BIO 5406 Notes, 2/19/01
 

METAMORPHOSIS
 

I.  Introduction.                                                                               [Hadley, pg. 332]

     A.  Metamorphosis  =  A series of abrupt postembryonic changes involving
           structure, function, and ecological niche of an animal.

     B.  Occurs in most insects, fishes, and amphibians.

     C.  Cells in all stages of development contain the same genetic information  ---->
           different sets of genes must be sequentially activated.

           1.  What is the switch that activates a previously inactive set of genes?
 

II.  Anuran (Frog) Metamorphosis.                                            [Hadley, pp. 331-334]

    A.  Transition from aquatic to terrestrial or semi-terrestrial environment.

    B.  Stages.

         1.  Premetamorphosis (weeks or years).
              a.  Tadpole grows with no morphological changes.

        2.  Prometamorphosis (2 - 6 weeks).
             a.  Accelerated growth of hind limbs.

       3. Metamorphic climax (1 week).

            a.  Morphological changes.

                 1.  Forelimbs emerge.
                 2.  Tail is resorbed.
                 3.  Shape of head changes.

            b.  Physiological changes.

                1.  Gills are replaced by lungs.
                2.  Gut shortens (herbivore to carnivore).
                3.  Ocular muscles develop.

          c.  Biochemical changes.

               1.  Change larval hemoglobin (higher O₂ affinity) to adult form.
               2.  Change from ammonia excretion to urea excretion.

     C.  Endocrine control.

           1.  Thyroid hormones.

                a.  1912  --  Friedrich Gudernatsch fed horse thyroid to tadpoles, causing
                     premature metamorphosis.

                b.  TSH, T₃, or T₄ will cause premature metamorphosis.

                c.  T₃ and T₄ levels are low during premetamorphosis and peak during
                     metamorphic climax.

                d.  Thyroidectomy or antithyroid drugs inhibit metamorphosis.

                e.  Hypophysectomy also inhibits metamorphosis.

           2.  Prolactin.

                a.  High in premetamorphosis.

                b.  Prolactin antagonizes thyroid hormones.

                c.  Inhibits metamorphosis.

          3.  Hypothalamic maturation model (or Etkin hypothesis).

               a.  Proposed by William Etkin in 1968.

               b.  During premetamorphosis, hypothalamus is immature.

                    1.  Prolactin secretion is high  ---->  promotes growth.

               c.  Prometamorphosis.

                    1.  Hypothalamus matures.

                         a.  Increased  TRH  ---->  stimulates TSH secretion  ---->
                              increased T₃ and T₄ secretion.

                         b.   Increased dopamine (PIF)  ---->   inhibits prolactin secretion.

                    2.  T₃ and T₄ promote development of the hypothalamohypophysial
                         portal system  ---->  more TRH reaches anterior pituitary  ---->
                         positive feedback loop.

                    3.  Increase in number of nuclear T₃ receptors in target tissues  ---->
                         increased sensitivity to thyroid hormones.

               d.  Metamorphic climax results from high T₃ and T₄ and low prolactin levels.
                    1.  Negative feedback loop begins.
 

III.  Insect Metamorphosis.

     A.  Hemimetabolous vs. holometabolous development.

          1.  Hemimetabolous development.

               a.  Undergo incomplete metamorphosis.
               b.  Examples:  Grasshoppers and cockroaches.
               c.  Young nymph resemble adults in appearance and diet.
               d.  Goes through a series of molts, finally emerging as adult

          2.  Holometabolous development.

               a.  Undergo complete metamorphosis.
               b.  Examples:  Flies, butterflies and moths.
               c.  Larvae do not resemble adults.
               d.  Undergo a series of molts, increasing in size at each instar.
               e.  Pupation  --  quiescent stage, incapable of feeding.
               f.   Pupa undergoes a complete structural reorganization during metamorphosis
                    ---->  emerges as adult.

     B.  Stages represent different functional periods.

          1.  Larval stage  --  eating and growing.
          2.  Pupation  --  overwintering.
          3.  Adult  --  reproducing.

     C.  Ecdysone or molting hormone.

          1.  Chemical structure was determined from extracts of 4 tons of silkworm pupae.

          2.  Steroid hormone.

               a.  Insects are incapable of synthesizing the steroid ring.
               b.  Obtain plant sterols in the diet.

          3.  Produced and secreted by prothoracic gland.

               a.  Located in the first thoracic segment.
               b.  Ecdysone secreted during larval and pupal stages.
               c.  Gland regresses in adult.

          4.  Causes differentiation of tissues in direction of adult form.

               a.  Ecdysone causes swelling of specific regions of chromosomes from
                    midge salivary gland.
               b.  These puffs are areas of increased RNA synthesis.
               c.   First direct evidence that a steroid hormone directs the activity of specific
                    genes.

          5.  Prothoracic gland is stimulated by prothoracotropic hormone (PTTH).

               a.  Kopec's neurohormone.
               b.  Peptide hormone.
               c.  Dependent on an external signal, such as changing environmental
                    temperature.

     D.  Corpora allata.

          1.  Paired glands located just posterior to the brain.

          2.  Experiments.

               a.  Remove corpora allata in third instar larva  ---->   pupates.

               b.  Transplant corpora allata of a young larva into a fifth instar larva  ---->
                    pupation is delayed.

               c.  Conclusion:
 
 

          3.  Secrete juvenile hormone.

               a.  Sort of a fatty acid structure.
               b.  Secretion diminishes progressively with each molt.

     E.  Interaction of ecdysone and juvenile hormone.

          1.  Larval molts stimulated by ecdysone in the presence of juvenile hormone.

          2.  Pupation stimulated by ecdysone in the diminishing presence of juvenile
               hormone.

          3.  Development of pupa into adult depends on ecdysone in the absence of
               juvenile hormone.