Marsupials,
including Australia’s iconic kangaroo, are distantly
related to humans and other placental mammals. This
makes for many key differences in the genomes of marsupials
and mammals which can be compared to each other and
studied to better understand the biology of health
and disease.
Proteins,
the building blocks of life, are very similar between
marsupials and humans, although the branches of mammals
they represent last shared a common ancestor 180 million
years ago. But 20 per cent of the protein switches
that control genes in humans are new since marsupials
split from other mammals.
Many
of these switches evolved using the DNA patterns left
behind by viruses. “Ancient viruses left their
DNA all over the genome, and these became new switches
to turn on or off banks of genes,” explained
Dr Matthew Wakefield, a KanGO investigator now at the
Walter and Eliza Hall Institute and third author on
the Nature paper.
One
of these new switches is involved in turning off one
of the two X chromosomes in females to preserve a balance
with males (with only one X chromosome). X chromosome
inactivation occurs in marsupials as well as humans
and mice. But marsupials lack the gene that switches
off the human X, and also lack the pattern of retrovirus
DNA on the X chromosome that this switch is thought
to use, meaning that the way scientists have understood
the basic gene silencing mechanism to work needs to
be rethought.
Other
surprises from the sequencing project came from the
study of immune genes led by KanGO investigator Dr
Kathy Belov from the University of Sydney.
Though
Australian scientists were first to propose sequencing
a marsupial, a lack of Australian support saw the South
American opossum chosen as model. The opossum last
shared a common ancestor with kangaroos about 80 million
years ago – the same time that humans and mice
diverged.
However,
with support from the Victorian Government, the kangaroo
genome is now being sequenced by the Australian Genome
Research Facility in Melbourne, with help from collaborators
in Texas. Kangaroo sequence will be critical in understanding,
conserving and managing our own wildlife.
“The
kangaroo sequence is even more exciting now,” Professor
Graves said. “Comparing the two distant species
tells us what is peculiar to marsupials and what to
humans and other placental mammals. This will help
us to understand what makes mammals different, as well
as to solve conservation problems like the Tasmanian
Devil Facial Tumour disease.”
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