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 haploid, non-motile spores are the site of meiosis and karyogamy in oomycetes.
The Oomycetes (/ˌoʊ.əˈmaɪsiːts/), or Oomycota, form a distinct phylogenetic lineage of fungus-like eukaryotic microorganisms within the Stramenopiles. They are filamentous and heterotrophic, and can reproduce both sexually and asexually. Sexual reproduction of an oospore is the result of contact between hyphae of male antheridia and female oogonia; these spores can overwinter and are known as resting spores. Asexual reproduction involves the formation of chlamydospores and sporangia, producing motile zoospores. Oomycetes occupy both saprophytic and pathogenic lifestyles, and include some of the most notorious pathogens of plants, causing devastating diseases such as late blight of potato and sudden oak death. One oomycete, the mycoparasite Pythium oligandrum, is used for biocontrol, attacking plant pathogenic fungi. The oomycetes are also often referred to as water molds (or water moulds), although the water-preferring nature which led to that name is not true of most species, which are terrestrial pathogens.
Oomycetes were originally grouped with fungi due to similarities in morphology and lifestyle. However, molecular and phylogenetic studies revealed significant differences between fungi and oomycetes which means the latter are now grouped with the stramenopiles (which include some types of algae). The Oomycota have a very sparse fossil record; a possible oomycete has been described from Cretaceous amber.
The egg cell or ovum (pl.: ova) is the female reproductive cell, or gamete, in most anisogamous organisms (organisms that reproduce sexually with a larger, female gamete and a smaller, male one). The term is used when the female gamete is not capable of movement (non-motile). If the male gamete (sperm) is capable of movement, the type of sexual reproduction is also classified as oogamous. A nonmotile female gamete formed in the oogonium of some algae, fungi, oomycetes, or bryophytes is an oosphere. When fertilized, the oosphere becomes the oospore.
When egg and sperm fuse together during fertilisation, a diploid cell (the zygote) is formed, which rapidly grows into a new organism.
Pythium oligandrum is an oomycete. It is a parasite of many fungi and other oomycetes including Botrytis, Fusarium and Phytophthora. It has been licensed as a biocontrol agent in the form of an oospore soil treatment, which reduces pathogen load and concomitant plant disease. P. oligandrum have been found to express several genes belonging to the CAZy-family when feeding on prey. P. oligandrum can grow within the roots of certain plants, including tomato and sugar beet. Production of auxin-like substances stimulate plant growth. Defense responses can be induced in the plant, which primes the plant from further infection by pathogenic fungi, oomycetes or bacteria.