In an experiment, activin-secreting beads were placed on unspecified cells from an early Xenopus embryo. The activin then diffused from the beads. If the beads contained 1nM of activin, it elicited expression of Xbra gene in cells near to the beads. If the beads contained 4nM activin, the expression of Xbra was elicited in cells, but only at a distance of several cell diameters away from the beads. In the latter case, expression of goosecoid gene was observed near the source bead. Beads with no activin did not elicit the expression of the two genes.​

Which of the following statements are correct?

A. High concentration of activin activates goosecoid, whereas lower concentrations activate Xbra
B. Lower concentrations of activin help specify the dorsal-most structures of the frog’s embryo
C. Concentrations of activin that do not lead to expression of the two genes specify the cell to become blood vessels and heart​

​Select the correct answer:​​

Correct option is C

Explanation-

Activin-secreting beads are placed in embryonic cells. Activin diffuses and forms a concentration gradient.
At low activin concentration (1 nM):
→ Xbra (Xenopus Brachyury) is expressed near the bead.
At high activin concentration (4 nM):
→ goosecoid is expressed near the bead.
→ Xbra is expressed farther away, where activin concentration is lower.
No activin: Neither gene is expressed.
This demonstrates that activin acts as a dose-dependent morphogen. Specific genes are turned on at specific thresholds of activin.
Analyzing the statements -
Statement A: High concentration of activin activates goosecoid, whereas lower concentrations activate Xbra. - CORRECT
goosecoid is a dorsal mesodermal marker, typically induced by high concentrations of activin. Xbra is a mesodermal marker as well, but is induced by intermediate to low concentrations. This observation is consistent with classic studies in Xenopus where goosecoid expression occurs near the source, and Xbra is expressed at a distance, matching activin gradient theory.
Statement B: Lower concentrations of activin help specify the dorsal-most structures of the frog’s embryo. - INCORRECT
The dorsal-most structures, such as the notochord and head mesoderm, are induced by high concentrations of activin and are marked by genes like goosecoid. In contrast, lower concentrations of activin induce ventral or lateral mesoderm, not dorsal-most structures. Therefore, this statement misrepresents the gradient-dependent differentiation model.
Statement C: Concentrations of activin that do not lead to expression of the two genes specify the cell to become blood vessels and heart. - CORRECT
​In Xenopus and other vertebrates, very low or absent levels of activin fail to induce mesodermal markers like goosecoid or Xbra. Such cells typically adopt a ventral mesodermal fate, contributing to structures like blood, blood vessels, and heart. This reflects the idea that default fates emerge in the absence of strong morphogen signals.

Final Conclusion:
Correct statements: A and C (Option c)