Excuse me, which way is the bank?

Ever wondered how fish find their way in the vastness of our seas, lakes and rivers? Researchers from one of the Max Planck Institutes and the European Synchrotron Radiation Facility (ESRF) have found the answer.

Scientists from Max Planck Institute for Developmental Biology in Germany and the French-based ESRF teamed up to study how a single gene in fish influences the way they orientate in their watery home. The secret, which made front-page news in Science magazine, is what the research team call the “biomineralisation of otoliths”.

These stone-like otoliths are located in the fish’s ear hole. Just like humans, fish have an inner ear which is critical not just in hearing and sensing movement, but also in orientation. The newly discovered gene has been given the name ‘starmaker’ in honour of its role in creating star-shaped otoliths.

Christian Riekel, one of the researchers and co-author of the Science article ‘Control of crystal size and lattice formation by starmaker in otolith biomineralisation’ (Vol. 302), said this is an important breakthrough for all scientists working in the field of biomineralisation. “The final aim [of this work] is to develop artificial materials based on information from biological systems – that is, to mimic nature,” he explained.

Crystalline materials formed in living creatures are often different in make-up and physical appearance than their synthetic cousins. Although these materials have the same basic structure, they can differ in shape and strength because the organism controls their crystal morphology. Teeth, bones, kidney stones and mussels are all examples of biomineralisation. If scientists knew how to mimic nature, biologically inspired materials could be used in fields such as the development of replacement teeth, bones and even tissues.

How Nemo finds his way

Testing a theory, the scientists first had to reduce the starmaker gene’s activity – and its encoded protein – in order to see how it influenced the otolith formation in the distinctly marked zebra fish. From this modification, the team observed that the normally smooth and round otoliths became elaborate, star-shaped crystals.

Synchrotron radiation experiments carried out at the ESRF in France showed that this change is also associated with a change in calcium carbonate crystal structure. Most of the fish with the altered gene had difficulty orienting themselves quickly in fast-moving water.

Using x-rays and crystals, the team performed a highly specialised form of diffraction on different types of otoliths of less than 30 microns in diameter. The goal was to determine which crystal polymorph was present in each type of otolith. By modifying one single gene, they detected a change of the calcium carbonate polymorphs at room temperature.

Learning more about this gene offers insight into the human equivalent responsible for hearing loss and teeth formation; dentin sialophosphoportein or DSPP. “The mutation of the DSPP protein is probably involved in biomineralisation of teeth in a similar way that starmaker mediates crystal formation in otoliths,” the principal investigator, Teresa Nicolson is quoted as saying.