Fungal-like Protists and Important Organisms

Fungal-like Protists

Overview

  • Definition: Fungal-like protists are a group of eukaryotic organisms that share features with fungi, primarily in their mode of nutrition and life cycle.

Slime Molds

  • Slime Molds Overview

    • Fungal-like heterotrophs

    • Assemblages belonging to various supergroups, focusing on two groups within the supergroup Amoebozoa:

    • Phylum Myxomycota (plasmidial slime molds and cellular slime molds)

      • Plasmodial Slime Molds (Myxomycetes)

      • Cellular Slime Molds (Dictyosteliomycetes)

Phylum Myxomycota

  • Characteristics of Myxogastria (Myxomycetes):

    • Approximately 700 species

    • Nutritional mode:

    • Feed on microorganisms (bacteria, yeasts) and decaying vegetation through phagocytosis

    • Cells are coenocytic, meaning they have multiple nuclei within a shared cytoplasm.

    • Life Cycle:

    • Invariably sexual, involving the fusion of two single cells of opposite mating types, forming a diploid zygote

    • Gametes produced are isogamous

    • Meiosis leads to spore formation

  • Plasmodium

    • The diploid stage exists as a creeping mass of protoplasm with many nuclei.

    • Examples:

    • Fuligo septica (commonly known as dog vomit slime mold)

    • Physarum swarm cells with multiple nuclei

Frutifications in Myxomycetes

  • When food becomes scarce and conditions are right, the plasmodium transforms into fruiting bodies, also called frutifications that can form within hours.

  • Types of frutifications:

    • Sporangia: Stalked and have unique shapes

    • Aethalia: Thickened, cushion-shaped

    • Plasmodiocarp: Most primitive form of frutification

Sclerotium Formation

  • Sclerotium: Resting stage of plasmodial slime mold, defined as a compact mass of hardened fungal mycelium with food reserves, surviving adverse environmental conditions through this structure.

  • Macrocysts: Individual aggregations within sclerotium, leading to the emergence of a plasmodium when environmental conditions become favorable.

Cellular Slime Molds (Dictyosteliomycetes)

  • Class features:

    • Less than 50 species

    • Organisms resemble free-living amoeba-like cells called myxamoebas, with cellulose-based cell walls.

    • Engages in normal mitosis and possesses centrioles.

    • Habitat: Common in litter-rich soils, feeding on bacteria via phagocytosis.

    • Reproduction: Isogamous gametes known; attracted by acrasin produced in starvation conditions.

Myxamoebas

  • When nutrients are scarce, myxamoebas aggregate, forming a slug-like mass called pseudoplasmodium, which has both anterior and posterior ends, directing movement towards light and/or away from heat sources.

Ammonia in Development

  • Waste product of pseudoplasmodium is ammonia, contributing to phototaxis and potentially activating genes related to cell fate.

  • Cell differentiation at rest:

    • Anterior cells produce a delicate stalk

    • Posterior cells differentiate into spores forming a terminal mass on the stalk.

Sorocarp Formation

  • Sorocarp: Structure formed for spore dispersal within the life cycle of Dictyosteliomycetes, facilitating asexual reproduction.

    • Components of sorocarp:

    • Sorus: Spore mass

    • Sorophore: Stalk

    • Basal disk: Support base

Life Cycle Overview

  • Sequence of development:

    • Solitary cell (feeding stage)

    • Aggregated amoebas migrate, forming pseudoplasmodium

    • Fruiting bodies develop, producing spores for dispersal

Fungal-like Protists: Oomycota

  • Phyly Oomycota:

    • Approximately 700 species

    • Significance: Mostly saprobes and parasites, with oosporic reproductive cycles.

    • Characteristics:

    • Cellulose-based cell walls

    • Biflagellate spores with dimorphic (heterokont) flagella

    • Osmotrophic feeding style via enzyme secretion into the substrate

Water Molds and Downy Mildews

Order Saprolegniales

  • Aquatic order, some parasitic species affecting fish and fish eggs

  • Definitions:

    • Saprophyte: Organism deriving nutrients from dead or decaying matter

    • Parasite: Organism living on a host, typically harming it

Order Peronosporales

  • Most advanced order, includes economically significant parasitic downy mildews.

  • Members: Plasmopara and Phytophthora are crucial genera.

  • Sporangia formed at stalk ends (sporangiophores), aids in propagation via zoospores.

  • Unique reproductive feature: Oogonia contain only one egg.

Historical Context of Downy Mildew

  • Notable impact on the French wine industry in the 1870s due to Plasmopara viticola.

  • Bordeaux Mixture (1882): First chemical preventative measure using copper sulfate and lime to combat downy mildew, significant for broader agricultural applications.

  • Phytophthora infestans responsible for the potato blight, leading to the Great Irish Famine in the late 1840s.

    • Originated in central Mexico, reached the USA circa 1843 and Europe by infected potato shipments in 1845.