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Polyomaviruses are a family of DNA tumor viruses that are known to infect mammals and birds. To investigate the deeper evolutionary history of the family, we used a combination of viral metagenomics, bioinformatics, and structural modeling approaches to identify and characterize polyomavirus sequences associated with fish and arthropods. Analyses drawing upon the divergent new sequences indicate that polyomaviruses have been gradually co-evolving with their animal hosts for at least half a billion years. Phylogenetic analyses of individual polyomavirus genes suggest that some modern polyomavirus species arose after ancient recombination events involving distantly related polyomavirus lineages. The improved evolutionary model provides a useful platform for developing a more accurate taxonomic classification system for the viral family Polyomaviridae.

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SUMMARY Antarctic fish Pagothenia borchgrevinki in McMurdo Sound,Antarctica, inhabit one of the coldest and most thermally stable of all environments. Sea temperatures under the sea ice in this region remain a fairly constant –1.86°C year round. This study examined the thermal plasticity of cardiac function in P. borchgrevinki to determine whether specialisation to stable low temperatures has led to the loss of the ability to acclimate physiological function. Fish were acclimated to–1°C and 4°C for 4–5 weeks and cardiac output was measured at rest and after exhaustive exercise in fish acutely transferred from their acclimation temperature to –1, 2, 4, 6 and 8°C. In the–1°C acclimated fish, the factorial scope for cardiac output was greatest at –1°C and decreased with increasing temperature. Increases in cardiac output with exercise in the –1°C acclimated fish was achieved by increases in both heart rate and stroke volume. With acclimation to 4°C, resting cardiac output was thermally independent across the test temperatures; furthermore, factorial scope for cardiac output was maintained at 4, 6 and 8°C, demonstrating thermal compensation of cardiac function at the higher temperatures. This was at the expense of cardiac function at –1°C, where there was a significant decrease in factorial scope for cardiac output in the 4°C acclimated fish. Increases in cardiac output with exercise in the 4°C acclimated fish at the higher temperatures was achieved by changes in heart rate alone, with stroke volume not varying between rest and exercise. The thermal compensation of cardiac function in P. borchgrevinki at higher temperatures was the result of a change in pumping strategy from a mixed inotropic/chronotropic modulated heart in –1°C acclimated fish at low temperatures to a purely chronotropic modulated heart in the 4°C acclimated fish at higher temperatures. In spite of living in a highly stenothermal cold environment, P. borchgrevinki demonstrated the capacity to thermally acclimate cardiac function to elevated temperatures, thereby allowing the maintenance of factorial scope and the support of aerobic swimming at higher temperatures.

Specialization to a particular environment is one of the main factors used to explain species distributions. Antarctic fishes are often cited as a classic example to illustrate the specialization process and are regarded as the archetypal stenotherms. Here we show that the Antarctic fish Pagothenia borchgrevinki has retained the capacity to compensate for chronic temperature change. By displaying astounding plasticity in cardiovascular response and metabolic control, the fishes maintained locomotory performance at elevated temperatures. Our falsification of the specialization paradigm indicates that the effect of climate change on species distribution and extinction may be overestimated by current models of global warming.

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