Myxomycotina

Myxomycotina-General account

1. Myxomycotina : General account Vaishali S.Patil Professor, Department of Botany Shri Shivaji College of Arts, Commerce & Science Akola

2. Introduction- 1.The Myxomycota or slime molds , are fungus-like organisms . 2.They are characterised by the absence of cell wall from their amoeboid,’ animal-like vegetative or assimilatory phase. 3.The amoeboid assimilatory phase may consist of a free-living multinucleate mass of protoplasm called a plasmodium or simple uninucleate amoeboid cells, the myxamoebae (sing, myxamoeba ) often aggregating into a pseudoplasmodium or amoeboid cells interconnected by slime filaments giving rise to a structure known as net plasmodium or filoplasmodium . 4.They exhibit characteristic of both animals and plants .   5.The true slime molds are mostly saprophytic (formation of

3. fructifications) with a few parasitic ones.(absence of development of fructifications, the plasmodium is converted into masses of spores). 6. All the true slime molds produce flagellated cells, the swarm cells . 7. The formation swarm cells is absent in the cellular slime molds . In the cellular slime molds fructifications are developed from the pseudoplasmodia . 8. In the net slime molds each cell is converted into a mucilaginous sporocyte in which spores are developed. 9.The spores escape as biflagellate zoospores which after swimming for some time, lose their flagella become surrounded by delicate envelope and give rise to new net plasmodium. 10. They are found on moist dead wood, rotting logs, damp soil, leaf mold , moist sawdust, bark of trees, decaying fleshy fungi, or other

4. organic matter . They often spend most of their lives within the substrate and emerge only when about to produce sporangia. Following periods of rainy weather they may occur on leaves of grasses or other plants on lawns . moist dead wood rotting logs damp soil, leaf mold , moist sawdust bark of trees decaying fleshy fungi on leaves of grasses or other plants on lawns. Other organic matter

5. Phylogeny- Macbride (1899) believed that the slime molds are fungi. Karling (1944) suggested that the slime molds are closely related to Protozoa than to fungi. Martin (1932, 1960-61) has pointed out that if the algal origin of fungi is accepted, then the slime molds are not fungi; but if the fungi have been descended from colourless flagellates, the slime molds can well be regarded as fungi. Sparrow (1958) proposed that the parasitic slime molds be regarded as a separate Class the Plasmodio­phoromycetes . Talbot (1971), Webster (1983), and others grouped all the four classes of slime molds under the Division Myxomycota close to fungi. Bonner considers the Acrasiomycetes to have had their origin from the free-living amoebae of the soil. 

6. Classification- Kingdom Fungi Division Myxomycota (Plasmodium or pseudoplasmodium present) Eumycota Classes Acrasiomycetes Hydromyxomycetes Myxomycetes Plasmodiophoromycetes (Amoeboid cells aggregating into a pseudoplasmodium ) (Amoeboid cells spindle-shaped to oval interconnected by slime filaments forming a net commonly known as ‘net plasmodium’ or ‘ filo -plasmodium’) (Free-having plasmodium bearing haploid or diploid nuclei, Plasmodium non-parasitic, fructification present.) Plasmodium parasitic, fructification lacking

7. A. Class: Acrasiomycetes : Acrasiomycetes are commonly known as cellular or amoeboid slime molds , and are found profusely in the upper layer of humus in deci­duousforests and also in cultivated lands. The characteristic features are: Somatic phase commonly consists of amoe­boid cells or myxamoebae . 2. Myxamoebae aggregate to form a pseudo­plasmodium , which develops fruit body. 3. Lack of flagellated cells, except in Pocheina rosea . 4. Spore wall contains cellulose . 5. Fruit bodies may be sorocarps (in Dictyo­stelium ) or sporocarp (in Protostelium ). Raper (1973) divided the class into 3 subclasses: Protostelidae , Acrasidae and Dictyostelidae . Later, Olive (1975) raised the rank subclasses into classes as Protosteliomycetes , Acrasiomycetes and Dictyosteliomycetes . Reproduction- When resources such as water or food become limiting, the amoeba will release pheromones such as  acrasin  to aggregate  amoebal  cells in preparation for movement as a large (thousands of cells)  grex  or  pseudopod . When in the grex , the amoeboids reproduce, resulting in fruit-like structures called spores, which develop into unicellular  molds  of the same species.

