Evolution of individuality

The key to understanding some of biology’s most profound problems – the origins of multicellularity; the origins of soma/germ differentiation, of reproduction, of development, even of cancer – could be contained in a better understanding the emergence of primordial life cycles.

 

Led by Distinguished Professor Paul Rainey, a team of scientists are looking for that understanding. They are investigating the transition from single cells to cooperative groups, with a particular focus on the emergence of life cycles that allow collectives to leave offspring. 

Today the biological world around us is dominated by multicellular plants and animals. All of these intricate forms have evolved from far simpler, single celled ancestors.

The group are using simple cooperating lineages of bacteria and propagating them under regimes that reward collective-level fecundity. 

Beginning with single cells, the research has shown how simple cooperating groups of bacteria can reproduce via a life cycle that incorporates ‘cheating’ cells as a primitive germ line.

Cheats are cells that do not contribute to the integrity of the group, but still take advantage of the benefits of being part of a collective. 

In one set of experiments the possibility was explored that cheating cells may serve as a primitive germ line allowing reproduction of groups. After a period of evolution, groups evolved whose fitness became decoupled from that of the lower level cells that comprise the groups.

The research, reported in the science journal Nature in 2014, involvedjoint first authors of the paper, Dr Caroline Rose and Dr Katrin Hammerschmidt performing painstaking experiments over the course of five years in which they tested the idea that cheats might play a constructive role in evolution. They allowed simple microbial groups to evolve via a life cycle in which cheats were either embraced, or purged.

“When cheats were embraced we discovered something surprising,” Dr Rose says. “Evolution saw a new kind of entity — a group comprised of two different cell states: cheating and cooperating cells. Evolution couldn’t focus on just one state or the other; for lineages to persist, evolution had to see both types — it had to work on a developmental programme.”

Dr Hammerschmidt explains: “When this happened, the groups became better adapted, but they did so at the expense of the individual cells that made up the groups. This might seem nonsensical, but it is precisely what is thought to happen during major evolutionary transitions: the higher (group) level subsumes the lower (cell) level, with the lower level eventually coming to work for the good of the collective. Nothing so remarkable happened when we performed the same experiment, but with a life cycle in which we got rid of cheats.”

“Little is known”, explains Professor Rainey, “but life cycles involving at least two different states are almost universal in the world of multicellular organisms. I suspect that this is because multiphase life cycles generate an organismal configuration that delivers to natural selection a machine-like entity with which it can really work.

Original publication
: Hammerschmidt, K., Rose, C. J., Kerr, B. & Rainey, P. B. (2014). Life cycles, fitness decoupling and the evolution of multicellularity.  Nature xx 2014 doi:10.1038/nature13884

http://www.nature.com/nature/journal/v515/n7525/full/nature13884.html

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