Ecdysteroid-dependent molting in tardigrades.

Area of Science:

• Developmental Biology
• Evolutionary Biology
• Zoology

Background:

• Molting is a key characteristic of Ecdysozoa, but its evolutionary origins are not fully understood.
• In insects and crustaceans, molting is controlled by the ecdysteroid (Ecd) hormone.
• The presence and role of Ecd signaling in molting outside of arthropods remain unclear.

Purpose of the Study:

• To investigate the role of the ecdysteroid (Ecd) hormone in the molting process of tardigrades.
• To determine if Ecd-dependent molting is an ancestral trait in panarthropods.
• To identify the neural regulation of molting in tardigrades.

Main Methods:

• Measured endogenous Ecd levels during the molting cycle of *Hypsibius exemplaris*.
• Administered exogenous Ecd and an Ecd receptor antagonist to observe effects on molting.
• Performed spatial and temporal gene expression analysis to identify regulatory pathways.

Main Results:

• Endogenous Ecd levels in *H. exemplaris* increase periodically during molting.
• Exogenous Ecd induced molting, while receptor antagonists suppressed it.
• Identified putative Ecd regulatory organs, downstream cascades, and a neural center for molting.

Conclusions:

• Tardigrade molting is regulated by the ecdysteroid (Ecd) hormone, supporting its ancestral role in panarthropods.
• The identified neural center may be homologous to that in pancrustaceans, representing an ancestral state.
• Ecd-dependent molting likely evolved earlier than previously thought, in the Ediacaran period.