24 August 2011

Secret to the kangaroo’s bounce revealed

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The Australian tammar wallaby is the first hopping mammal to have its genetic make-up sequenced, helping scientists to explain how it evolved its jump.
tammar wallaby

The tammar wallaby is found on offshore islands on the South Australian and Western Australian coast. Credit: Keng Yih Chew, University of Melbourne

tammar wallaby jumping

A new genetic study is helping scientists to explain how it evolved its jump. Credit: Marilyn Renfree, University of Melbourne

newborn wallaby

A newborn wallaby stays permanently attached to the mother’s teat for its first 100 days. Credit: Geoff Shaw, University of Melbourne

BRISTOL: The Australian tammar wallaby is the first hopping mammal to have its genetic make-up sequenced, helping scientists to explain how it evolved its jump.

An international team lead by scientists from The Australian Research Council Centre of Excellence in Kangaroo Genomics examined the wallaby’s genome sequence, revealing the gene thought to be responsible for limb development, a region termed HOXD13.

The researchers believe this research will also shed light on this highly specialised kangaroo’s unique biology, including its heightened sense of smell and the female’s ability to produce anti-microbial milk during motherhood.

“What is interesting is the surprising similarities as well as the differences in the genes uncovered in this study,” said lead author Marilyn Renfree from the University of Melbourne of the study published in the current issue of Genome Biology.

“The genetic sequence of the tammar wallaby has provided new insights into marsupial early development, lactation and the immune system.”

Finding jumping genes

The iconic kangaroo, Macropus eugenii, is a member of the marsupial family and is highly abundant in the islands of Southern and Western Australia as well as small pockets found in the south western corner of the continental mainland.

The research team used a combination of progressive genome sequencing techniques to compare the similarities and differences between the HOX gene region in the tammar wallaby with the same region in both the human and mouse genome.

They found that the region HOXD13, which is strongly expressed in the anterior and posterior points on the hind legs, could be responsible for the highly developed muscular back limbs distinctive of the tammar wallaby. These strong back legs lead to the animal’s unique hopping gait.

Protective mother’s milk

During genome sequencing, the team also found 14 cathelicidin genes expressed in the mammary gland during milk production. These genes are responsible for producing powerful antimicrobial peptides found in the milk. This range of protective genes may help to explain why the tammar is able to secrete milk of differing antimicrobial compositions from each of its teats.

When born the baby tammar is only 16 to 17 mm long, similar in size to a kidney bean. Equipped with well developed forearms, the tiny newborn is able to climb up into its mother’s pouch. To protect the young from dangerous pathogens the mother produces milk with strong antimicrobial properties.

“The new information that the genome sequence provides will help to contribute to our understanding of Australian wildlife health and conservation,” said co-lead author Tony Papenfuss from Melbourne’s Walter and Eliza Hall Institute.

“Using the genetic sequence we have discovered many new marsupial genes vital to the survival of the young, including genes that make antimicrobial proteins that kill bacteria in the dirty pouch.”

Learning from the wallaby

While many of the genes in the tammar sequence are shared with humans, the study also revealed a new human gene which was previously unknown.

“As a smaller member of the kangaroo family that breeds well in captivity and is easy to handle, the tammar wallaby was the obvious macropodid species to sequence,” said Elizabeth Murchison, a genetic specialist from the Wellcome Trust Institute in Cambridge, Britain.

“The tammar genome sequence provides a window on the evolution of mammals and represents a critical resource for the scientific community.”

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