Lecture Notes for Monday, April 8



Please watch this excellent video showing metamorphosis in a frog and in a flatfish:
YouTube video link

Tadpole ----> Frogs & toads

1* Self-destruction of tail - Apoptosis

NOT analogous to cutting off blood supply

2* Formation of legs
also: loss of ability to regenerate legs (?analogy to salamanders?)

3* Change in retina: different light-detecting pigment
Also formation of new optic nerve fibers

4* Movement of eyeballs to more anterior location
Carnivores (e.g. adult frogs) typically have binocular 3-D vision with the eyes at the front of the head, whereas herbivores (tadpoles) have their eyes on the side, which helps protect them from predators.

Dany Adams of Tufts University did experiments in which an eye was grafted onto the tail of a tadpole. When metamorphosis occurred, the eye shifted its location.

5* Formation of lungs

6* Degeneration of gills

7* Changed hemoglobin; different cells blood-producing stem cells for breathing air

8* Kidney changes - adult uses urea instead of ammonia

9* loss of lateral line system

10* Digestive tract herbivorous--> carnivorous (shorter in length)

11* Skin thicker epidermis and dermis


Important that these changes occur in the right sequence:
"Don't destroy your tail until you have formed your new legs"

Threshold theory
steady increase in thyroxine concentration

the lower the threshold, the sooner the stage will occur
(sudden, high concentration will cause everything to happen at once)

maybe an over-simplification


Stimulation of amphibian metamorphosis by thyroxine :
two tyrosines, attached end to end, with 3 or 4 Iodine atoms
(We use this same chemical as a stimulant for metabolism.)

The thyroid gland evolved from a mucus-secreting groove in primitive chordates,
which then became separated from the floor of the throat.
For example, sea squirts
are filter-feeders that blow water out through their gill slits and capture food in the mucus.

Thyroxine is made from mucus inside epithelial spheres; covalent binding of iodine
to tyrosines + binding of pairs of tyrosine.

This polymerization occurs in the mucus inside the epithelial spheres.

? related to evolution of thyroid from the mucus-secreting gland used for filter-feeding by sea squirts
and larval lampreys


Salamanders also have larvae, with legs, that undergo metamorphosis,
in a sub-group of salamanders called newts

(common in this area: more in New England; very poisonous skin)

Larva -> Eft --> Newt


Neoteny   -   Axolotl

Reach sexual maturity and breed when anatomically still larvae.

Treating axolotls with thyroxin can force metamorphosis into something similar to a spotted salamander.

There are also some neotenic species of sea squirts.


Also there are species of frogs and salamanders with "direct development",
where the egg develops directly into an animal with legs. These live in areas where
there isn't enough fresh water for a tadpole stage to survive.


Invertebrate examples of metamorphosis

I) Ammocoetes larva ---> Lamprey

II) Sea Squirt "tadpole" ----> Sea Squirt

III) Pluteus ----> Sea urchin (and star-fish, etc.)

IV) Sponges have a swimming larval form, whose cells then rearrange to form a sponge
(in which cells keep on rearranging forever)

V) Trochophore larvae, Pilidium larva, and several other planktonic larvae

VI) Larvae of barnacles look like shrimp, etc.


VII) Caterpillar ----> ----> ----> Butterfly, or moth

Maggot ----> ----> ----> fly

grasshopper -> grasshopper -> grasshopper -> grasshopper -> grasshopper

silverfish -> silverfish -> silverfish-> -> silverfish -> silverfish


Metamorphosis by molting (shedding outer layer of skin)

All arthropods molt. Some insects have a metamorphic molt.

The new body surface is made by differentiation of imaginal discs.

Hormonal control of arthropod molting

    Ecdysone (a steroid) stimulates molting; found in all arthropods

    Juvenile hormone (a lipid) inhibits metamorphosis

If you keep treating caterpillars with juvenile hormone, they will molt into bigger & bigger caterpillars.

Precor - chemical name methoprene - is an analog of juvenile hormone that eliminates fleas by preventing them from going through metamorphosis to become breeding adults. It is very effective for treating a house that's infested with fleas, and is much less toxic than insecticides or the flea preventatives that are applied to dogs' skins. It isn't usually sold in pet stores etc. but it can be ordered on line. To use it, you mop the floor with a diluted solution, or spray it on carpet, and let it dry. It keeps flea larvae from developing into adults. (A version is also sold that contains an insecticide in addition to the hormone analog.)





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