The species lost the glands that nourished the semen sperm more than 50 million years ago. A study with the participation of researchers from the National Institute for Agricultural and Food Research and Technology, of the Higher Council for Scientific Research (INIA-CSIC), reveals that the sperm of the dolphins had to adapt to allow reproduction in the marine environment.
Unlike their terrestrial relatives, which use glucose as a source of energy, the sperm of the dolphin metabolize fatty acids to allow their motility and acquire the ability to fertilize the egg. The work has been published in the journal 'Current Biology'.
50 million years ago, when some herbivores decided to return to the sea, they had to evolve and change their morphology to adapt to swimming. His metabolism changed drastically by substituting a plant-based diet for a diet rich in fat and protein, based on the consumption of fish.
This transformation contributed to the adaptation to the new conditions of lack of oxygen for long periods of time.
Alfonso Gutiérrez-Adán, one of the study authors and an INIA-CSIC researcher said, by changing the diet of vegetables and polysaccharides of plant origin for proteins and fat, they began to use fatty acids as an energy substrate. The muscles adapted to use fat as an energy source, while glucose was reserved for some specific tissues such as the brain.
In these new conditions, their organs and reproductive strategies also underwent great transformations. Among them, the dolphins lost the seminal glands that produce the seminal fluid that nourishes the sperm in their ejaculate, so the energy source to be able to move and fertilize the oocyte had to be accumulated inside.
The scientist clarified, we have discovered that many of the enzymes of the glycolytic pathway, responsible for metabolizing glucose in the testis, are inactivated in the dolphin. This is because the pathway that sperm use to produce energy and move is oxidative lipid phosphorylation. , which supposes that the species underwent an extraordinary adaptation, essential to reproduce in the new marine conditions.
To reach these conclusions, the INIA-CSIC team analyzed the dolphin sperm and, especially, the glucose or pyruvate requirements for movement, as well as their motility by inactivating the mitochondrial fatty acid beta-oxidation pathway. They also performed metabolomic analyzes to verify their differences with the sperm of terrestrial mammals such as the bull.
Researchers from the Center for Marine and Environmental Research at the University of Porto, responsible for identifying mutations in glycolytic genes, also participated in the study. The Veterinary Faculty of the Complutense University of Madrid (UCM) and the Oceanogràfic of the City of Arts and Sciences of Valencia have also collaborated, which contributed the dolphin sperm samples.
An adaptation of dolphins, but no whales
The cetaceans are divided into two large groups, the odontocetes (toothed cetaceans) and the mysticetos (baleen whales). While the former have teeth, like dolphins and killer whales, the latter have barbs to filter, swallow and expel water through their barbs.
Researchers have noted that the mutations experienced by dolphins have also been observed in other species within the group of odontocetes.
Gutiérrez-Adán said, the change seems essential for their adaptation to the sea and to a diet of proteins and fats. However, the diet of baleen whales is based on krill, small marine crustaceans of various species that are part of plankton and whose composition is rich in a carbohydrate: chitin.
Although it is difficult to collect sperm from these animals, he said, and scientists still do not know much about their metabolism, these mutations in glycolytic genes have not been observed in mysticetes.
In the next phase of the study, the researchers will focus on analyzing the energy source and the strategy used by dolphins in the sperm training process, since understanding the entire sperm adaptation process could serve to apply this knowledge to reproductive biotechnologies of livestock species and humans.