Slime molds

Five phyla of organisms (we will discuss two) that are not in the Kingdom Fungi, but Kingdom Protoctista Vegetative thallus – – lacks a cell wall, – amoeba like, – phagotrophic, i.e. ingests food particles by phagocytosis – May be multinucleate = plasmodium Have primarily been studied by mycologists, found in habitats of fungi, some produce fruiting structures that resemble fungi

Slime mold phyla Dictyosteliomycota – cellular slime molds – Vegetative thallus – amoebae that aggregate to form pseudoplasmodium – 3 genera, 50 spp. Myxomycota – true slime molds – Vegetative thallus – plasmodium – 71 genera, 500 spp.

Dictyosteliomycota – cellular slime molds Widely used in studies of eukaryotic cell development Make transition from population of individual amoeboid cells to multicellular structure Occurrence – widespread in forest soils, dung, decaying plant matter Feed on bacteria in soil as amoeboid cells

Dictyostelium discoideum life cycle Vegetative thallus – unicellular amoebae that feed on bacteria by phagocytosis Asexual reproduction – cell division Can form microcysts – form thin cellulose cell wall & withstand unfavorable environmental conditions

Developmental changes When food supply becomes exhausted or population reaches certain size, amoebae enter a starvation period Amoebae undergo developmental changes – Metabolic changes – shift from facultative aerobes to obligate aerobes – Use endogenous reserves – Cell surface antigens change – cells become more cohesive – Certain amoebae secrete a chemotactic substance - acrasin

Aggregation Acrasin in Dictyostelium is cyclic AMP, other species produce other substances Acrasin causes other amoebae to migrate toward the center of production in pulsating streams – aggregation stage

Aggregation

Pseudoplasmodium Amoebae aggregate to form pseudoplasmodium (slug, grex) Transition from population of independent cells to a multicellular structure Pseudoplasmodium in D. discoideum is 1-2 mm long and moves along gradients of temperature, light, humidity Is surrounded by a sheath of polysaccharide and protein, Leaves a trail of slime as it migrates

Pseudoplasmodium Amoebae do not feed or divide If food is added, may be de- aggregated up to a certain point after which they are committed to development As the slug migrates, it becomes polarized and cells begin to differentiate

Differentiation Two cytologically and biochemically distinct types of cells are forming in slug Prestalk cells – anterior portion (1/3) of slug – Swell, form a cell wall, become vacuolate and eventially die as they become stalk cells Prespore cells – posterior portion (2/3) of slug – Form prespore vacuoles – involved in cell wall synthesis

Culmination Slug migration ceases and becomes globose Prestalk cells form the beginning of the stalk

Sorocarp Stalk cells are formed, prespore cells migrate up the stalk Prespore cells form cellulose cell wall become spores Structure formed is a sorocarp with spores in the sorus (droplet containing spores) – not enclosed by wall, not a sporangium Spores – are uninucleate – remain dormant – Germinate to form an amoeba

Formation of a Sorocarp

Sorocarp

Formation of sorocarp – for dispersal of spores Asexual reproduction occurs as a result of cell division by amoebae before sorocarp formation Ca. one third of amoebae lost in sorocarp formation (produce stalk) Stalk is cellular

Sexual reproduction Not well understood Giant cells (zygotes) formed from fusion of two amoebae (gametes) Large number of amoebae migrate to zygote, secrete wall to enclose amoebae and zygote Zygote feeds on amoebae Other wall layers produced to form macrocyst

Macrocyst Meiosis occurs in macrocyst Cytoplasm cleaves to produce uninucleate amoebae Amoebae released through broken cyst walls Both homothallic and heterothallic strains are known from different species

Macrocyst formation

Myxomycota – true slime molds Produce a true plasmodium at some point in their life cycle Plasmodium – multinucleate mass of protoplasm that feeds by phagocytosis Great variability in size – some are microscopic, others may grow to meters

Habitats Commonly occur in cool, moist shady habitats, e.g. decaying logs Can occur on lawns if weather is moist Have also been found on bark of trees and in deserts Feed on bacteria, protozoa, small pieces of organic matter Generally not of great economic importance

