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Zorluk: ZorGrowth, Development, and Metamorphosis

Tadpoles of the African bullfrog (*Pyxicephalus adspersus*) were raised in two separate aquatic environments. Environment 1 contained natural pond water, while Environment 2 contained mineral-free water deficient in dissolved iodine salts. The tadpoles in Environment 1 developed limbs, absorbed their tails, and successfully metamorphosed into juvenile frogs. In contrast, the tadpoles in Environment 2 increased in body size as giant tadpoles but failed to develop limbs or reabsorb their tails. Which hormone deficiency accounts for the halted metamorphosis in Environment 2, and which gland synthesizes this hormone?

  1. Thyroxin deficiency, synthesized by the thyroid glandCevap
  2. B
    Juvenile hormone deficiency, synthesized by the corpus allatum
  3. C
    Ecdysone deficiency, synthesized by the prothoracic gland
  4. D
    Oxytocin deficiency, synthesized by the pituitary gland

Cevap

Thyroxin deficiency, synthesized by the thyroid gland
Amphibian metamorphosis is under strict endocrine control mediated by thyroxin, a hormone secreted by the thyroid gland. Thyroxin synthesis requires dietary or environmental iodine. When iodine is absent, thyroxin levels remain insufficient to trigger metamorphic events such as limb development, lung formation, and tail reabsorption, resulting in neotenic or oversized larval tadpoles.

Adım Adım Çözüm

1
Analyze the experimental observation
Tadpoles deprived of iodine failed to undergo structural transformation (metamorphosis) into frogs despite growing larger in size.
Iodine is an indispensable elemental component required for the biochemical synthesis of thyroid hormones.
2
Identify the specific hormone and endocrine gland involved
The thyroid gland requires iodine to produce thyroxin, which regulates gene expression for limb bud growth, lung maturation, and tail resorption.
Without iodine, thyroxin cannot be produced, halting amphibian metamorphosis at the larval stage.
3
Differentiate amphibian hormones from insect metamorphic hormones
Juvenile hormone and ecdysone govern arthropod/insect metamorphosis, whereas thyroxin governs amphibian metamorphosis.
Conflating arthropod endocrine systems with vertebrate amphibian systems is a common conceptual misconception.

Anahtar Kavram

Hormonal Regulation of Amphibian Metamorphosis
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Konular

Growth, Development, and Metamorphosis

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