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An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae, fungi, and Oomycetes. They are believed to have evolved either through the fusion of two species or the chemically-induced stimulation of mycelia, leading to oospore formation.
In Oomycetes, oospores can also result from asexual reproduction, by apomixis. These are found in Fungi as the sexual spores; these help in the sexual reproduction of Fungi.These are non motile spore these are also haploid spores these are site of meiosis and karyogamy in oomycetes