8. B. Class: Hydromyxomycetes : The thallus forms net plasmodium or filoplasmodium . They are commonly aquatic and saprobic , but rarely parasitic. The characteristic features are: 1.The thallus consists of uninucleate spindle shaped cells , forming extensive filaments. 2. The filaments are tubular and form net-like structure, the net-plasmodium or filoplas - modium . 3. Reproduction by cyst formation, zoospore formation or by congregation. The class consists of two orders Hydro- myxales and Labyrinthulales . But Hawksworth et al. (1983) excluded Hydromyxales from fungi and included it in Protozoans . The members of Labyrinthulales are parasitic on higher plants and also on marine algae. Labyrinthula macrocystis is a very common species, found as parasite on Zostera marina, the eel-grass.

9. Class- Myxomycetes /true slime molds /slime molds / slime fungi / Mycetozoa . / plasmodial slime molds (Fig. 4.10). They are commonly found in damp places, espe­cially on old wood and other decomposing plant parts. T. Panckow in 1654 was the first to discover the Myxomycetes . The characteristic features are: 1. The vegetative body is a free-living plasmo­dium . 2. They feed on yeast cells, protozoa, fungal spores and other substances. 3. Reproduction takes place by asexual and sexual means . The reproductive process is plant­like producing spores with definite cell walls. 4. Asexual reproduction takes place by fragmentation of plasmodium or binary fission in myxamoebae . 5.Sexual reproduction takes place by fusion between flagellated zoospores or myxamoeba to form zygote, from which multinucleate plasmodium develops by mitotic divisions. They develop diffe­rent types of fructification. These are sporangium, aethalium and plasmodiocarp (Fig. 4.11D-G). Meiosis takes place during spore formation in the fructification. Alexopoulos (1973) divided the class Myxomycetes into three subclasses: Ceratio - myxomycetidae , Myxogastromycetidae and Stemonitomycetidae .

10. D. Class: Plasmodiophoromycetes : This group is commonly known as endo - parasitic slime molds . They are obligate para­sites, grow on algae, aquatic fungi and higher plants (commonly in the roots). The characte­ristic features are: Members of this class are obligate (i.e., biotrophic ) parasites of fresh water algae, aquatic fungi and higher plants (commonly in the roots). 2. Somatic body consists of a naked holo­carpic plasmodium . 3. Plasmodia are of two types in their life cycle: sporangiogenous plasmodium (form sporangia) and cytogenous plas­modium (gives rise to cysts i.e., resting spores). 4. Zoospores biflagellate , having unequal flagella of whiplash type, situated in opposite direction, the shorter one in anterior and longer one in posterior side. The class consists of a single order Plasmodiophorales with a single family Plasmo - diophoraceae . The class consists of 16 genera and 45 species. Some like Plasmodiophora brassicae causes club-root of crucifer; Spongospora subterranea causes pow­dery scab of potat o.

11. Sexual reproduction- The myxamoebae or swarm cells , behave as gametes and as such take part in the sexual reproduction. Fusion may take place between the two swarm cells or two myxamoebae or between a swarm cell and a myxamoeba resulting in the formation of a zygote, when plasmogamy is followed by karyogamy . Fusion between the two swarm cells or between a swarm cell and a myxamoeba leads to the development of a flagellated zygote which sheds its flagella and becomes amoeboid. The zygote formed in any one of the processes grows, along with which the diploid nucleus also divides mitotically giving rise to a multinucleate amoeboid structure, the plasmodium. According to some, karyogamy takes place during zygote formation. To them, the plasmodium represents the diplophase in the life history of a myxomycete and meiosis takes place during the formation of spores in the fructification (fruit body).Whereas, others are of opinion that karyogamy takes place just before meiosis during the development of spores in the fructification and as such the plasmodium is in the dikaryophasic condition.

12. Life cycle- The spores on germination produce one to four swarm cells or myxamoebae . The swarm cells or myxamoebae behave as gametes. They fuse in pairs, plasmogamy is followed by karyogamy . The zygote so formed grows accompanied with repeated mitotic division of the diploid zygotic nucleus resulting in the development of a plasmodium. Many zygotes may also coalesce to produce a plasmodium. A mature plasmodium thickens and gives rise to the fructification of varied nature. During the development of the fructification (fruiting body) and spore formation the diploid nuclei divide reductionally . Each haploid nucleus with a portion of cytoplasm enveloped by a thick wall develops into a spore. The spores vary greatly in colour and characteristics of the wall which are of taxonomic importance.(Figure 327).

13. Economic Importance of Myxomycota : 1.They have been the subject of intensive laboratory studies . 2. They contribute to the carbon and nitro­gen cycles by using various organic matter including bacteria as food . 3. They provide a large amount of protoplasm free from cell walls which has been used as an ideal medium to solve variety of fundamental problems of biochemists, biophysicists, myco­logists and even the geneticists .