Some protists superficially resemble fungi in that they are not photosynthetic and some have bodies formed of threadlike structures called hyphae. However, fungus-like protists are not fungi for several reasons. Many of these protists have centrioles and produce cellulose as a major component of their cell walls, whereas fungi lack centrioles and have cell walls of chitin. Two major groups of fungus-like protists are: slime molds and water molds (oomycotes).
(i) Slime molds or Myxomycota
The feeding stage of a slime mold is a plasmodium, a multinucleate mass of cytoplasm that can grow to 30 cm (1 ft) in diameter. The plasmodium, which is slimy in
Fig. 7.14Slime mold physarum (a) the plasmodium is a naked mass of cytoplasm having many nuclei. (b) Reproductive structures are stalked sporangia.
Appearance, streams over damp, decaying logs and leaf litter. It often forms a network of channels that cover a large surface area. As it creeps along, it ingests bacteria, yeasts, spores and decaying organic matter (Fig. 7.14)
During unfavorable condition, slime mold forms resistant haploid spore by meiosis within stalked structures called sporangia. When conditions become favourable again, spores germinate into biflagellated or amoeboid retroactive or swarm cells which unite to form diploid zygote. Zygote produces multinucleate plasmodium, each nucleus being diploid.
The plasmodial slime mold physarum polycephalum is a model organism that has been used to study many fundamental biological processes, such as growth and differentiation, cytoplasmic streaming, and the function of cytoskeleton.
Water molds or Oomycotes
Oomycotes show close relations with the fungi and have a similar structure, but are now regarded as more ancient group. Their cell walls contain cellulose, not chitin. Their hyphae are aseptate (without cross walls). Oomycotes include a number of pathogenic organisms, including phytophthora infestans, which have played infamous roles in human history.
Fig. 7.15Phytophthora infesians growing in a diseased potato leaf, with sporangiophores emerging from the underside of the leaf.
Phytophthora infestans was the cause of Irish potato famine of the 19th century. It causes a disease commonly known as late blight of potatoes. Because of several rainy, cool summers in Ireland in the 1840’s, the water mold multiplied uncheched, causing potato tubers to rot in the fields. Since potatoes were the staple of Irish peasants’ diet, many people (250,000 to more than 1 million) starved to death. The famine prompted a mass migration out of Ireland to such countries as the United states (Fig. 7.15).