Love is in the Air (or Water)

(November 14th, 2014) Not only human males put on the best perfumes if they want to impress the ladies, tilapia fish pursue the same strategy. Portuguese and German researchers identified and characterised one of the fish’s pheromones.

Think of your partner. What made you talk to each other for the first time? While there are no strict laws of attraction between humans, there are apparently strict laws of mating between tilapia fish. Tina Keller-Costa’s team from Portugal and Yoko Nakamura’s team from Germany, discovered that a specific molecule – 17,20β-P - prepares female Mozambique tilapia fish (Oreochromis mossambicus) for spawning.

The molecule, which was identified in male urine, is a steroid. Steroids are small compounds made mainly of carbon and hydrogen atoms. 17,20β-P is also a pheromone because it acts outside the fish’s body, in contrast to hormones that act inside the organism. The Eureka moment for Tina Keller-Costa, from the Centre of Marine Sciences at the University of Algarve in Portugal, came when the team discovered “what type of steroid it is that the males are releasing”. This steroid exists as two epimers: 17,20α-P and 17,20β-P. Epimers are the same molecules but with a different orientation of atoms in space. “A major aim for the future is the identification of the olfactory receptor(s) that bind(s) the identified steroid glucuronates. This will be novel and challenging since no fish pheromone receptor has as yet been identified,” says Keller-Costa.

When the male fish urinates, it releases the two pheromone epimers into the water and the female fish detects them as an odour. This ‘perfume’ wakes up her endocrine system to induce egg maturation. One could say that tilapia males have the magic power over females; however: only dominant males are selected by females. You might ask why? It’s because dominant males have larger bladders, with thicker muscular walls, which can accommodate more urine and release more pheromone into the water.

Another important factor to consider is: “as the urine is released to the water it is diluted” and so is the pheromone concentration. When both a dominant and a subordinate male urinate, the female will smell only the pheromones of the dominant male. Thus, females distinguish between healthier dominant males with better quality sperm, compared to weaker subordinate males with potentially lower quality sperm. This is a great example of how the social status of an individual determines evolution of that particular species in nature.

Ultraperformance liquid chromatography coupled to high-resolution mass spectrometry was used to identify the driving molecule for female priming and proton-nuclear magnetic resonance to identify the precise molecular structure of the pheromone. This high-tech equipment, together with complex statistical analyses strengthens the reliability of the findings.

Tilapia fish are a popular food choice in human diet, however their invasive behaviour damages the ecosystems they inhabit. Understanding Tilapia reproduction mechanisms will prove essential in Tilapia population control. The authors conclude that ‘The chemical identification of a sex pheromone in male tilapia urine that primes the female reproductive system and possibly promotes spawning synchrony will stimulate further research into chemical communication and behaviour, in particular how different sensory information is integrated.’

Nadejda Capatina

Picture: Fotolia/Sean Gladwell

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