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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