Extreme environments select for performance and tolerance traits that are vital for population fitness. Biologists have always been fascinated by how animals perform in their natural environment and how this relates to physiological mechanisms, with the hope that knowledge gleaned from such studies will help us to protect endangered species and maintain biodiversity in this era of global climate change. With a view to understanding how environments select physiological traits and the associated molecular and genomic mechanisms, Zhongqi Chen and colleagues from the University of British Columbia, Canada, and the Columbia River Inter-Tribal Fish Commission, USA, studied how the redband trout, a subspecies of rainbow trout, adapts to a desert stream and explored the evolutionary limit of the fish's thermal adaptation.

The researchers first collected newly emerged (3 months old) redband trout fry from Little Jack Creek (hot desert climate), Keithley Creek (cool mountain climate) and Fawn Creek (cold mountain climate) in southern Idaho, USA. Having allowed the fish to adapt to their new lab home for at least 6 weeks, the team measured their physiological performance over a range of acclimation temperatures (12–24°C). The researchers discovered that the desert population had the best thermal tolerance, the highest maximum heart rate during acute warming and maintained aerobic capacity above 24°C. However, the difference between maximum and minimum aerobic metabolism (known as the aerobic scope) – which is the maximum amount of oxygen available for any aerobic activity above basic maintenance needs – of the two mountain populations collapsed by nearly 50% beyond 24°C. Therefore, the ability of the cardiovascular system to deliver oxygen to tissues appears to be crucial for redband trout when living in a desert stream.

The team then explored which molecular mechanisms responded to an acute warming and contributed to the outstanding physiological function of the desert redband trout population. Collecting samples of the heart tissue at 15°C, 20°C and the temperature at which the heart began to fail (24–25°C), and minute clips from the fins to identify genes that responded to the higher temperatures, the team discovered that the expression of genes associated with cardiac function, energy production pathways and protection from stress were upregulated. To determine whether these candidate genes contributed to the desert fish's ability to tolerate heat, the researchers calculated upper thermal tolerance values, based on the gene expression patterns that they had measured in the desert trout, and found that they were very similar to the measured values. Thus, these genes likely contribute to the sustained cardiac function and aerobic capacity that support the fish's upper thermal tolerance, which is essential for the population to thrive in a hot desert stream.

Biologists are definitely in an exciting time now that they can reveal the molecular mechanisms of thermal adaption. This study by Chen and colleagues demonstrates one survival strategy used by the Little Jack Creek redband trout population in a desert stream: they maintain cardiac function and aerobic capacity in a high temperature environment. However, it is likely that there are many other species out there with their own unique responses to their particular conditions awaiting the attention of intrigued biologists.

Chen
,
Z.
,
Farrell
,
A. P.
,
Matala
,
A.
and
Narum
,
S. R.
(
2018
).
Mechanisms of thermal adaptation and evolutionary potential of conspecific populations to changing environments.
Molecular Ecology.