in this book at the very beginning, but there doesn't seem to be a difference as for which is the most ancestral.