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| The standard definition
of cloning refers to the development of offspring that are genetically
identical to the parent. Traditionally, an artificial process called
somatic cell nuclear transfer creates a clone of an adult animal.
This term refers to the transfer of the nucleus from a somatic cell to
an egg cell (oocyte). A somatic cell is any
cell in the body other than germ cells, e.g., skin, liver, heart, etc.
In this process, the nucleus of a somatic cell is removed and inserted
into an unfertilised egg that has had its original nucleus removed.
The egg, with its donated nucleus, is then nurtured and divides until it
becomes an embryo. The embryo is then placed inside the uterus of
a surrogate mother to complete its development. |
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"Dolly" the sheep (taxidermy
specimen).
Courtesy: National Museum
of Scotland (Edinburgh).
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There are two variants to this technique; the Roslin technique by
which "Dolly" the sheep was created, and the Honolulu technique.
The principle of nuclear transfer is essentially the same with both techniques.
Unlike the case with
"Dolly", the cloning process with respect to the thylacine would be further
complicated by the fact that the unfertilised egg cell would have been
donated by another species, and the inserted nucleus would be comprised
entirely of artificially created |
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| chromosomes. This cross-species
egg donation would create another major problem; the thylacine's closest
relatives have been extinct for millions of years, and the thylacine line
split from its evolutionary cousins some 30 million years ago. |
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| An egg from either
Sarcophilus
or Dasyurus would still contain mitochondrial DNA following the
removal of its nucleus. Mitochondria are found within the cytoplasm,
and are the power houses of the cell. They contain their own compact,
specialised genome that is separate from the executive genetic program
in the cell nucleus. The nucleus and mitochondria normally maintain
a constant, intimate biochemical cross-talk, exchanging vital proteins
and enzymes. It is this mitochondrial
DNA |
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the donated nuclear DNA from the thylacine once that had been introduced
into the donor egg. The chances of this communication taking place
with cousins distanced by some 30 million years is highly unlikely, and
consequently, the necessary triggers to initiate cell division would remain
switched off and the cell would fail to divide and continue its development
into becoming an embryo. This was not a factor in the successful
cloning of "Dolly", as mitochondrial recognition is not relevant with same-species
cloning.
Cross-species mitochondrial
recognition poses a significant hurdle that would need to be overcome to
produce a viable thylacine embryo, but even in regard
to this, some progress has been made.
In 1998, Professor Roger
Short, one of Australia's premier reproduction physiologists, participated
in the 20th century's most amazing hybridization experiment, one that spanned
an ocean and at least 11 million years in time: a successful crossing between
a guanaco (Lama guanicoe) (the small, South American progenitor
of both the domesticated llama and alpaca), and an Arabian dromedary camel
(Camelus dromedarius). The scientists artificially inseminated
a number of female camels with |
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Rotating
computer model of the DNA double helix. Image: Richard Wheeler.
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| guanaco sperm; they also inseminated female
guanacos with camel sperm. Two of the female camels became pregnant,
but both aborted in late pregnancy. Six of the guanacos conceived,
but two reabsorbed their embryos in early pregnancy, and three others produced
stillborn late-term foetuses. Short believes that a communication
problem between the sperm genes and the eggs' mitochondrial genes was the
reason for the failures. However, on 14th January 1998, after a 328-day
pregnancy (the usual gestation period for a guanaco), one of the guanacos
gave birth to the world's first camel-guanaco
hybrid. |
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| The common ancestor
of camels and llamas, Procamelus, evolved in North America around
11 million to 14 million years ago, and its descendants migrated to Asia
and Africa where they eventually developed into modern camels. The
ancestor of the guanaco entered South America by way of the newly formed
isthmus of Panama some 5 million years ago during the Great
American Interchange. Geneticists estimate the minimum divergence
time between the Asian and American camelids at about 11 million years. |
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| "A camel
is six times the size of the wild guanaco mother, but the baby was the
size of a normal guanaco calf", says Short. "The mother's
system somehow overrides any tendency to produce a huge baby. But
since the hybrid was born, it has grown like a rocket - it's already much
bigger than the mother".
The thylacine's closest relatives became extinct millions of years ago.
The fossil remains of many of these earlier thylacinids have been found,
along with a multitude of other extinct marsupials, in the extraordinary
limestone formation that Prof. Archer and his colleagues discovered in
the early 1980s at Riversleigh Station, in northwest Queensland. |
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"Site
D" at Riversleigh. The boulders sitting atop the hill are weathered
remnants of the Carl Creek Limestone, from which have come a significant
number of Miocene vertebrate fossils. The rocks of Riversliegh have
provided a unique glimpse into the life of the Australian rainforests which
existed here 12-25 million years ago, a very imporant time time period
in the evolution of Australia's marsupial fauna.
Photo
courtesy: University of New South Wales.
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