A tadpole larva of the ascidian Ciona. Image credit: Hozumi et al. (CC BY 4.0)
Ascidians (also known as sea squirts) are small marine animals. Ascidian larvae have tadpole-like bodies and swim by beating their tail, but the adults have a vase-like shape, no tail and are attached to rocks or other solid surfaces, unable to move. The process by which an ascidian larva transforms into an adult, known as metamorphosis, starts when the larvae stick to a surface using parts of their body known as adhesive papillae.
A group of ascidians known as Ciona are widely studied due to the availability of molecular biology methods for understanding their genetics. Previous studies have shown that Ciona larvae must remain stuck to a surface for approximately 30 minutes to trigger metamorphosis. This time gap between attaching to a surface and metamorphosis is thought to be a system for ensuring an ascidian is attached securely to a suitable surface before it loses its ability to move. However, it remains unclear how Ciona can measure time in this situation.
To address this question, Hozumi, Totsuka et al. combined genetics, pharmacology and live imaging approaches to study Ciona larvae as they prepared for metamorphosis. The experiments found that Ciona larvae made a molecule known as cyclic adenosine monophosphate (or cAMP, for short) in their adhesive papillae once they had attached to a surface. Over time, the levels of cAMP in the papillae gradually increased, providing the larvae remained attached. When the levels of cAMP reached a threshold, Ciona initiated metamorphosis.
These findings demonstrate that Ciona larvae use cAMP levels as a timer to measure how long they have been attached to a surface. This molecule is found in all living organisms; therefore, other animals may use similar mechanisms to measure time. Researchers need to be able to artificially control the timing of biological events to efficiently culture cells and tissues in the laboratory. Therefore, the genes that regulate cAMP production in Ciona and other organisms may be useful targets for developing new methods for growing organisms in the laboratory.
