Can DNA methylation explain the mysterious metamorphosis of tuna?

New Zealand’s freshwater eels, or tuna, are enigmatic animals. After spending most of their lives wallowing happily in rivers, or awa, they travel thousands of kilometres downstream into the ocean to spawn. To prepare for this, they undergo a dramatic transformation, known as silvering, during which their bodies are altered to tolerate saltwater. After centuries of study, we still don’t know what triggers silvering or other major life transitions in tuna. Why do some eels transition after 15 years, while others wait for 50? 

Dr Sugrue has received a Marsden Fund Fast-Start grant to investigate whether silvering is triggered by DNA methylation – chemically tagging of DNA with methyl groups. DNA methylation profiles can vary over time, in response to internal and external cues. Changes in DNA methylation can turn genes on or off, leading to shifts in an organism’s morphology, physiology, or behaviour. 

Dr Sugrue will compare methylation patterns in the genomes of tuna at different life stages and look for regions that change during silvering. She will also develop the world’s first “epigenetic clock” for tuna, based on methylation data. This will allow assessment of the age and health of tuna without harming them. 

Tuna are taonga species for Aotearoa New Zealand, with deep cultural and ecological significance.  This research will advance our fundamental understanding of metamorphosis and ageing. It will also provide an innovative method for monitoring populations ethically and effectively, addressing a critical barrier to conservation of tuna, and to sustainable management of waterways and fisheries.

Otoliths (ear bones) mounted on a slide. Otolith rings are counted to estimate the eel's age (photo credit: Duncan Coutts)