ScienceCast: Life History, Mutation Rates, and Male Mutation Bias

For the first time, scientists have used large-scale
DNA sequencing data to investigate a long-standing evolutionary assumption: DNA mutation rates
are influenced by a set of species-specific life-history traits. These traits include
metabolic rate and the interval of time between an individual’s birth and the birth of its
offspring, known as generation time. The team of researchers led by Kateryna Makova, a Penn
State University associate professor of biology, used whole-genome sequence data to test life-history
hypotheses for 32 mammalian species, including humans. To find correlations between life history
and mutation rates, the team first focused on generation time. The more generations a
species has per unit of time, the more chances there are for something to go wrong; that
is, for mutations or changes in the DNA sequence to occur. For example, for mice, every 100
years equates to about 200 generations, whereas for humans, there are only five generations
every 100 years. After comparing 32 mammalian species, Makova’s team found that the strongest,
most significant life-history indicator of mutation rate was, in fact, the average time
between a species member’s birth and the birth of its first offspring. Makova’s team also found that generation time
affects male mutation bias — a higher rate of DNA mutation in the male sperm versus the
female egg. “It has been known for a while that the mutation
rates are higher in males than in females and this can be explained by the difference
in the number of cell divisions. There are many more cell divisions that lead to the
formation of a sperm as compared to just a few that lead to the formation of an egg.” The second life-history trait that Makova’s
team identified was metabolic rate — the amount of energy expended by an animal daily
— and how it correlates with genetic mutations. Some of the team’s 32 test species, such as
rodents, fell into the high-metabolism category, while elephants fell into the low-metabolism
category. Previous researchers had hypothesized that sperm cells should be more affected than
egg cells by a higher metabolic rate. A sperm cell is very active and constantly moving,
and, in addition, its cell membrane is not very dense. The combination of high energy
and meager protection leaves sperm cells more susceptible to bombardment by free radicals,
which can increase mutations. The hypothesis is that a high metabolism greatly increases
this already volatile situation, especially for sperm. Makova’s team found that, unlike
generation time, metabolic rate appeared to be only a moderate predictor of mutation rates
and of male mutation bias. However, the team members suspect that further studies may provide
stronger evidence that metabolic rate exerts an important influence on mutation rates and
male mutation bias. “Our study has lots of interesting implications.
One is that just by sequencing the genome of an ancient species for which we don’t have
any information about the life-history traits we can predict those life-history traits just
by comparing the mutation rates between sex chromosomes and the other chromosomes within
the genome.• For ScienceCast, I’m Katrina Voss.

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