[Campbell Biology P.1112] The experiment in Figure 47.26 involved transplanting ZPA tissue fr... | Practice Question

The experiment in Figure 47.26 involved transplanting ZPA tissue from a donor chick embryo to the anterior margin of a host limb bud. What was the most significant result observed, and what did it suggest about the ZPA's function?

  • A: The host limb bud developed normally, indicating ZPA only functions in its original posterior location.
  • B: The host limb bud failed to develop, suggesting ZPA is crucial for initiating limb formation.
  • C: The host limb bud developed extra digits in a mirror-image arrangement, suggesting ZPA secretes a diffusible signal that specifies posterior structures based on concentration.
  • D: The host limb bud developed into a hindlimb instead of a forelimb, indicating ZPA determines limb identity.

Explanation

The 'Results' section of Figure 47.26 explicitly states: 'The host limb bud developed extra digits from host tissue in a mirror-image arrangement to the normal digits.' The 'Conclusion' further explains this, stating that 'the mirror-image duplication observed in this experiment suggests that ZPA cells secrete a signal that diffuses from its source and conveys positional information indicating “posterior.” As the distance from the ZPA increases, the signal concentration decreases, and hence more anterior digits develop.' This directly supports option C.