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Secrets from beyond extinc­tion: The Tas­man­ian tiger

pub­lished 23 Decem­ber 2017 | mod­i­fied 23 Decem­ber 2017

The entire thy­lacine genome has now been sequenced, reveal­ing the apex mar­su­pial preda­tor was in poor genetic health and may have strug­gled to fight dis­ease had it survived.

Author: Dr Ner­issa Han­nink, Uni­ver­sity of Mel­bourne

Specimen C5757; Picture: Museums VictoriaFloat­ing in a small jar of alco­hol sits one of Australia’s rarest spec­i­mens. The jar, labelled col­lec­tion num­ber C5757, holds a juve­nile Tas­man­ian tiger or thy­lacine, one of the best-​preserved extinct species, now held in Muse­ums Victoria’s Collection.

As the ani­mal became rarer, muse­ums every­where clam­bered to have a thy­lacine on show, and they are now its last refuge after being hunted to extinc­tion in 1936.

Using tech­niques never imag­ined when the last thy­lacine died in Hobart Zoo last cen­tury, a team led by the Uni­ver­sity of Mel­bourne have now sequenced the genome of the Tas­man­ian tiger (Thy­lac­i­nus cyno­cephalus), mak­ing it one of the most com­plete genetic blue­prints for an extinct ani­mal. The results are pub­lished online on 11 Decem­ber in the jour­nal Nature Ecol­ogy & Evo­lu­tion.

For project leader Pro­fes­sor Andrew Pask, the thy­lacine is his labour of love. Over ten years ago, he and an inter­na­tional team first res­ur­rected a Tas­man­ian tiger gene from pre­served pelt, but the DNA was too frag­mented to obtain the whole genome.

So, they searched muse­ums’ world-​wide data­bases and found spec­i­men C5757 in Muse­ums Victoria’s col­lec­tion — a young thy­lacine pup. Because the Tas­man­ian tiger was a mar­su­pial, which are mam­mals with a pouch, this pup spec­i­men could be pre­served in its entirety, allow­ing the research team to extract DNA and use cutting-​edge tech­niques to sequence the thy­lacine genome.

Asso­ciate Pro­fes­sor Andrew Pask says the results pro­vide the first full genetic blue­print of the largest Aus­tralian apex preda­tor to sur­vive into the mod­ern era.

ThylacineThe now-​extinct Tas­man­ian tiger’s genome has now been sequenced, reveal­ing the species had low genetic diver­sity.
Pic­ture: TMAG Tas­man­ian Museum and Art Gallery

The genome allows us to con­firm the thylacine’s place in the evo­lu­tion­ary tree. The Tas­man­ian tiger belongs in a sis­ter lin­eage to the Dasyuri­dae, the fam­ily which includes the Tas­man­ian Devil and the dun­nart,” says Asso­ciate Pro­fes­sor Pask, from the School of Biosciences.

Impor­tantly, the genome has also revealed the poor genetic health, or low genetic diver­sity, the thy­lacine expe­ri­enced before it was over-​hunted. The Tas­man­ian Devil is now also fac­ing a ‘genetic bot­tle­neck’ which is a likely result of their genetic iso­la­tion from main­land Aus­tralia for the last 10,000 to 13,000 years.

How­ever, the genome analy­sis sug­gests that both ani­mals were expe­ri­enc­ing low genetic diver­sity before they became iso­lated on Tas­ma­nia. This, in turn, sug­gests that Tas­man­ian tigers may have faced sim­i­lar envi­ron­men­tal prob­lems to the Dev­ils, had they sur­vived, such as a dif­fi­culty over­com­ing disease.

Our hope is that there is a lot the thy­lacine can tell us about the genetic basis of extinc­tion to help other species.

Andrew J. Pask, co-​author, Asso­ciate Pro­fes­sor School of Bio­Sciences, The Uni­ver­sity of Mel­bourne, and Muse­ums Vic­to­ria, Mel­bourne, Australia

As this genome is one of the most com­plete for an extinct species, it is tech­ni­cally the first step to ‘bring­ing the thy­lacine back’, but we are still a long way off that possibility.”

We would still need to develop a mar­su­pial ani­mal model to host the thy­lacine genome, like work con­ducted to include mam­moth genes in the mod­ern ele­phant. But know­ing the Tas­man­ian tiger was fac­ing lim­ited genetic diver­sity before extinc­tion means it would still have strug­gled sim­i­larly to the Tas­man­ian Devil if it had survived.”

The genome pro­vides other impor­tant new insights into the biol­ogy of this truly unique marsupial.

The thy­lacine is often described as a long dog with stripes, because it had a long, stiff tail and a big head. A fully grown thy­lacine could mea­sure 180cm from the tip of the nose to the tip of the tail and stand 58cm high. Its thick black stripes extended from the shoul­ders to the base of the tail. Like the dingo, the thy­lacine was a very quiet ani­mal. But they were reported to be relent­less hunters who pursed their prey until it was exhausted.

Sci­en­tists con­sider the thy­lacine and the dingo as one of the best exam­ples of ‘con­ver­gent evo­lu­tion’, the process whereby organ­isms that are not closely related inde­pen­dently evolve to look the same as a result of hav­ing to adapt to sim­i­lar envi­ron­ments or eco­log­i­cal niches.

It appears that because of their hunt­ing tech­nique and diet of fresh meat, their skulls and body shape became extremely sim­i­lar. Work­ing with Dr Christy Hip­s­ley from Muse­ums Vic­to­ria, the team analysed the char­ac­ter­is­tics of the thylacine’s skull — such as eye, jaw and snout shape.

We found the Tas­man­ian tiger had a more sim­i­lar skull shape to the red fox and gray wolf than to its clos­est rel­a­tives,” Dr Hip­s­ley says. “The fact these groups have not shared a com­mon ances­tor since the Juras­sic period makes this an astound­ing exam­ple of con­ver­gence between dis­tantly related species.”

Asso­ciate Pro­fes­sor Pask says that the thy­lacine looked almost like a dingo with a pouch.

When we looked at the basis for this con­ver­gent evo­lu­tion, we found it wasn’t actu­ally the genes that pro­duced the same skull and body shape, but the con­trol regions around them that turn genes ‘on and off’ at dif­fer­ent stages of growth. This reveals a whole new under­stand­ing of the process of evo­lu­tion. We can now explore these regions of the genome to help under­stand how two species con­verge on the same appear­ance, and how the process of evo­lu­tion works.”

In this case, it seems the need to hunt led the thy­lacine to trans­form its appear­ance into one sim­i­lar to the wolf over the past 160 mil­lion years.

Sci­en­tists can now start to under­stand the genet­ics that have dri­ven this process and uncover more about the biol­ogy of this unique mar­su­pial apex predator.

The research team also included sci­en­tists from the Uni­ver­sity of Mun­ster, Muse­ums Vic­to­ria, Uni­ver­sity of Ade­laide and Uni­ver­sity of Con­necti­cut. Some of the work was funded by the Research @ Mel­bourne Accel­er­a­tor Program.

(Source: The Uni­ver­sity of Mel­bourne; This arti­cle was first pub­lished on Pur­suit, 12.12.2017; Read the orig­i­nal arti­cle)

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