Embryology Biology 441 Vertebrate Embryology, Spring 2015
Metamorphosis
Tadpole ----> Frogs & toads
1* Self-destruction of tail - Apoptosis
Tail onto which the eye was grafted
The eye remains intact, and shifts to the location where the tail had been
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
binocular vision 3-D predator
5* Formation of lungs
7* Degeneration of gills
8* Changed hemoglobin; different cells blood-producing stem cells
for breathing air
9* Kidney changes - adult uses urea instead of ammonia
10* loss of lateral line system
11* Digestive tract herbivorous--> carnivorous (shorter in length)
12* Skin thicker epidermis and dermis
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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.
Gilbert regards as an over-simplification
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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,
which then became separated from the floor of the throat
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
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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
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Neoteny - Axolotl
Reach sexual maturity and breed when anatomically still larvae.
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Also there are species of frogs and salamanders with
"direct development"
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Invertebrate examples of metamorphosis
II) Ammocoetes larva ---> Lamprey
III) Sea Squirt "tadpole" ----> Sea Squirt
IV) Pluteus ----> Sea urchin (and star-fish, etc.)
V) Sponges have a swimming larval form, whose cells then rearrange to form a sponge (in which cells keep on rearranging forever)
VI) Trochophore larvae, Pilidium larva, and several other planktonic larvae
VII) Larvae of barnacles look like shrimp, etc.
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VIII) 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
If you keep treating caterpillars with juvenile hormone, they will molt into bigger & bigger caterpillars.
Precor - chemical name methoprene - analog of juvenile hormone
Eliminates fleas by preventing them from going through metamorphosis to become breeding adults
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