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?