Adapted from Mason Inman, “Jumping Genes Reveal Kangaroos’ Origins,” © 2010 by Mason Inman.
Nothing is more Australian than kangaroos. But these
marsupials—along with a variety of others including the
Tasmanian devil—have ancient roots in South America, a
new study suggests.
Exactly how these various marsupials, both living and
extinct, are related has been murky. There are marsupials found
today in both Australia and the Americas, with the opossum the
most familiar to Americans.
Some older studies suggested that marsupials first arose in
Australia and that some marsupial lineages might have been
split in two when these landmasses separated 80 million years
ago. But there are few fossils from either South America or
Australia of long-extinct marsupials, so debates have raged for
decades about how to arrange the branches of the marsupial
family tree. Genetic studies—looking both at genes in cells’
nuclei and in mitochondria, the cells’ powerhouses that carry
their own DNA—have come up with contradictory results
about which lineages are most closely related and which split
off first.
Maria Nilsson, Jürgen Schmitz, and colleagues present the
first study to use the sequences of retroposed elements—a kind
of “jumping gene”—to reconstruct marsupials’ family tree.
Retroposed elements make up a bigger portion of kangaroos’
and other marsupials’ genomes than any other mammal that’s
had its genome sequenced. The sequences appear to serve little
or no purpose to these animals, but that’s exactly what gives the
new technique its strength.
Retroposons use their own enzymatic machinery, or that of
other retroposons, to copy their own RNA and create DNA
copies of themselves. Instead of making copies to spread from
cell to cell and organism to organism, as, for example, viruses
do, retroposed elements are deposited in other parts of the same
genome in the same cell—including in the germ line cells—
cutting a gap in a DNA strand and inserting themselves there.
These copies remain in their new locus. It is extremely rare that
a retroposed element is cleanly excised sometime after
insertion. After millions of years, hundreds of thousands of
them are now littered throughout marsupials’ genomes.
Also, the way they spread through the genome means they
can occur in idiosyncratic patterns. Jumping genes are so
widespread in marsupial genomes that when a copy jumps, it
often lands in the middle of an older jumping gene, creating
one retroposon nested within another one. Retroposons, and
especially nested ones, are unlikely to arise independently in
another species in exactly the same part of the genome by
chance. So if different species share a few of the same nested
retroposed elements, chances are overwhelming that they all
got them from a long-lost ancestor.
In two marsupial genomes that were recently sequenced,
Nilsson, Schmitz, and colleagues identified thousands of these
nested retroposed elements—more than 8,000 in the genome of
the South American opossum (Monodelphis domestica), and
nearly 4,500 in the genome of a kangaroo, the Australian
Tammar Wallaby kangaroo (Macropus eugenii).
Nilsson, Schmitz, and colleagues sorted all these nested
retroposed elements into three categories: those unique to the
kangaroo, those unique to the opossum, or those shared
between the two. They pared down the thousands of jumping
elements to 53 that would serve as markers of how various
marsupials branched off from each other. They found that all
living marsupials must have come from one branch of
mammals, since they all share jumping elements in 10
particular spots in their genomes, which are not found in any
other mammals.
They then searched through the DNA of 20 marsupial
species—including the wallaroo, the common wombat, and the
marsupial mole—to see which of these markers they carry. It
hadn’t been clear which lineage of marsupials split off first, but
the new study found this first branch gave rise to the
Didelphimorphia lineage, which includes several species of
opossums of South America. Further branches gave rise to
other South American marsupials. All of today’s Australian
marsupials appear to have branched off later, all arising from a
single lineage that branched from a South American
microbiotherian-like ancestor to form varied forms—
kangaroos, the rodent-like bandicoots, and the Tasmanian
devil.
It’s still a bit of a mystery, Nilsson, Schmitz, and colleagues
say, why the marsupial family tree sorted out so cleanly. They
found two distinct branches—one for South America and one
for Australia—despite these landmasses having formed parts of
the larger landmass of Gondwana for tens of millions of years
around the time that marsupials arose.
