Adaptive evolution and inherent tolerance to extreme thermal environments

Abstract

Background: When introduced to novel environments, the ability for a species to survive and rapidly proliferate corresponds with its adaptive potential. Of the many factors that can yield an environment inhospitable to foreign species, phenotypic response to variation in the thermal climate has been observed within a wide variety of species. Experimental evolution studies using bacteriophage model systems have been able to elucidate mutations, which may correspond with the ability of phage to survive modest increases/decreases in the temperature of their environment.

Results: Phage PhiX174 was subjected to both elevated (50 degrees C) and extreme (70 degrees C+) temperatures for anywhere from a few hours to days. While no decline in the phage's fitness was detected when it was exposed to 50 degrees C for a few hours, more extreme temperatures significantly impaired the phage; isolates that survived these heat treatments included the acquisition of several mutations within structural genes. As was expected, long-term treatment of elevated and extreme temperatures, ranging from 50-75 degrees C, reduced the survival rate even more. Isolates which survived the initial treatment at 70 degrees C for 24 or 48 hours exhibited a significantly greater tolerance to subsequent heat treatments.

Conclusions: Using the model organism PhiX174, we have been able to study adaptive evolution on the molecular level under extreme thermal changes in the environment, which to-date had yet to be thoroughly examined. Under both acute and extended thermal selection, we were able to observe mutations that occurred in response to excessive external pressures independent of concurrently evolving hosts. Even though its host cannot tolerate extreme temperatures such as the ones tested here, this study confirms that PhiX174 is capable of survival.

Figure 1

Experimental design for short-term treatment of phage lysate to elevated temperatures. Each virus lysate tube was submerged in water and treated at either 50°C or 70°C for 1, 2, 3, or 4 hours, after which it was plated with E. coli C.

Figure 2

The average survival rates and standard error of the mean for the 70°C treatments.

Figure 3

Experimental design for long-term treatment of phage lysate to elevated temperatures. Each virus lysate tube was submerged in water and heated, after which it was either entirely plated with E. coli C or an aliquot was plated and the remaining lysate was used for subsequent heating. In the case of the former, the plate was harvested after overnight incubation and used for subsequent treatments (as indicated in the figure by the gray dashed line).

Figure 4

The survival rate of Line A and Line B isolates throughout extended thermal selection. This rate was calculated based upon the PFU values at time 0 (ancestral control lines) and the number of PFU after each treatment. The PFU values measured for treatment 4 (open square and triangle for Line A and B respectively) are approximated values due to the less than exact amounts of treated solution available for plating. The asterisks are the treatments in which the phage lysate was collected from the previous treatment's plating and incubation at 37°C for 24 hours.

Figure 5

Nonsynonymous mutations observed in the ΦX174 genome due to elevated temperature conditions. Those denoted in black are from other studies [16,17,19,20]; those denoted in blue are from the acute thermal selection experiments presented here; and those in gold are from the extended thermal selection isolates.