Hydra is a model organism for studying regeneration due to its capability for morphallactic regeneration, where lost parts are replaced primarily by re-patterning of existing tissues. A key component in this process is the establishment and maintenance of the body’s apical-basal axis, involving distinct signaling gradients crucial for guiding regeneration and pattern formation.

The Importance of Wnt/β-catenin Signaling in Hydra

• Wnt/β-catenin pathway orchestrates axial patterning:
  The pathway regulates the formation of the head organizer (hypostome) and postulates a gradient of head-activating signals. The β-catenin protein accumulates in the nuclei of cells in the hypostome, activating Wnt target genes critical for head regeneration.

• Head activation gradient:
  The highest activity of Wnt signaling is localized at the hypostome (apical end), creating a gradient that diminishes along the body axis. This gradient guides where the head and tentacles form during regeneration and development.

• Role in foot formation:
  Opposing gradients influenced by Wnt and other signals regulate basal disc (foot) formation, ensuring polarity along the apical-basal axis.

• Experimental evidence:
  Manipulation of Wnt/β-catenin signaling through inhibitors or activators alters regenerative patterning, demonstrating its pivotal role in regenerating the correct body parts and maintaining polarity.

Why Other Pathways Are Less Prominent

• Retinoic acid pathway is not a major player in hydra axial regeneration but is important in vertebrate limb and craniofacial development.

• FGF pathway is crucial for vertebrate development and regeneration but less characterized in Hydra regeneration.

• Delta-Notch pathway mainly regulates differentiation and patterning in various developmental contexts but is not the central pathway for Hydra axis formation during morphallaxis.

Summary

Morphallaxis in Hydra depends critically on the Wnt/β-catenin signaling pathway to pattern the body axis during regeneration. By establishing gradients of activation, this pathway enables precise head and foot formation, maintaining organismal polarity and facilitating complete regeneration from tissue fragments.

Final Answer:
(1) Wnt/β-catenin pathway