Slime molds

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

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 CaCO3

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 CaCO3

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