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 nuclei surrounded by a single membrane (Figure 9.16).This retains 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 it is 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 plasmodium is a mass of protoplasm containing several nuclei and bounded by a cytoplasmic membrane.
Fruiting body (sporangiophore)
Development of fruiting body
Figure 9.17 The cellular slime moulds. Fruiting bodies develop from the pseudoplasmod-ium or 'slug' and release haploid spores that develop into individual amoebas. Only haploid forms participate in this cycle, which is therefore asexual. Sexual reproduction can also occur, involving the production of dormant diploid spores called macrocysts. Note that the pseudoplasmodium of cellular slime moulds is entirely cellular
Figure 9.18 A modern view of eucaryotic taxonomy. A possible scheme for the relationship between protistan groups based on 18S RNA data. The positions of the fungi, plants and multicellular animals (Metazoa) are also shown. Note that some protistan groups placed together in traditional schemes (e.g. kinetoplastids and choanoflagellates) are very distant in phylogenetic terms. The diplomonads and parabasilians are shown as diverging from the main stock before the acquisition of mitochondria through endosymbiosis with bacteria. This hypothesis may need to be revised in light of recent evidence that these organisms did once possess mitochondria but have since lost them
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