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April 28, 2009

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Needy amoebae reach out to “family”

Nov. 24, 2008
Courtesy Baylor College of Medicine
and World Science staff

When in need, some amoe­bae seek the sup­port of kin, re­search­ers say.

The mi­crobes, when starving, seek each oth­er out and form groups in which some sac­ri­fice them­selves. As a re­sult of their deeds, oth­ers are able to trav­el some dis­tance away, where they may find more food.

Al­though this phe­nom­e­non has been long doc­u­mented, it was­n’t clear the ex­tent to which these as­socia­t­ions are based on re­lat­ed­ness, ac­cord­ing to the sci­en­tists, from Bay­lor Col­lege of Med­i­cine and Rice Uni­ver­s­ity in Texas. 

“These sin­gle cells ag­gre­gate based on ge­net­ic si­m­i­lar­ity,” though “not true kin­ship,” said re­search team mem­ber Gad Shaul­sky, a ge­net­icist at Bay­lor. The pro­cess none­the­less demon­strates a dis­crimina­t­ion be­tween “self” and “non-self” si­m­i­lar to that seen in im­mune sys­tem cells of high­er or­gan­isms, he added. The study ap­pears Nov. 24 in the on­line re­search jour­nal PLoS Bi­ol­o­gy.

The amoe­ba Dic­tyostelium dis­coideum starts life as a sin­gle-celled or­gan­ism. As long as it has am­ple food and a pleas­ant en­vi­ron­ment, it’s hap­py to stay that way. 

But when food is scarce, the crea­tures band to­geth­er and form what’s ef­fec­tively a mul­ti­-cel­lu­lar or­gan­ism. The cells then sort them­selves in­to two roles. Some cells be­come spores, which can sur­vive and re­pro­duce. Oth­ers per­ish, and in the pro­cess form an in­an­i­mate stalk that holds the live spores aloft. This helps the spores to dis­perse far away—far enough to reach new sur­round­ings, with pos­si­bly new food op­por­tun­i­ties.

Re­search in­to these mi­cro­bi­al group­ings, called ag­gre­gates, helps sci­en­tists un­der­stand bio­films—te­na­cious col­o­nies of bac­te­ria or fun­gi that can harm hu­mans and oth­er mam­mals, said Jo­an Strass­mann of Rice Uni­ver­s­ity, anoth­er of the re­search­ers. For exam­ple, peo­ple with cyst­ic fi­bro­sis are vulnera­ble to the forma­t­ion of lung-dam­aging bio­films.

In pre­vi­ous work, Strass­man and col­la­bo­ra­tors found that some Dic­ty­os­te­lium cells “cheat” their social sys­tem: they con­sist­ently avoid the bio­chem­i­cal chain of events that leads to death, and in­stead be­come spores. In the new stu­dy, the re­search­ers found that by band­ing to­geth­er based on ge­net­ic si­m­i­lar­ity, amoe­bae re­duce the ben­e­fit of cheat­ing. That’s be­cause even though some cells will die, they get to prop­a­gate some of their genes an­y­way, be­cause the cells that do live are rel­a­tives.

The cells’ predilection for re­lat­ed com­pan­ions is­n’t “ex­clu­sive,” said Shaul­sky, “but it’s a pref­er­ence.” That way, “they min­i­mize the risk that cells of their ge­net­ic si­m­i­lar­ity will die.” In the lab­o­r­a­to­ry, the sci­en­tists mixed cells from ge­net­ic­ally dis­tinct strains and found that they seg­re­gate in­to clus­ters of ge­net­ic­ally si­m­i­lar in­di­vid­u­als af­ter they’ve joined in­to ag­gre­gates.

Strass­man said the cells use mo­lec­u­lar mech­a­nisms to dis­tin­guish more and less re­lat­ed peers. Si­m­i­lar mech­a­nisms seem to op­er­ate in mul­ti­cel­lu­lar or­gan­isms like hu­mans, al­low­ing cells to rec­og­nize each oth­er as be­ing part of the body or al­ien. Dic­ty­os­te­lium is thus a mod­el for un­der­standing oth­er mul­ti­-cel­lu­lar or­gan­isms, said Eliz­a­beth Os­trow­ski, a post-doctoral re­searcher at Rice.

“Coopera­t­ion is one of the suc­cess sto­ries of the ev­o­lu­tion of life,” said Strass­mann. “Part of that suc­cess in­volves al­low­ing coop­era­t­ion in a way that con­trols con­flict. One of the best ways to con­trol con­flict is co­op­er­at­ing with ge­net­ic­ally si­m­i­lar in­di­vid­u­als.”

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When in need, some amoebae seek the support of kin, researchers say. The starving microbes seek each other out and form groups in which some sacrifice themselves. As a result of their deeds, others are able to travel some distance away, where they may find more food. Although this phenomenon has been long documented, it wasn’t clear the extent to which these associations are based on relatedness, according to the scientists, from Baylor College of Medicine and Rice University in Houston “These single cells aggregate based on genetic similarity,” though “not true kinship,” said research team member Gad Shaulsky, a geneticist at Baylor. The process nonetheless demonstrates a discrimination between “self” and “non-self” similar to that seen in immune system cells of higher organisms, he added. The study appears online Nov. 24 in the online research journal PloS Biology. The amoeba Dictyostelium discoideum starts life as a single-celled organism. As long as it has ample food and a pleasant environment, it’s happy to stay that way. But when food is scarce, the creatures band together and form what’s effectively a multi-cellular organism. The cells then sort themselves into two roles. Some cells become spores, which can survive and reproduce. Others perish, and in the process form an inanimate stalk that holds the live spores aloft. This helps the spores to disperse far away—far enough to reach new surroundings with, possibly, new food opportunities. Research into these microbial groupings, called aggregates, helps scientists understand biofilms—tenacious colonies of bacteria or fungi that can harm humans and other mammals, said Joan Strassmann of Rice University, another of the researchers. For example, people with cystic fibrosis are vulnerable to the formation of biofilms that can damage the lungs. In previous work, Strassman and collaborators found that some Dictyostelium cells “cheat” the system. They consistently avoid the biochemical chain of events that leads to death, and instead become spores. In the new study, the researchers found that by banding together based on genetic similarity, amoebae reduce the benefit of cheating. That’s because even though some cells will die, they get to propagate some of their genes anyway, because the the cells that do live are relatives. The preference for related companions isn’t “exclusive,” said Shaulsky, “but it’s a preference.” That way, “they minimize the risk that cells of their genetic similarity will die.’ In the laboratory, the scientists mixed cells from genetically distinct strains and found that they segregate into clusters of genetically similar individuals after they’ve joined into aggregates. Strassman said the cells use molecular mechanisms to distinguish more and less related peers. Similar mechanisms seem to operate in multicellular organisms like humans, allowing cells to recognize each other as being part of the body or alien. Dictyostelium is thus a model for understanding other multi-cellular organisms, said Elizabeth Ostrowski, a post-doctoral researcher at Rice. “Cooperation is one of the success stories of the evolution of life,” said Strassmann. “Part of that success involves allowing cooperation in a way that controls conflict. One of the best ways to control conflict is cooperating with genetically similar individuals.”