Science Daily
Cincinnati
- What's missing might turn out to be as important as what's actually
there in uncovering the roots of the mammalian tree of life.
A team of biologists led by Mark Springer at the University of
California, Riverside and including Ronald DeBry of the University
of Cincinnati report in the Feb. 1 issue of Nature that an intensive
analysis of DNA sequences provides strong support for a grouping
Springer dubs "Afrotheria." The group includes a variety of placental
mammals from elephants to elephant shrews. And add in aardvarks,
manatees, and hyraxes to boot.
"One of the problems with mammal phylogenies is there hasn't been
a lot known," explained DeBry. "We were searching for the basic
outline of the tree of life."
Traditional phylogenies, or evolutionary trees, were based on
fossil evidence and physical similarities. To complicate things
further, there was a huge explosion of mammalian groups right
after dinosaurs went extinct.
"Finding the base of the tree has been difficult," said DeBry.
"There are lots and lots of questions."
Over the last 10 years, DNA studies have confirmed some patterns
proposed by those studying fossil evidence and physical similarities.
Other DNA studies turned up new and unexpected relationships.
The picture quickly got muddier and muddier.
"What we needed was a BIG data set," said DeBry. "Our data set
has six different genes and 8600 base pairs." Two of the genes
are found in mitochondrial DNA. The other four are found in the
chromosomes of the nucleus, including BRCA-1, commonly known as
the breast cancer gene.
Springer, DeBry and the other co-authors report that a specific
deletion of nine base pairs in BRCA-1 is shared by 12 groups of
placental mammals. These are the groups Springer puts together
in "Afrotheria."
In addition, the exhaustive comparison helps to answer a more
recent question: How closely are rabbits and guinea pigs related
to rodents? The results in Nature indicate those groups should
remain together, in contrast to previously published molecular
results.
"Our data show a clearer picture," said DeBry. "The rodents, including
the guinea pig, belong together. And rabbits probably do go with
rodents."
On the other hand, a group known as Archonta should be split apart
according to Springer et al. "The molecular data are really convincing
that this isn't a group," said DeBry. "Bats are somewhere completely
different. They're closer to pigs and cows than rodents and primates.
Micro- and mega-bats go together with hedgehogs."
If this is starting to sound a bit confusing, the biologists have
a very simple explanation for the divergence and resulting evolutionary
tree. The relationships which evolved closely parallel the movement
of continental land masses during geologic time.
That's why Springer named one group Afrotheria for its African
origins and another Laurasiatheria after the land mass which gave
rise to North America, Europe and Asia.
"Our results give a really strong division right at the base of
the tree," noted DeBry. "Where did each group originate? One in
the southern hemisphere and the other in the northern hemisphere.
On the classic trees, the northern and southern groups are mixed
up."
Geological evidence supports the phylogenies as well. Mammals
first appeared in the fossil record about the time continents
were splitting apart. This would give the different groups separate
evolutionary histories, which is documented in the DNA analyses.
"The DNA sequences tell us a nice biogeographic story," said DeBry.
"We're not the first to see these relationships, but the evidence
really hammers home the point that there is a group from Africa
that is closely related."
The co-authors are: Ole Madsen and Wilfried deJong (University
of Nijmegen, The Netherlands), Mark Scally, Christophe Douady,
Heather Amrine and Mark Springer (University of California, Riverside),
Ronald Adkins (University of Massachusetts), and Michael Stanhope
(Queen's University of Belfast).
Funding was provided by the National Science Foundation and the
European Commission's Training and Mobility of Researchers program.
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