Cosmos - - Feature - JOHN PICKRELL is a Syd­ney-based science writer and author.

CLONING A THYLACINE will be more chal­leng­ing than Church’s project to res­ur­rect the mam­moth us­ing the Asian ele­phant. Their an­ces­tors di­verged just six mil­lion years ago, and they share about 99% of their genes. There is no equiv­a­lent species for the thylacine.

Pask sug­gests Western Aus­tralia’s num­bat, whose genome he plans to se­quence, might pro­vide the best start­ing DNA blue­print. It is one of the thylacine’s clos­est liv­ing rel­a­tives, last shar­ing a common an­ces­tor 30 mil­lion years ago. The diminu­tive ter­mite-eat­ing crea­ture has stripes, but that’s where the sim­i­lar­ity ends. Adult num­bats are slightly big­ger than a squir­rel, whereas adult thy­lacines weighed about 30 kg. De­spite this, Pask says as much as 95% of their DNA may be iden­ti­cal.

That still leaves an aw­ful lot of num­bat DNA to edit, mak­ing it an ex­pen­sive propo­si­tion. But, as with all other ge­netic tech­nolo­gies, the costs are likely to fall fast. Pask will wait and watch while other de-ex­tinc­tion projects, par­tic­u­larly that of the mam­moth and a sim­i­larly ad­vanced ef­fort to res­ur­rect the pas­sen­ger pi­geon of North Amer­ica, per­fect the tech­nolo­gies.

The next se­ries of steps are the most un­pre­dictable: cloning an em­bryo, im­plant­ing it into a sur­ro­gate and ges­tat­ing the pouch young.

Get­ting cloning to work is a ma­jor chal­lenge. The tech­niques used to cre­ate Dolly are no­to­ri­ously dif­fi­cult to ap­ply to dif­fer­ent species. It was only in 2017 – more than 21 years af­ter Dolly – that it was suc­cess­fully repli­cated in a pri­mate, with Chi­nese sci­en­tists pro­duc­ing two ge­net­i­cally iden­ti­cal long­tailed macaques.

Once re­searchers get a thylacine-re­coded num­bat egg to start de­vel­op­ing into an em­bryo, ges­tat­ing it is also far from straight­for­ward. For hu­mans and sheep, both pla­cen­tal mam­mals, the science of im­plant­ing em­bryos into a womb is well-es­tab­lished. Not so for mar­su­pi­als, where im­plan­ta­tion takes place much later. In pla­cen­tals we know how to prime a mother with hor­mones to ac­cept an em­bryo, but this knowl­edge is com­pletely lack­ing in mar­su­pi­als.

To mas­ter as­sisted re­pro­duc­tion in mar­su­pi­als, Pask has turned to a dif­fer­ent thylacine rel­a­tive, the tiny mouse-like dun­nart. They breed well in cap­tiv­ity and pro­duce a lit­ter of up to 20 young twice a year. Nev­er­the­less, he says, “it will be a decade be­fore we get a re­ally good han­dle on a lot of this stuff in mar­su­pi­als”.

Preg­nancy is also a very dif­fer­ent propo­si­tion to pla­cen­tal mam­mals. A mar­su­pial still looks some­thing like a foe­tus when it is born, typ­i­cally two weeks af­ter con­cep­tion. About the size and shape of a pink jelly­bean, it must crawl up its mother’s ab­domen and into her pouch, where it latches onto a teat to suckle. Its mother’s milk, like a pla­centa, changes its com­po­si­tion to guide most of the joey’s devel­op­ment.

This two-stage ges­ta­tion does of­fer in­trigu­ing pos­si­bil­i­ties. A thylacine em­bryo might be ges­tated in the uterus of a smaller mar­su­pial, and then trans­ferred to the pouch of a larger one – per­haps a kan­ga­roo. Cross-fos­ter­ing is a well-es­tab­lished tech­nique to help bol­ster the pop­u­la­tions of en­dan­gered rock wal­la­bies. In 2014 a rock wal­laby suc­cess­fully fos­tered a baby tree kan­ga­roo in its pouch.

An­other op­tion is hand rear­ing, al­ready widely em­ployed for res­cued kan­ga­roos and also for Tas­ma­nian devils cap­tive bred to save the species from the devil fa­cial tu­mour dis­ease (DFTD) that has dec­i­mated wild pop­u­la­tions.

ONCE A THYLACINE joey has weaned, at about nine months, there would be a new set of hur­dles. Would it be­have like a thylacine?

Lit­tle is known about nat­u­ral be­hav­iours, such as hunt­ing or mat­ing, as the thylacine was scarcely ob­served in the wild. “Many be­hav­iours are in­nate,” Pask says, “but there would be a large sub­set that they prob­a­bly learnt from in­di­vid­u­als around them. Learned be­hav­iour is more common in species that use com­plex de­ci­sion-mak­ing to hunt prey, and pre­served thylacine brains re­veal a well-de­vel­oped frontal cor­tex, in­di­cat­ing good mem­ory and ca­pac­ity to learn.”

