26 Feb 2019

CIIMAR unravels the genetic basis of whale and dolphins "special" sleep

A team of researchers CIIMAR, makes significant progress in understanding the unique sleep behavior of whales, dolphins and sea cows. Using comparative genomic analysis, the portuguese team was able to deduce that the vigilant behavior and uni-hemispheric sleep characteristic of whales, dolphins and sea cows are due, partially, to the loss of genes involved in the production and signaling of a hormone called melatonin . A striking example of convergent evolution.



Melatonin is a hormone with a remarkable biological importance. It participates, among others, in the physiological control of several circadian (daily) rhythms as the wake-sleep cycles in mammals. Melatonin is produced in the pineal gland in a very specific pattern and at particularly high levels at night. However there are some exceptions that stand out as for some aquatic mammals like whales, dolphins and also in sea cows. These animals exhibit asymmetrical sleep. Interestingly, the pineal gland that typically produces melatonin has been suggested as being vestigial or even absent in several species of cetaceans and in sea cows. Despite this, melatonin has already been measured and found in the blood of dolphins. To understand this intricate biological puzzle was the objective of this investigation of the group of researchers from CIIMAR.


The study now published in the journal Genes investigated for the first time the endogenous ability of cetaceans and sea cows to biosynthesize and signal the hormone melatonin. Through the comparative analysis of the genome and bioinformatic comparisons of the genes involved in melatonin biosynthesis, the team was able to determine that the melatonin-producing enzymes as well as the receptors capable of translating the biological effects of this hormone are not functional in cetaceans and sea cows. Interestingly, since whales and dolphins do not share a direct ancestor with sea cows, it is also possible to "infer that this solution - to lose genes involved in the synthesis and signaling of melatonin - has evolved in a convergent and independent way in these two lineages of aquatic mammals! ", probably driven by the pressures of adaptation to the ecosystems in which they live.


It remains to clarify the last piece of the puzzle: if they do not produce melatonin how was this hormone measured in the blood of dolphins? The study suggests that that food is the most obvious source for the levels found.