General Mechanisms of Variability

• Hybridization
• Mutation
• Cytoplasmic Inheritance

If one takes the position of a Darwinian that the ability to be genetically plastic and adapt to changing needs is the key to survival then one must postulate that all organisms have some mechanism to introduce genetic variability. The classic Mendelian scheme applies generally to eukaryotes but alternate schemes must occur in some prokaryotes that do not undergo sexual reproduction.

Specialized Mechanisms of Variability in Pathogens

• Sexual-like Processes in Fungi

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  • Heterokaryosis

    The most common way that heterokaryotic mycelium is formed is by the anastomosis of somatic hyphae of different genetic constitutions. The foreign nucleus of nuclei introduced into a mycelium multiplies and its progeny spread through the mycelium, rendering the latter heterokaryotic.

   
  • Parasexualism
    1. Formation of heterokaryotic mycelium,
    2. 2.Fusion between two nuclei.
       • a. Fusion between like nuclei.
       • b. Fusion between unlike nuclei.
    3. Multiplication of diploid nuclei side by side with the haploid nuclei.
    4. Occasional mitotic crossing-over during the multiplication of the diploid nuclei.
    5. Sorting out of diploid nuclei.
    6. Occasional haploidization of the diploid nuclei.
    7. Sorting out of new haploid strains.

• Sexual-like Processes in Bacteria

  
  • Transformation

    A condition in which bacterial cells are transformed genetically by absorbing and incorporating in their own cells genetic material secreted by, or released during rupture of other compatible bacteria.

    
     

    

  • Transduction

    A condition in which a bacterial virus (Phage) transfers genetic material from the bacterium in which the phage was produced to the bacterium it infects next.

     

    Conjugation

    

• Genetic Recombination in viruses

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