Five people are walking through a field. They all move with agility along the flat plains, easily withstand the dewey grass and feel comfortable in the 60° F weather. But suddenly, natural selection appears in front of them in the form of...a fiery pit of liquid magma. In order to reproduce, they must cross the pit. Who will be the evolutionary victor?
Well, obviously it's the one who has a genetic resistance to extreme temperatures.
Unfortunately for the doomed travelers, humans don't tend to rapidly develop such beneficial mutations. Bacteria may fare better. According to an article recently published in Nature Genetics, bacteria obtain genetic material through horizontal transfer—their equivalent of sex—which allows them to adapt to novel environments. The study estimates that about 25 of E. coli's 900 metabolic genes were gained via this method, over the past 100 million years.
Advertisement
"It is the first time we understand how bacteria change their complete metabolism over evolutionary time," according to lead researcher Martin Lercher of the University of Bath.
Horizontal transfer—where genes are transmitted between contemporaries instead of via reproduction—is a common method of gene transfer, best illustrated by erroneous viral infection.
"If a virus attacks a bacterium, it multiplies its own genome inside the bacterial cell, packs it into a shell, and then destroys the bacterium to get out, in order to infect new bacteria," Lercher said via e-mail. "Sometimes the virus accidentally packs not its own genome, but part of the genome from the bacterial host, into a shell."
If this shell infects another bacterium, the genes from the first host can be integrated into the next. The second bacterium may then include genes from an entirely different species; these genes can provide new metabolic functions.
Bacteria don't gain the new genes with the express purpose of environmental versatility, but by accident. It's only when the bacteria are faced with a new environment—maybe they move from the ground into your intestines—that the new genes may provide a critical evolutionary advantage. Even if only a few bacteria win out, the species can survive.
"That's the advantage of bacteria: There are so many of them that if each tries out something new from time to time, together they can try a lot and evolve," Lercher said.
The researchers looked at over 50 fully-sequenced strains of bacteria, all related to E. coli. They then constructed an evolutionary family tree of these species and observed the distribution of each gene across the tree, inferring when it was lost by mutation or gained by horizontal transfer. The researchers focused on the part of the tree linking E. coli and salmonella, which diverged from a common ancestor about 100 million years ago. Lercher said anywhere from 15 to 32 metabolic genes were gained since the split from Salmonella.
A bacteria's adaptability to new environments could be exploited by engineers, Lercher said. "Extensions of the work will look at individual functions—e.g., the ability to chew up oil spils in the sea—and predict which bacteria are best suited to adapt to that new function and which genes it would need to do so."