Nothing is more Australian than kangaroos. But these
marsupials—along with a variety of others including the
Tasmanian devil—have ancient roots in South America, a
new study suggests.
Exactly how these various marsupials, both living and
extinct, are related has been murky. There are marsupials found
today in both Australia and the Americas, with the opossum the
most familiar to Americans.
Some older studies suggested that marsupials first arose in
Australia and that some marsupial lineages might have been
split in two when these landmasses separated 80 million years
ago. But there are few fossils from either South America or
Australia of long-extinct marsupials, so debates have raged for
decades about how to arrange the branches of the marsupial
family tree. Genetic studies—looking both at genes in cells’
nuclei and in mitochondria, the cells’ powerhouses that carry
their own DNA—have come up with contradictory results
about which lineages are most closely related and which split
off first.
Maria Nilsson, Jürgen Schmitz, and colleagues present the
first study to use the sequences of retroposed elements—a kind
of “jumping gene”—to reconstruct marsupials’ family tree.
Retroposed elements make up a bigger portion of kangaroos’
and other marsupials’ genomes than any other mammal that’s
had its genome sequenced. The sequences appear to serve little
or no purpose to these animals, but that’s exactly what gives the
new technique its strength.
Retroposons use their own enzymatic machinery, or that of
other retroposons, to copy their own RNA and create DNA
copies of themselves. Instead of making copies to spread from
cell to cell and organism to organism, as, for example, viruses
do, retroposed elements are deposited in other parts of the same
genome in the same cell—including in the germ line cells—
cutting a gap in a DNA strand and inserting themselves there.
These copies remain in their new locus. It is extremely rare that
a retroposed element is cleanly excised sometime after
insertion. After millions of years, hundreds of thousands of
them are now littered throughout marsupials’ genomes.
Also, the way they spread through the genome means they
can occur in idiosyncratic patterns. Jumping genes are so
widespread in marsupial genomes that when a copy jumps, it
often lands in the middle of an older jumping gene, creating
one retroposon nested within another one. Retroposons, and
especially nested ones, are unlikely to arise independently in
another species in exactly the same part of the genome by
chance. So if different species share a few of the same nested
retroposed elements, chances are overwhelming that they all
got them from a long-lost ancestor.
In two marsupial genomes that were recently sequenced,
Nilsson, Schmitz, and colleagues identified thousands of these
nested retroposed elements—more than 8,000 in the genome of
the South American opossum (Monodelphis domestica), and
nearly 4,500 in the genome of a kangaroo, the Australian
Tammar Wallaby kangaroo (Macropus eugenii).
Nilsson, Schmitz, and colleagues sorted all these nested
retroposed elements into three categories: those unique to the
kangaroo, those unique to the opossum, or those shared
between the two. They pared down the thousands of jumping
elements to 53 that would serve as markers of how various
marsupials branched off from each other. They found that all
living marsupials must have come from one branch of
mammals, since they all share jumping elements in 10
particular spots in their genomes, which are not found in any
other mammals.
They then searched through the DNA of 20 marsupial
species—including the wallaroo, the common wombat, and the
marsupial mole—to see which of these markers they carry. It
hadn’t been clear which lineage of marsupials split off first, but
the new study found this first branch gave rise to the
Didelphimorphia lineage, which includes several species of
opossums of South America. Further branches gave rise to
other South American marsupials. All of today’s Australian
marsupials appear to have branched off later, all arising from a
single lineage that branched from a South American
microbiotherian-like ancestor to form varied forms—
kangaroos, the rodent-like bandicoots, and the Tasmanian
devil.
It’s still a bit of a mystery, Nilsson, Schmitz, and colleagues
say, why the marsupial family tree sorted out so cleanly. They
found two distinct branches—one for South America and one
for Australia—despite these landmasses having formed parts of
the larger landmass of Gondwana for tens of millions of years
around the time that marsupials arose.