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NEWS
Got Milk? Exploring the Evolutionary Connection between Milk
Drinking, Lactose Digestion, and Sunlight
News
Milk, as the popular slogan goes, does a body good. It contains
essential nutrients including fat, protein, sugar, as well as calcium, other minerals, and vitamin D needed for bones. Most
people in the world lose the ability to digest lactose, the main
sugar in milk, shortly after weaning. For these people, drinking
fresh milk can lead to unpleasant bloating, flatulence, and
cramps. However, about one-third of people in the world—
mostly those whose ancestors originate in Europe, the Middle
East, Africa, and southern Asia—continue to produce the
enzyme lactase, which is responsible for lactose digestion,
throughout adulthood. This trait is called lactase persistence,
and recent genetic evidence has shown that it evolved independently in different parts of the world over the last 10,000
years as a result of strong natural selection.
Why lactase persistence has evolved under such strong
natural selection remains something of a mystery. The most
widely cited explanation is that in the absence of dietary
sources for vitamin D and with insufficient sunlight to
make vitamin D in the skin, early northern European farmers
were at risk of bone disease. Milk is an excellent source of
calcium and an adequate source of vitamin D. So, as this
“calcium assimilation hypothesis” proposes, having the ability
to drink fresh milk into adulthood could have led to a major
survival advantage.
In a new article in Molecular Biology and Evolution journal,
Sverrisdóttir et al. (2014) looked for the mutation that causes
lactase persistence in Europeans (referred to as 13,910*T) in
the bones of early farmers from sunny Spain. They didn’t find
it! They then used computer simulations to estimate how
much natural selection would be needed to push the frequency of 13,910*T up to what is seen in Iberia today
(about one-third have the mutation). To their surprise, the
answer was “a lot!”
What does this tell us about the calcium assimilation
hypothesis? Well in Iberia, there is plenty enough sunlight
to produce vitamin D in the skin, so calcium deficiency
shouldn’t have been a problem for those early farmers. As
Sverrisdóttir et al. (2014) reason, if selection was a necessary
drive up for lactase persistence frequency in people for whom
calcium deficiency was not an issue, then the calcium assimilation hypothesis could not be the main explanation for the
observed frequencies of lactase persistence in the Iberian
Peninsula today, and so not the only explanation for the
evolution of lactase persistence in Europe. They conclude
that other evolutionary selective pressures must have been
at work to explain the presence of this trait in modern
Europeans.
“Using ancient DNA and computer simulations, we show
that strong natural selection has acted on lactase persistence
in Iberia over the last 7,000 years. Sunlight in Iberia is sufficient
to allow the synthesis of vitamin D in the skin for most of the
year. It is therefore unlikely that the risk of calcium deficiency
was the main driver for the evolution of lactase persistence
(the calcium assimilation hypothesis) in this region. Additional evolutionary forces need to be identified to explain
this example of strong natural selection in the genome of
Europeans.”
Reference
Sverrisdóttir OO, Timpson A, Toombs J, Lecoeur C, Froguel P, Carretero
JM, Arsuaga Ferreras JL, Götherström A, Thomas MG. 2014. Direct
estimates of natural selection in Iberia indicate calcium absorption
was not the only driver of lactase persistence in Europe. Mol Biol
Evol. 31(4):975–983.
Joseph Caspermeyer*,1
1
MBE Press Office
*Corresponding author: E-mail: [email protected]
doi:10.1093/molbev/msu052
Advance Access publication February 10, 2014
New Sequencing Tools Give a Close Look at Yeast Evolution
The baker’s yeast Saccharomyces cerevisiae has been associated with human activities for thousands of years, being the
primary biological agent in baking, brewing, winemaking, and
other fermentation processes. It is also one of the most
important model organisms in molecular biology and genetic
research. For a long time, the history and evolution of this
important yeast has been a complete mystery, but recent
advances in genome sequencing technologies allow it to be
studied in greater detail.
Using next-generation sequencing, the authors Bergström
et al. (2014) provide a detailed characterization of the genetic
variation present within the baker’s yeast species. They sequenced the genomes of 42 strains of S. cerevisiae and its
closest relative S. paradoxus, which is an entirely wild species
ß The Author 2014. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please
e-mail: [email protected]
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Mol. Biol. Evol. 31(4):1056–1057
that has not had any contact with humans. A central finding
of this study is that even though strains in S. paradoxus are
separated by much greater genetic distances in terms of single
nucleotide polymorphisms, the S. cerevisiae strain genomes
harbor more variation in terms of absence and presence and
copy number of genes. It has previously been observed that
trait variation is also much larger in S. cerevisiae than in its
wild relative. These new results therefore raise the intriguing
hypothesis that this variation in the content of the genome,
rather than single-nucleotide differences, underlies the large
phenotypic variation in S. cerevisiae.
The authors find that the subtelomeric regions of the
genomes, located just before the telomeres at each chromosome end, are highly enriched for genome variation that is
likely to contribute to differences in traits between strains.
This includes loss-of-function mutations that likely disrupt
the function of whole genes. As an example of functional
variation, they describe how differences in the copy number
of a subtelomeric gene cluster controls the ability of strains to
grow under arsenic stress and demonstrate that this variation
is the product of convergent evolution in yeast lineages in
different parts of the world.
“These genome sequences allowed us to expose surprising
differences between the evolutionary histories of the
common baker’s yeast and its wild relative. Our results
suggest that the very large diversity in traits observed between
strains of baker’s yeast might mostly be due to the presence or
absence of entire genes rather than differences in single DNA
letters.”
The study provides intriguing insights into the recent
history of this important organism and the relationship between genome variation and trait variation. Future research
will further elucidate what role humans have played in shaping the evolution of baker’s yeast, for example, the extent to
which the genomic variation is a consequence of yeast strains
moving into novel habitats and niches opened up by human
activities.
Reference
Bergström A, Simpson JT, Salinas F, Barre B, Parts L, Zia A, Nguyen Ba
AN, Moses AM, Louis EJ, Mustonen V, et al. 2014. A high-definition
view of functional genetic variation from natural yeast genomes.
Mol Biol Evol. 31(4):872–888.
Joseph Caspermeyer*,1
1
MBE Press Office
*Corresponding author: E-mail: [email protected]
doi:10.1093/molbev/msu060
Advance Access publication February 17, 2014
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