Slime Molds

1. Slime Molds

2. Kingdom Protoctista • Phyla of slime molds: • Plasmodiophoromycota—endoparasitic slime molds • Dictyosteliomycota—cellular slime molds • Acrasiomycota—cellular slime molds • Myxomycota—plasmodial slime molds

3. Slime molds • An organism that produces a trophic stage that lacks a cell wall; phagotrophic • Trophic stages: • amoebae • plasmodia

4. Amoeba or plasmodium? • Amoebae are uninucleate • Plasmodia are multinucleate • Both lack cell walls, engulf food, and can multiply http://www.planet-pets.com/plntamba.htm

5. Cellular Slime Molds • Two phyla • Dictyosteliomycota (dicytostelids) • Primarily in soil • Acrasiomycota (acrasids) • On dead plant parts, tree bark, dung and soil • Trophic stage comprises uninucleate cells (myxamoebae) that aggregate

6. Dictyostelids • Three genera, 50 species • First discovered in 1869 by Oskar Brefeld • Dictyostelium discoideum isolated by Kenneth Raper (1935) • important model organism for study of cytokinesis, signalling, chemotaxis, phagocytosis, motility, cell sorting, cell-type determination • See DictyBase http://dictybase.org/dicty.html

7. http://web1.manhattan.edu/fcardill/plants/protoc/dicty.html

8. Dictyostelium life cycle • Free-living, uninucleate haploid myxamoebae with filose pseudopodia emerge from spores www.image1.com/images/ timelapse-movie.gif

9. Dictyostelium life cycle • Myxamoebae aggregate in response to chemical signal (cAMP); aggregating amoebae adhere end-to-end www-biology.ucsd.edu/labs/ loomis/agg-stream2.jpeg

10. Dictyostelium life cycle • Pseudoplasmodium (non-feeding stage), also called slug, formed of 10- to 50,000 individual amoebae encased in cellulose sheath • Migrate in response to temperature, light, relative humidity www.germany-info.org/relaunch/ education/new/edu_genome.html

11. Life cycle continued • Culmination results in formation of sorocarp http://niko.unl.edu/bs101/pix/dd1.gif

12. Spores Cells in anterior direct the pseudoplasmodium, but are destined to become stalk cells Cells in posterior will become spores Cellulose sheath secreted by amoebae Direction of movement of pseudoplasmodium

13. Dictyostelid life cycle http://www.zi.biologie.uni-muenchen.de/zoologie/dicty/dicty.html

14. Plasmodial Slime Molds

15. http://www.ucmp.berkeley.edu/chromista/chromistasy.html Ribosomal RNA phylogenies place slime molds as unrelated, early branching eurkaryote lineages

16. Baldauf and Doolittle, 1997. PNAS 94 (22): 12007-12012 Actin, elongation factor, and β-tubulin phylogenies place the plasmodial and cellular slime molds as a monophyletic group close to Animal + FungI

17. Myxomycota • Myxomycetes--5 orders, 13 families, 62 genera, 800 species • Characterized by plasmodium • Engulfs bacteria, fungal spores, small pieces of organic matter

18. Physarum life cycle meiosis karyogamy microcyst Synchronous mitotic divisions macrocyst

19. Stages in Life Cycle • Spores (2nn) • 4-20 µm, pigmented ornamented; meiosis in spore = 4 nuclei; 3 degenerate

20. Stages in Life Cycle • Myxamoebae (n) • feed, divide, convert to swarm cells, function as gametes; form microcysts under adverse conditions www.uoguelph.ca/~gbarron/ MISCE2002/myxamo2.jpg

21. Stages in Life Cycle • Swarm cells (n) • 1-4 anterior whiplash flagella, amoeboid posterior; feed (absorption and engulfment), function as gametes • can’t undergo cell division http://www.botany.hawaii.edu/faculty/wong/Bot201/Myxomycota/Swarm_Cells2.jpg

22. Stages in Life Cycle • Zygote (2n) • Formed by fusion of myxamoebae or swarm cells; enlarges through synchronous nuclear division • Plasmodium (2n) • Multinucleate, wall-less protoplasm Photo by Stephen Sharnoff http://www.plant.uga.edu/mycology-herbarium/myxogal/Physros3.jpg

23. Stages in Life Cycle • Sporophore (2n) • Entire plasmodium is converted into sporophore(s) • Sclerotium or macrocyst (2n) • Resistant stage formed by plasmodium

24. Sporocarp—stalked or sessile May have columella 

25. Plasmodiocarp • Sporophore developing along veins of phaneroplasmodium; takes on reticulate shape of veins

26. Photo by Stephen Sharnoff

27. Liceales-- Pale or brown spores, capillitium and lime absent, pseudocapillitium may be present Dictydium Lycogala Photo by Stephen Sharnoff

