Slime Moulds

Slime moulds (or slime molds) are earlier grouped under fungi but later, they are kept in the kingdom Protista with other unicellular and small multicellular eukaryotic organisms.

They are saprophytic and feed on dead and decaying organic matter.

The name ‘slime’ comes from the gelatinous appearance of macroscopic slime moulds. Under unfavourable conditions, they form aggregates, this is common in acellular or plasmodial slime moulds. Their size varies from a few centimetres to several square metres.

They can live as a single-celled organism when there is abundant food, mainly cellular slime moulds.

General Characteristics of Slime Moulds

  • Slime moulds are found creeping on debris, decaying leaves or twigs, in soil, on the forest floor, on tree canopies and moist, dark and cool conditions
  • The protoplast is not surrounded by a cell wall in the vegetative phase
  • They are saprophytic and lack chlorophyll. They feed on microorganisms such as bacteria, fungi and yeasts and decompose dead organic matter
  • Some of the slime moulds are parasitic and found in the roots of cabbage and other plants of Brassicaceae family
  • The plasmodial stage resembles protozoa and fruiting bodies form spores resembling fungi
  • Spores have a cell wall made up of cellulose and are resistant to adverse conditions. They can survive for many years

Slime Moulds Classification

Slime moulds are classified under kingdom Protista. They resemble fungi as well as protozoa. In modern taxonomy, the true slime moulds come under Mycetozoa. They are further classified in different classes. The main classes of slime moulds are the following:

Myxomycetes- True slime moulds or acellular slime moulds. They are characterised by syncytial (multinucleated), plasmodial stage

Dictyostelia- Cellular slime moulds. They do not form huge coenocytes

Protostelia- Simple, minute, amoeboid slime moulds

Acrasia- Cellular slime moulds similar to dictyostelids but have eruptive pseudopodia

Plasmodiophomycetes- Parasitic slime moulds. They are found as an internal parasite in cabbage roots. They cause various diseases in plants such as clubroot disease of cabbage

Labyrinthulae- Net slime moulds. Form a network of tubes in which amoeba without pseudopodia can swim freely

Fonticula- Form volcano-shaped fruiting bodies

Slime moulds are mainly of two types: Acellular or Plasmodial slime moulds and Cellular slime moulds

Acellular or Plasmodial Slime Moulds

  • The feeding stage is a multinucleate mass of protoplasm, i.e. plasmodium
  • They can grow up to 1 ft in diameter
  • They are found creeping as a slimy mass over leaf litter, moist and decaying logs. It feeds on dead and decaying organic matter and microorganisms
  • When the food is scarce and moisture is less, they reproduce asexually
  • Examples: Physarum, Cribaria, Lycogala, Fuligo, Tubifera

The life cycle of acellular slime moulds

  • Drying plasmodium forms stalked fruiting bodies, which bear sporangia
  • Haploid spores are produced in sporangia by meiosis. They have a thick cell wall and are extremely resistant to adverse conditions
  • Under favourable conditions, these spores germinate to form haploid cells
  • These haploid cells act as a gamete. Different kinds of cells are formed depending on moisture availability
  • Biflagellated swarm cells are formed when the conditions are wet or otherwise amoeboid myxamoebae are formed. These cells act as gametes
  • These haploid gametes fuse to form a diploid zygote
  • The diploid zygote undergoes multiple mitotic division forming multinucleated plasmodium as it doesn’t undergo cytoplasmic division (cytokinesis)

Cellular Slime Moulds

  • The feeding stage is a single-celled amoeboid, which lives as a solitary organism
  • They have a close resemblance to amoebas
  • Individual cells feed on microorganisms and other food matter while creeping on decaying log or freely swimming in freshwater
  • Just like an amoeba, each cell has a haploid nucleus and divides mitotically
  • When the food is less they form aggregate but retain their individuality due to the presence of a thin plasma membrane and reproduce asexually by spore formation
  • When the food or moisture is depleted, they send out a cAMP-mediated chemical signal
  • The chemical diffuses out and binds to the receptors present on the surface of nearby cells resulting in the movement of cells towards cAMP (cyclic adenosine monophosphate)
  • Examples: Dictyostelium, Acytostelium, Polysphondylium

The lifecycle of cellular slime moulds

  • Thousands of cells aggregate forming a multicellular slug, which creeps for a short distance and settles
  • Each cell in the slug retains its individuality and retains its plasma membrane
  • After settling down, fruiting bodies are formed in the slug, which contains spores
  • Spores are released and germinate under favourable conditions into a single haploid amoeboid cell, which is the vegetative stage
  • Formation of spore is an asexual method of reproduction
  • They mostly lack sexual reproduction and lack the flagellated gamete stage

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