The antonym for the word "cross-fertilize" is "self-fertilize" or "self-pollinate." This refers to the process in which a plant pollinates itself without the aid of another plant. Unlike cross-fertilization, self-fertilization is a less diverse process, which limits genetic variation and adaptability.
Another antonym for "cross-fertilize" is "keep separate" or "isolate." This refers to the deliberate separation of different groups or ideas, rather than allowing them to mix and interact with each other.
Both antonyms suggest a lack of diversity and a preference for staying within one's own group or ideas.