Downy cells9/27/2023 When the oospores are produced in plant tissue, they may occupy a large portion of the tissue ( Figure 11). The fertilized oogonium develops into a thick-walled oospore ( Figure 10). Some Phytophthora species are homothallic. Unlike the heterothallic species, homothallic individuals do not require distinct mating types, but can reproduce sexually by selfing. In other species, sexual reproduction occurs within a single individual (these are homothallic individuals). In heterothallic oomycetes, the gametangia are produced only in the presence of both mating types due to the fact that a hormone produced by one thallus stimulates the other to produce gametangia. In some species, two distinct mating types occur and both are required for sexual reproduction (these are heterothallic as opposed to homothallic species). There may be differences in “femaleness” and “maleness” and sexual preference is relative to other individuals. Typically each individual produces both antheridia and oogonia. In some genera the antheridium is attached to the side of the oogonium ( paragynous, Figure 8), but in other genera, the antheridium surrounds the base of the oogonium ( amphigynous, Figure 9). The morphology of antheridium attachment has been an important feature in morphological taxonomy of some genera. Because meiosis does not occur until the formation of gametangia occurs, the vegetative nuclei are diploid. Sexual reproduction occurs via the production of gametangia: oogonia and antheridia. In yet other species, sporangia can germinate directly to produce germ tubes or “indirectly” to produce zoospores, a trait which is often temperature dependent, with zoospores being produced at cooler temperatures. In this case, the sporangia are sometimes termed “conidia”. In some species, the ability to produce zoospores has been lost, and sporangia are thought to have evolved into structures that germinate directly to produce germ tubes. They may be terminal or intercalary (within a hyphal filament), bulbous or not, and if terminal, caducous (sporangia detach readily) or not. Sporangia of different taxa within the group are of diverse shapes and characteristics ( Figures 3-8, 29). After a time of free swimming the zoospores settle on a surface, retract their flagella, and secrete a mucilaginous matrix which affixes them to the surface. Oomycetes can often be “baited” from soil water, streams or ponds, and it is thought that zoospores are attracted to the baits. Zoospores can swim in water films on leaf surfaces, in soil water, in hydroponic media and in natural bodies of water. Although wall-less, zoospores retain a consistent but flexible shape. The anterior flagellum of a zoospore is a tinsel type, while the posterior flagellum is a whiplash type both are typically attached in a ventral groove ( Figure 2). One of the most distinguishing characteristics is the production of Morphological characteristics of oomycetes Many species produce wall-less, biflagellated swimming spores (zoospores) in structures called sporangia. The cell wall is composed of β-1,3, and β-1,6 glucans, and not of chitin (the polymer of N-acetyl glucose amine, found in the walls of true fungi). The nuclei of vegetative cells are typically diploid. Septa (cell walls) in the hyphae are rare, resulting in a multinucleate condition (termed coenocytic). There are many features distinguishing oomycetes from fungi. The data from these molecular analyses have been particularly convincing to non-systematist plant pathologists. Phylogenetic analyses using genes and intergenic regions have confirmed the assertions of earlier systematists that the oomycetes are different from fungi. Indeed, fungi appear more closely related to animals than to oomycetes, and oomycetes are more closely related to algae and to green plants ( Figure 1). However, as our understanding of evolutionary relationships has grown, it is now clear that this group of organisms is unrelated to the true fungi. Because of their filamentous growth habit, nutrition by absorption, and reproduction via spores, oomycetes were long regarded by plant pathologists as lower fungi. Some notable diseases are the late blight of potato, downy mildew of grape vine, sudden oak death, and root and stem rot of soybean. The diseases they cause include seedling blights, damping-off, root rots, foliar blights and downy mildews. The oomycetes, also known as “water molds”, are a group of several hundred organisms that include some of the most devastating plant pathogens.
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