A survey carried out by the Fisheries Institute (IP-APTA), an agency of the Secretariat of Agriculture and Supply of the State of São Paulo, showed that trout kept at temperatures slightly higher than usual during the stage of sexual development were able to transmit thermal tolerance to the next generation of fish. The change, however, caused reproductive damage to males, leading to a decrease in the total number of viable offspring. Due to its relevance and originality, the study resulted in publication in the scientific journal Scientific Reports, linked to the prestigious nature.
“This work sought to emulate the effects of global warming on rainbow trout, under climatic conditions in Brazil”, explains IP researcher Neuza Takahashi. As he reveals, this was done by keeping the fish for 3 months in a 4ºC warmer than usual habitat, while they were still juveniles – a central stage in the formation of the individual’s sexual organs. After this period, the fish returned to normal temperature conditions for another 15 months, until they completed their sexual maturation. “Since reproduction is one of the essential steps for the perpetuation of the species, we tested the effect of temperature rise on reproductive parameters”, details Neuza.
The results found drew attention – and worried scientists. “This study demonstrated, in an unprecedented way, that heat stress in juvenile male rainbow trout causes deleterious effects on germ cells that persist into adulthood,” says Arno Butzge, a researcher who conducted the experiments at IP during his doctorate at Unesp-Botucatu, under the guidance of Claudio de Oliveira and Ricardo Hattori and support from FAPESP.
In other words, keeping the trout in warmer water than usual, even for a few months, was enough to make the adult males functionally sterile (they did not reach sexual maturation), or permanently less fertile (significant reduction in the amount of semen produced). “In addition to affecting testicular development, fecundity, sperm motility and morphology, the embryonic development of the progeny (the descendants) was also compromised”, explains Butzge. In the case of females, there were no significant changes.
Clearly, these early findings were not very encouraging for the future of trout. In an expected scenario of increasing global temperatures, a lower reproductive capacity of males could compromise not only the trout farming activity, but also the perpetuation of the species. However, like a light at the end of the tunnel, IP’s work ended up bringing an unexpected – and intriguing – discovery.
“Surprisingly, these negative effects could be offset by a gain in thermotolerance in the offspring after a single generation,” says Butzge. That is, although males “stressed” by the high temperature generate fewer offspring and with a higher incidence of deformities, those who were born healthy managed to acquire tolerance to warmer water through their parents. “Nature favored surviving males, when breeding adults, to generate offspring with a greater environmental thermal tolerance, ensuring better survival and growth for their offspring when subjected to the same stressful conditions imposed on their parents”, says Neuza.
As experts explain, in a jump, the children became more heat tolerant, only by the father’s exposure to warmer water when young. This foreshadows, according to the authors of the work, the occurrence of a biological phenomenon that is on the frontier of Science – epigenetics – in which a significant stress during the life of the parents is marked in the genetic inheritance by future generations, without any changes in the DNA – contrary to what is seen in conventional genetics.
According to the researchers, salmonid fish, such as trout, are typical of cold waters, and therefore the rise in temperature caused by climate change is a real threat to the continuity of the species (many have even been extinct or are under threat). In addition to the ecological damage, experts argue that this would lead to a decrease in the access of various human populations to fishery resources. “Considering the imminent increase in global average temperatures, there is a concern in terms of food security, due to the risks for fish production and fish farming, especially for species more susceptible to high temperatures”, warns Butzge.
In this scenario, although there is no guarantee that the future of trout is assured, the new research provides some indications of how we can help in their preservation. “This process that we have studied, developed in an ethical manner, makes it possible to prepare strains of fish with better tolerance to temperature changes, allowing the species to continue in the face of vital damage due to global warming”, points out Neuza.
So, if, on the one hand, the work of the IP raises a warning signal, on the other hand, it brings some hope. “These results have implications both for aquaculture, since this strategy can be used to improve tolerance to high temperatures in a much shorter period, and from an ecological point of view”, believes Butzge. “If fish with acquired thermal tolerance are able to reproduce and generate viable offspring, this process can be extremely beneficial to improve their adaptive capacity and withstand global climate change”, concludes the expert.
03 Setembro 2021