Questions 1-11 are based on the following
passage.
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.
5 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
10 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
15 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.
20 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
25 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
30 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
35 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
40 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
45 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
50 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
55 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
60 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
65 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
70 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
75 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
80 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
85 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.