If a non married bat Cohen donates an egg to a yisraelite couple who have a first born son from it, do they have to do a pidyon haben?
As I'd answered the other question, Rabbi Aaron Soloveichik and other American poskim of the late twentieth century would say that it is needed. "The egg is considered nothing more than a synthetic product made in Japan", writes Soloveichik! The halachic mother here is the Israelite birth mother.
Many rabbis in Israel, plus Rabbis Heinemann and Hopfer of Baltimore, would say the Cohen egg mother precludes the necessity of pidyon haben.
I'd suspect today a lot of people would recommend doing one just in case, or the like.