Life cycle Two amoeba-like vegetative phases – Plasmodium – Myxamoebae Complex fruiting structures – sporophores Few species have been cultured (dual cultures) and grown through all stages in life cycle Fewer have been grown in axenic culture

Spores Spores are haploid, spherical Thick walled with spines, teeth, other ornamentation Cell wall composition not well known – one report – galactosamine polymer & melanin Can remain dormant (to at least 75 yrs)

Spore germination Cell wall either splits or small pore is digested Germination produces amoeba-like cells - myxamoebae (one or several) – or swarm cells – that have 2 flagella Myxamoebae and swarm cells can be interconverted – when water present, flagella are produced

Myxamoebae Feed by phagocytosis of bacteria, other small particles Divide by mitosis – dissolution of nuclear membrane and formation of centrioles If unfavorable conditions occur – can encyst (form a cell wall) to form a microcyst

Sexual reproduction Plasmogamy occurs between myxamoebae or swarm cells (some species are heterothallic) Must also be a critical mass of cells in population Karyogamy occurs shortly after plasmogamy to form zygote (2n) Zygote feeds, can engulf other myxamoebae, coalesce with other zygotes

Plasmodium Zygote forms plasmodium – longest lived vegetative stage Variation in species – – Size – microscopic to meter across – Color – colorless, black, violet, red, yellow, etc No definite shape Move over surface engulfing particles Vein like network with viscosity differences in cytoplasm Rapid cytoplasmic streaming

Plasmodium Phagocytosis of particles Can absorb nutrients Nuclei divide in synchronous fashion In mitosis, nuclear envelope doesn’t break down, no centrioles

Sclerotium Unfavorable environmental conditions can induce plasmodia to form dormant structures - sclerotia Hardened mass containing spherules – cytoplasm and several nuclei surrounded by cell wall Favorable conditions – germinate to form plasmodia

Sporulation Entire plasmodium differentiates to form reproductive structures Environmental conditions trigger – moisture, light, temperature, pH, exhaustion of food supply Sporulating structures = sporophores, 3 types – Sporangium (pl. sporangia) – Aethallium (pl. aethallia) – Plasmodiocarp

Sporophores In all sporophores, the multinucleate cytoplasm is cleaved into many spores Membranes are laid down around nuclei Cell walls are formed around cell membrane This differentiates a sporangium from a sorus

Sporangia Most common type of sporophore One plasmodium may form many sporangia Parts of a sporangium – Hypothallus – secretion of plasmodium that is left on substratum, base of the sporangium, may be a thin, cellophane-like secretion or a crust of CaCO 3

Parts of a sporangium Stalk – supports sporangium, may or may not be present, may be hollow or filled with material Stalks formed from secretions of plasmodium and are acellular (in contrast to cellular slime molds)

Parts of a sporangium Peridium – outer covering of sporangium Ranges from delicate membrane to tough covering May be impregnated with CaCO 3

Parts of a sporangium Columella – an extension of the stalk into the sporangium

Parts of a sporangium Capillitium – nonliving threads that intermingle but are not attached to spores May be attached to peridium or columella May be ornamented Formed during spore cleavage Involved in dispersal

Parts of a sporangium Spores – main function of sporophore is to form spores Formed from a multinucleate mass of cytoplasm – vacuoles form and fuse to form membranes around nuclei, cell walls formed in vacuoles – called cleavage

Spores Are uninucleate and diploid at first Meiosis occurs to form 4 haploid nuclei Three nuclei may disintegrate to form uninucleate haploid spores In some, spore may contain more than one nucleus – germinate to produce more than 1 myxamoebae

Other types of sporophores Aethallium – fairly large cushion shaped structure (does not differentiate into individual sporangia)

Other types of sporophores Plasmodiocarp – similar in appearance to plasmodium, plasmodial veination is retained, stalkless

Classification Myxomycota classified on the characteristics of their sporophores – presence or absence of capilltium, stalk, nature of the peridium, etc. Many form brightly colored sporophores