28. Trichiales--Pale spores (yellow, orange or red) and abundant, conspicuous capillitium Trichia Arcyria

29. Physarales--Purplish-brown spores, usually with abundant and conspicuous lime on or in sporophore Badhamia Leocarpus Fuligo

30. Stemonitales--Violet-brown spores, lime absent Lamproderma Diachea

31. Photo by Stephen Sharnoff Stemonitis

32. Ceratiomyxales--Exosporous sporophores; probably belongs in protostelids (one genus, Ceratiomyxa) Photo by Stephen Sharnoff Photo by David Geiser

33. Fungus-like Protista Myxomycota, Myxomycetes, true "Slime Molds", Plasmodial slime molds The plasmodial slime molds are a group of fungus-like organisms usually present and sometimes abundant in terrestrial ecosystems. The myxomycete life cycle involves two very different trophic (feeding) stages, one consisting of uninucleate amoebae, with or without flagella, and the other consisting of a distinctive multinucleate structure, the plasmodium. Under favorable conditions, the plasmodium gives rise to one or more fruiting bodies containing spores. The spores germinate to produce the uninucleate amoebae. The vegetative stage is essentially a multinucleate mass of protoplasm (called a plasmodium) composed of numerous diploid nuclei. Depending on the species, the plasmodium may be only a few millimeters in diameter to large pizza-sized slime molds. Plasmodia typically occur in cool, moist, shady places such as within crevices of decaying wood and in leaf litter on the forest floor. The plasmodium moves like a giant amoeba, flowing over the surface as it ingests organic matter, such as dead leaves and wood. It may be brightly colored, such as the yellow Physarum polycephalum. Slime molds are important heterotrophs in the decomposition of dead organic matter in temperate and tropical forests.

34. The transformation of the plasmodium into spore-bearing "fruiting bodies" or fructifications can occur within hours. The fruiting bodies produced by myxomycetes are somewhat suggestive of those produced by higher fungi, although they are considerably smaller (usually no more than 1-2 mm tall). The life cycle of a slime mold begins with a haploid spore which is produced inside the fructification. The germinated spore can transform into an amoeba. Myxamoeba divide into large populations. Two myxamoeba cells may join together in a cellular fusion (plasmogamy) followed by nuclear fusion (karyogamy). The combining of their cell contents (protoplasts) represents a very primitive form of sexual reproduction. The fusion of two haploid cells results in a diploid zygote which transforms into a developing plasmodium. As the diploid nuclei divide, the plasmodium grows larger and larger, slowly moving (creeping) along the forest floor and feeding like a giant amoeba. There are approximately 1000 recognized species of myxomycetes. The majority of species are probably cosmopolitan, but a few species appear to be confined to the tropics or subtropics and some others have been collected only in temperate regions.

35. Acrasiomycota, cellular slime molds Cellular slime molds spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, the individual cells aggregate into a great swarm. Cellular slime molds are thus of great interest to cell and developmental biologists, because they provide a comparatively simple and easily manipulated system for understanding how cells interact to generate a multicellular organism. Cellular slime molds, the second major group of slime molds, exist as minute "amoeba" during their growth phase. Each amoeba contains a single nucleus. The amoebae crawl through dung, soil, rotting mushrooms, decaying leaves and other organic material at an average speed of 1 mm per hour. When conditions are right, all the amoebae in an area join together to create a pseudoplasmodium, a "fake plasmodium".

36. Dictyostelium discoideum Genome Project Dictyostelium discoideum, a soil-living amoeba, is an excellent organism for the study of the molecular mechanisms of cell motility, signal transduction, cell-type differentiation and developmental processes. Genes involved in any of these processes can be knocked-out rapidly by targeted homologous recombination. Since Dictyostelium is haploid, mutants are readily isolated. The determination of the entire information content of the Dictyostelium genome will be of great value to those working with this organism directly, as well as to those who would like to determine the functions of homologous genes from other species.         The hereditary information is carried on 6 chromosomes with sizes ranging from 4 to 7 Mb resulting in a total of about 34 Mb of DNA, a multicopy 90 kb extrachromosomal element that harbours the rRNA genes, and the 55 kb mitochondrial genome. The number of genes in the genome is about 12,500 and many of the known genes show a high degree of sequence similarity to homologues in vertebrate species.         The Dictyostelium discoideum genome project is an international collaboration. We are pleased to announce the completion of the genome sequence of the social amoeba Dictyostelium discoideum (Eichinger et al., Nature 435, 43-57, 2005). DNA microarrays of about 6.000 Dictyostelium genes are constructed.