History and Symetries in Evolution and Development

• Important cellular symetries changed during evolution:

  
  - JGK_6379 Garcia-Bellido (1996) On biological symetry (pdf available)
  • Molecular recognition provides a profound inertia to innovation
    • D-isomer sugar dominance in biological systems (Pasteur).
      - corresponding asymetry of enzyme active site
      - based on evolutionary descent of metabolic enzymes by gene duplication
      
    • L-isomer prevalence of phospholipids
    • L- dominance over D-amino acids
      - shape of proteins is forever based on the alpha-helical structure of L-aa's
      - D-amino acids are constrained to appear only through dedicated enzyme pathways
  • Primary helicity of DNA is based on its mode of synthesis but can be reversed and sequence is unconstrained.
  • DNA and RNA palindromic sequences can mutate but selective forces on RNA hairpin-loop structures constrain the RNA and thus the DNA sequences (tRNA, rRNA conserved structure)
    - secondary and tertiary stucture of RNA and DNA provide binding sites for novel polypeptides and polynucleotides that have frozen the structures of the DNA and RNA polymers
  • Cell Membranes provide the universal polarity of inside vs outside
    • Inside and Out of cells, organelles (mitochondria, ER, nuclear membranes)
      - polarity problems, once solved, are solved for all posterity
        - secretion, cytolysosomes and tissue involution, gastrulation
  • Polarity of cells produced by cytoskeletal proteins
    - bacterial flagellum at one end of bacterial cell
    - budding region in fungi and yeast
    - based on asymetries of structural proteins like actin, myosin, tubulin
    - original polarities guide subsequent ones
    - basis for original polarity may be historical
  • Histological asymetry n epithelia develops with multicellularity
    - two cell organisms have a basal-apical polarity
    - flattened (planula) organisms have an A/P, D/V and Left/Right asymetry
      - revealed by transplantation and polar coordinate model phenomena
  • Cell differentiation of organules constrained by their common plan
    - insect glands, sensory bristles, omatidia, all have common origin.
  • Arthropod limbs are outgrowths of the already polarized body epithelium
    - anterior, posterior, dorsal/ventral, proximal/distal (L/R or R/L) are inherited
  • Bilateral or Helical organisms are motile - Radial are sedentary
    - medusa/polyp dominated/constrained by sessile polyp
    - radial cleavage gives rise to metamerism along a circumference (polyp)
    - orthogonal cleavage gives rise to bilateral organisms
    - helical symetry (moluscs) obtained from a modified radial cleavage.
    - secondary impositions or disruptions of polarity:
      - flatfish, insect genitalia (lock and key)

  
  

• Generative or Operational Symmetries

  • Directional reactions are at the root of biological syntheses
  • How was the DNA -> RNA -> Protein directional axis evolved?
    • RNA catalysis of RNA synthesis and splicing
      - results in more of the same.
    • Reverse transcription evolution
    • DNA -> RNA -> Protein informational axis solidified
      - Is this latter switch of temporal polarity heterochrony?
  • Introduction of mutation
  • Duplication and experimentation
  • Metamerism (physical duplication and experimentation)
  • Uniblastic to diploblastic to triploblastic organisms
  • The homologies of genes are part of the invisible evolution of pathways
    - insect storage proteins are decendents of hemocyanins of arthropods
    - or vice versa?
  • The evolution across several levels of complexity is due to a limited number of successful molecules or motifs:
    - Vitellogenin (LDL, arthropod fibrinogen); Yolk Protein (lipase); Hemocyanin (Hexamerin{loss of Cu⁺³ and O₂ binding}; tyrosinase{oxidase})
    - Zn finger, HMG-domain,
    - myoglobin, embryonic- , fetal-, adult- hemoglobins (+ pseudogenes)
    - EF-hand calcium binding proteins (calmodulin, troponin-C)
    - Immunoglobin family
    - mRNA differential/alternative splicing