Physically fit frogs have faster-changing genomes, says a new study of poison frogs from Central and South America.
Stretches
of DNA accumulate changes over time, but the rate at which those
changes build up varies considerably between species, said author Juan
C. Santos of the National Evolutionary Synthesis Center in Durham, North
Carolina.
In
the past, biologists trying to explain why some species have
faster-changing genomes than others have focused on features such as
body size, generation time, fecundity and lifespan. According to one
theory, first proposed in the 1990s, species with higher resting
metabolic rates are likely to accumulate DNA changes at a faster rate,
especially among cold-blooded animals such as frogs, snakes, lizards and
fishes. But subsequent studies failed to find support for the idea.
The
problem with previous tests is that they based their measurements of
metabolism on animals at rest, rather than during normal physical
activity, Santos said.
“Animals
rarely just sit there,” Santos said. “If you go to the wild, you’ll see
animals hunting, reproducing, and running to avoid being eaten. The
energetic cost of these activities is far beyond the minimum amount of
energy an animal needs to function.”
To
test the idea, Santos scoured forests in Colombia, Ecuador, Venezuela,
and Panama in search of poison frogs, subjecting nearly 500
frogs—representing more than 50 species—to a frog fitness test.
He
had the frogs run in a rotating plastic tube resembling a hamster
wheel, and measured their oxygen uptake after four minutes of exercise.
The
friskiest frogs had aerobic capacities that were five times higher than
the most sluggish species, and were able to run longer before they got
tired.
“Physically
fit species are more efficient at extracting oxygen from each breath
and delivering it to working muscles,” Santos said.
To
estimate the rate at which each species’ genome changed over time, he
also reconstructed the poison frog family tree, using DNA sequences from
fifteen frog genes.
When
he estimated the number of mutations, or changes in the DNA, for each
species over time, a clear pattern emerged—athletic frogs tended to have
faster-changing genomes.
Santos
tested for other factors as well, such as body and clutch sizes, but
athletic prowess was the only factor that was consistently correlated
with the pace of evolution.
Why
fit frogs have faster-changing genomes remains a mystery. One
possibility has to do with harmful molecules called free radicals, which
increase in the body as a byproduct of exercise.
During
exercise, the circulatory system provides blood and oxygen to the
tissues that are needed most—the muscles—at the expense of less active
tissues, Santos explained.
When
physical activity has stopped, the rush of blood and oxygen when
circulation is restored to those tissues produces a burst of free
radicals that can cause wear and tear on DNA, eventually causing genetic
changes that—if they affect the DNA of cells that make eggs or
sperm—can be passed to future generations.
Before
you ditch your exercise routine, Santos offers some words of caution.
The results don’t debunk the benefits of regular physical exercise,
which is known to reduce the risk of cancer, heart disease, and
diabetes.
“What
applies to cold-blooded animals such as poison frogs doesn’t
necessarily apply to warm-blooded animals such as humans,” Santos said.
The findings appeared in the April 10th issue of Molecular Biology and Evolution.
Fast molecular evolution associated with high active metabolic rates in poison frogs
