SAT Reading - Khan Diagnostic Quiz level 4 - reading 17

Questions 1-11 are based on the following

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

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

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.

Question 1 What does the study of retroposed elements mainly indicate about marsupials in South America and Australia?