We do know thy­lacines did not fare well in cap­tiv­ity. The Royal Zoo­log­i­cal So­ci­ety of NSW noted in 1939: “The thylacine does not take kindly to cap­tiv­ity, and rarely lives un­der such con­di­tions for any length of time.” From 1850 to 1931, 224 were kept at zoos in cities in­clud­ing Washington DC, New York, Ber­lin and Paris. Lon­don Zoo had 20 over the years. Some died dur­ing jour­neys, oth­ers stopped eat­ing and fell ill. None bred. While our skill at keep­ing an­i­mals has in­creased enor­mously, there is no guar­an­tee res­ur­rected thy­lacines would do bet­ter.

Un­der­stand­ing how a species might fare is im­por­tant, says Beth Shapiro, an evo­lu­tion­ary bi­ol­o­gist at the Univer­sity of Cal­i­for­nia, Santa Cruz, and author of How to Clone a Mam­moth: The Science of De-ex­tinc­tion (2015). “Pop­u­la­tions liv­ing in cap­tiv­ity, pos­si­bly for decades, need not only to sur­vive but must also learn how to live,” she says. “They need to learn how to feed and pro­tect them­selves, how to in­ter­act with oth­ers, how to avoid pre­da­tion, how to choose a mate, and how to pro­vide parental care.”

You also need a pop­u­la­tion with ge­netic va­ri­ety, Shapiro says. Pask sug­gests it might be pos­si­ble to edit such vari­a­tion into the genome. “If you can get over the hur­dle of mak­ing all those mil­lions of ed­its

to the genome to make it look like a thylacine in the first place,” he says, then in­tro­duc­ing vari­abil­ity into im­mune sys­tem genes “is noth­ing”.

IF ALL THESE HUR­DLES can be over­come, the end goal of any de-ex­tinc­tion ef­fort surely must be to rein­tro­duce an­i­mals to the wild. One po­ten­tial is­sue for some de-ex­tinc­tion can­di­dates – ap­pro­pri­ate habi­tat – is not a prob­lem. Re­serves cover about half of Tas­ma­nia to­day. “The habi­tat is the same, the an­i­mals they ate are still there,” says Archer. “There’s no ques­tion it could be put back into the bush of Tas­ma­nia.” There is also good rea­son to do so: “The thylacine was Tas­ma­nia’s key car­ni­vore. Get­ting it back is about resta­bil­is­ing ecosys­tems cur­rently un­der threat.”

That still may not be enough to con­vince ev­ery­one we should bring back thy­lacines. Many ar­gue de­ex­tinc­tion projects take the fo­cus away from the vi­tal work to save other species from ex­tinc­tion.

“If you have the mil­lions of dol­lars it would take to res­ur­rect a species and choose to do that, you are mak­ing an eth­i­cal de­ci­sion to bring one species back and let sev­eral oth­ers go ex­tinct,” Canadian con­ser­va­tion bi­ol­o­gist Joseph Ben­nett has said. “It would be one step for­ward, and three to eight steps back.”

Yet what is true to­day may not be true to­mor­row. Pask agrees that, right now, re­sources should go to sav­ing en­dan­gered mar­su­pi­als. “If, how­ever, in 10 to 15 years’ time it be­comes rel­a­tively in­ex­pen­sive, then I think it is def­i­nitely worth pur­su­ing.” Hav­ing hunted the thylacine to ex­tinc­tion, he says, “we owe it to the species to bring it back”.

It may not be en­tirely thylacine, but one day, a cen­tury or so from now, a crea­ture that looks and be­haves like one might be found qui­etly slip­ping be­tween piles of rusty rocks that bear its like­ness, etched mil­len­nia ago.

For mil­lions of years, Thy­lacines roamed across Aus­tralia and Pa­pua New Guinea. A dry­ing cli­mate led to the loss of for­est habi­tat and wiped out most thylacine pop­u­la­tions on the main­land around 3,000 years ago. The is­land of Tas­ma­nia re­mained the last refuge. Though thy­lacines looked much like dogs, they last shared an an­ces­tor with ca­nines about 160 mil­lion years ago. The re­sem­blance is an ex­am­ple of con­ver­gent evo­lu­tion, in which an­i­mals de­velop sim­i­lar fea­tures to fill sim­i­lar eco­log­i­cal niches. Thy­lacines were mar­su­pi­als, car­ry­ing their young in a pouch on their bel­lies like other iconic na­tive Aus­tralian an­i­mals in­clud­ing koalas and kan­ga­roos.

A few tweaks to turn a num­bat into a thylacine? The striped ter­mite- eat­ing num­bat, about the size of a large squir­rel, will have its DNA edited to re­sem­ble that of its long-lost cousin.IM­AGES 01 Nick Rains / Aus­tralian Ge­o­graphic 02 Tas­ma­nian Mu­seum and Art Gallery 03 Rod Start / Mu­se­ums Vic­to­ria 04 An­drew Pask 05 Vac1 / Getty Im­ages 06 Craigrjd / Getty Im­ages

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