Isogamous organisms

I was looking for a type of organism in which the was no differentiation between the gametes, both are 100% identical (of course, there could be mutations, but no allele differentiation in matching-type regions, like the positive + and negative - mathing types in some algaes), and with a sexual reproduction. In isogamous organisms we have a small genetic differentiation between the gametes, as the + and - one I said. Is there an organism like this in nature? Also, is there any isogamous organism in which the gametes are produced in the same organ or region?