A Col­lec­tion of News by Moos


Biol­o­gists repli­cate key evo­lu­tion­ary step

pub­lished 19 Jan­u­ary 2012 | mod­i­fied 05 Decem­ber 2012

More than 500 mil­lion years ago, single-​celled organ­isms on the Earth’s sur­face began form­ing mul­ti­cel­lu­lar clus­ters that ulti­mately became plants and ani­mals. Just how that hap­pened is a ques­tion that has eluded evo­lu­tion­ary biologists.

But sci­en­tists in the Uni­ver­sity of Minnesota’s Col­lege of Bio­log­i­cal Sci­ences have repli­cated that key step in the lab­o­ra­tory using nat­ural selec­tion and com­mon brewer’s yeast, which are single-​celled organ­isms. The yeast “evolved” into mul­ti­cel­lu­lar clus­ters that work together coop­er­a­tively, repro­duce and adapt to their envi­ron­ment – in essence, pre­cur­sors to life on Earth as it is today.

Their achieve­ment is pub­lished in the Jan­u­ary 17 issue of Pro­ceed­ings of the National Acad­emy of Sciences.

Bridg­ing the famous multi-​cellularity gap would be “just about the coolest thing we could do” researchers Will Rat­cliff and Michael Trav­isano said to each other over cof­fee, two years ago. They prob­a­bly were the first that had a go at it, and sur­pris­ingly, it wasn’t actu­ally that dif­fi­cult. Using yeast cells, cul­ture media and a cen­trifuge, it only took them one exper­i­ment con­ducted over about 60 days, says Trav­isano. Their achieve­ment has earned praise and admi­ra­tion from evo­lu­tion­ary biol­o­gists around the world.

To under­stand why the world is full of plants and ani­mals, includ­ing humans, we need to know how one-​celled organ­isms made the switch to liv­ing as a group, as mul­ti­celled organ­isms,” said Sam Scheiner, pro­gram direc­tor in the National Sci­ence Foun­da­tion (NSF)’s Divi­sion of Envi­ron­men­tal Biol­ogy. “This study is the first to exper­i­men­tally observe that tran­si­tion, pro­vid­ing a look at an event that took place hun­dreds of mil­lions of years ago.”

In essence, here’s how the exper­i­ments worked. The two chose brewer’s yeast or Sac­cha­romyces cere­visiae, a species of yeast used since ancient times to make bread and beer, because it is abun­dant in nature and grows eas­ily. They added it to a nutrient-​rich cul­ture media and allowed the cells to grow for a day in test tubes. Then they used a cen­trifuge to strat­ify the con­tents by weight. As the mix­ture set­tled, cell clus­ters landed on the bot­tom of the tubes faster because they are heav­ier. They removed the clus­ters, trans­ferred them to fresh media, and grew them up again. Sixty cycles later, the clus­ters – now hun­dreds of cells – looked roughly like spher­i­cal snowflakes.

Analy­sis showed that the clus­ters were not just groups of ran­dom cells that adhered to each other, but related cells that remained attached fol­low­ing cell divi­sion. That was sig­nif­i­cant because it meant they were genet­i­cally sim­i­lar, which pro­motes coop­er­a­tion. When the clus­ters reached a crit­i­cal size, some cells essen­tially com­mit­ted sui­cide (apop­to­sis) to allow off­spring to sep­a­rate. The off­spring repro­duced only after they attained the size of their parents.

A clus­ter alone isn’t mul­ti­cel­lu­lar,” Rat­cliff said. “But when cells in a clus­ter coop­er­ate, make sac­ri­fices for the com­mon good, and adapt to change, that’s an evo­lu­tion­ary tran­si­tion to multicellularity.”

In order for mul­ti­cel­lu­lar organ­isms to form, most cells need to sac­ri­fice their abil­ity to repro­duce, an altru­is­tic action that favors the whole but not the indi­vid­ual, Rat­cliff said. For exam­ple, all cells in the human body are essen­tially a sup­port sys­tem that allows sperm and eggs to pass DNA along to the next gen­er­a­tion. Thus, mul­ti­cel­lu­lar­ity is by its nature extremely coop­er­a­tive. “Some of the best com­peti­tors in nature are those that engage in coop­er­a­tion, and our exper­i­ment bears that out,” said Travisano.

Evo­lu­tion­ary biol­o­gists have esti­mated that mul­ti­cel­lu­lar­ity evolved inde­pen­dently in about 25 groups. Trav­isano and Rat­cliff won­der why it didn’t evolve more often in nature, since it’s not that dif­fi­cult to recre­ate it in a lab. Con­sid­er­ing that tril­lions of one-​celled organ­isms lived on the Earth for mil­lions of years, it seems as if it should have, Rat­cliff said.

Maybe that’s a ques­tion they will answer in the future, using the fos­sil record for thou­sands of gen­er­a­tions of their mul­ti­cel­lu­lar clus­ters, which is stored in a freezer in Travisano’s lab. Since the frozen sam­ples con­tain mul­ti­ple lines that inde­pen­dently became mul­ti­cel­lu­lar, they can com­pare them to learn if sim­i­lar or dif­fer­ent mech­a­nisms and genes were respon­si­ble in each case, Trav­isano said.

The research duo’s next steps will be to look at the role of mul­ti­cel­lu­lar­ity in can­cer, aging and other crit­i­cal areas of biology.

Our mul­ti­cel­lu­lar yeast are a valu­able resource for inves­ti­gat­ing a wide vari­ety of med­ically and bio­log­i­cally impor­tant topics
Trav­isano said, “Can­cer was recently described as a fos­sil from the ori­gin of mul­ti­cel­lu­lar­ity, which can be directly inves­ti­gated with the yeast sys­tem. Sim­i­larly the ori­gins of aging, devel­op­ment, and the evo­lu­tion of com­plex mor­pholo­gies are open to direct exper­i­men­tal inves­ti­ga­tion that would oth­er­wise be dif­fi­cult or impossible.”

- All of the above is an exact copy of the pub­li­ca­tion on the Uni­ver­sity of Minnesota’s web­site, besides some minor changes which were less rel­e­vant to this really excit­ing message -

(Source: Uni­ver­sity of Min­nesota News, 17.01.2012; PNAS, 17.01.2012)

Goal: 7000 tigers in the wild

Tiger range countries map

Tiger map” (CC BY 2.5) by Sander­son et al., 2006.

about zoos and their mis­sion regard­ing breed­ing endan­gered species, nature con­ser­va­tion, bio­di­ver­sity and edu­ca­tion, which of course relates to the evo­lu­tion of species.
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