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The slime moulds and water moulds (the fungus-like protists)
The final group to consider are the so-called ‘fungus-like’ protists. Its members are phylogenetically diverse, and as we’ll see, its two principal groupings, the slime moulds and the water moulds, are placed far apart from each other in modern classification systems.
Water moulds resemble true fungi in their gross structure, comprising a mass of branched hyphae. At the cellular and molecular level however, they bear very little resemblance, and are not at all closely related.
The Oomycota derive their name from the single large egg cell that is fertilised to produce a diploid zygote as part of the sexual reproduction cycle.
Many water moulds play an important role in the decomposition of dead plants and animals in freshwater ecosystems, while others are parasitic on the gills of fish. Terrestrial members of the Oomycota include a number of important plant pathogens, such as rusts and mildews, which can have a devastating effect on crops such as tobacco and potatoes.
At one stage in their life cycle, the plasmodial or acellular slime moulds exist as a single-celled amoeboid form. Two of these haploid amoebas fuse to give a diploid cell, which then undergoes repeated divisions of the nucleus, without any accompanying cell division; the result is a plasmodium, a mass of cytoplasm that contains numerous nucleisurrounded by a single membrane (Figure 9.16).This re-tains the amoeboid property of cytoplasmic streaming, so the whole multinucleate structure is able to move in a creeping fashion. This ‘feeding plasmodium’, which may be several centimetres in length and often brightly coloured, feeds phagocytically on rotting vegetation. Fruiting bodies develop from the plasmodium when itis mature or when conditions are unfavourable, and a cycle of sexual reproduction is entered. When favourable conditions return, meiosis gives rise to haploid spores, which germinate to produce the amoeboid form once more.
A unicellular amoeboid form also figures in the life cycle of the other group of slime moulds, the Dictyostelida (Figure 9.17). This haploid amoeba is the main vegetative form, but when food supplies are scarce, large numbers aggregate to form a slug-like blob, superficially not unlike the plasmodium described above. Unlike the plasmodium, however, this aggregate is fully cellular, so each component cell retains its plasma membrane.
Compare the life cycle of cellular slime moulds (Figure 9.17) with that of the plas-modial kind (Figure 9.16). Fruiting bodies again develop, giving rise to spores that germinate into new amoebas. No meiosis step is required, however, because the whole cycle comprises haploid forms, and this is therefore a form of asexual reproduction. A simple sexual cycle may also occur, when haploid amoebas fuse to give a diploid zygote.
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