04. Myxomycetes differ from fungi in not
having
Answers
Explanation:
Division: Myxomycota
Members of this division are commonly referred to as slime molds. Although presently classified as Protozoans, in the Kingdom Protista, slime molds were once thought to be fungi (=kingdom Mycetae) because they produce spores that are borne in sporangia, a characteristic common to some taxa of fungi. However, the assimilative stage in slime molds is morphologically similar to that of an amoeba. This assimilative stage has been designated a myxamoeba (Fig 1). The myxamoeba, as is the case of the amoeba, is a uninucleate, haploid cell which is not enclosed in a rigid cell wall, and ingests its food by means of phagocytosis. During this mode of ingestion, the food particles, usually bacteria, beceome surrounded by the pseudopodia of the myxamoeba. Once the food has been engulfed in this matter, it is surrounded by a membrane or food vacuole where hydrolytic enzymes are secreted that will digest the food. In fungi, the assimilative stages are mycelium and yeast, both of which are surrounded by a rigid cell wall and obtain their food by means of absorption. These are some of the reasons why mycologists no longer recognize slime molds as being fungi. However, organisms in this group continue to be studied in mycology as a matter of tradition and not because they are thought to be related to fungi. Within the Myxomycota, the class Myxomycetes, often referred to as the acellular slime molds, will be the only class that we will consider.
Class: Myxomycetes
There are approximately 500 species of Myxomycetes. They are found on moist soil, decaying wood, and dung. One of the more interesting characteristic about this group of organisms is that while the species of other organisms will vary in different geographical localities, i.e. you don't find the same species of plants and animals on the mainland that you find in Hawai‘i, this is not generally true in the Myxomycetes. Most species can be found throughout the world.
Life Cycle of Myxomycetes
Introduction
Physarum polycephalum and Didymium iridis will be used to represent the Myxomycete life cycle and when possible, pictures of these species will be used to illustrate the various parts of the life cycle. These two species were selected since a great deal of researach has been carried out with these species and their life cycles are well known. However, since other species may differ somewhat in their life cycles, some of these variations will also be described.
Spore Germination, Myxamoebae and Swarm Cells
The spores of Myxomycetes are normally globose, uninucelate and haploid. The spore surface may range from almost smooth to reticulate. Spores of P. polycephalum and D. iridis are spiny. The spore wall is composed primarily of cellulose and is only one of two stages where a cell wall is formed. The other stage that forms a cell wall is the microcyst, which is discussed below. Upon germination, the spore will crack open and release a single, uninucelate myxamoeba. The myxamoeba moves by amoeboid motion and ingest food, by phagocytosis, as it does so. As the myxamoebae feeds and grows, they will reproduce, asexually, by mitosis and cytokinesis.
The myxamoeba stage may continue to proliferate for an indefinite period of time if there is available nutrient and the environment remains favorable. In most species, the myxamoeba stage may also vary according to the environment. When free water is available myxamoeba can differentiate into flagellated swarm cells (Fig 4-5). Although two flagella are present, One the long, anteriorly directed flagellum is visible. The second, very short flagellum is usually not visible. During periods of unfavorable conditions, the protoplast of the myxamoeba or swarm cell can round up and form a thin, cellulose protecetive layer around itself, called the microcyst (Fig 6), which will protect it from the environment.