Monday, November 10, 2014

All for One and One for All - How Multicellular Life Began

Petri dishes growing various strains of Pseudomonas fluorescens bacteria. Researchers were able to watch individual cells evolve into multicellular entities capable of self-reproduction. Photo: Gayle Ferguson, Max Planck Institute for Evolutionary Biology, Plön, Germany.
Genetics studies show that multicellular life evolved from single-celled creatures. This required formerly independent cells to cooperate - but what might have induced them to override self interest and cooperate in the first place?

By integrating bacteria into a special two-phase life cycle, international researchers observed and recorded individual cells evolving into self-reproducing, oxygen-acquiring tissue - one that engages in a division of labor, generating reproductive cells.

Such life cycles were originally "...a spectacular gift to evolution. A developmental program evolved which merged these cells into a single organism - benefiting groups, as formerly independent cells came to work for the good of the many", says Max Planck Institute's Dr. Paul Rainey.

Pseudomonas fluorescens bacteria usually live singly, but some mutations result in cells which excrete adhesive glycoproteins - gluelike carbohydrate-protein compounds. These cadherins can attach cells to one another after they have divided. The resulting cellular assemblies are better able to survive under certain environmental conditions, despite the biologically expensive cost to the individual cells excreting the cellular glue.

In this experiment, Pseudomonas fluorescens was cultured in unagitated petri dishes, where oxygen would be most abundant at the surface. Cell colonies were best able to benefit from this condition by forming mats which floated upon the liquid surface.

The logical problem with this adaptation is that natural selection should favor "cheater cells" - those which don't produce biologically expensive adherons, but instead exploit the already-existing mat surface to promote their own quick growth.

In biology, cheaters are individuals which take advantage of communal efforts without expending any efforts themselves. In this case, however, the success of cheater cells means the defeat of the population as a whole: an abundance of cells which don't produce adherons clinging to a mat will cause it to collapse and fall to the bottom of the solution, where the entire community then suffers from oxygen deprivation.

This means, says Dr. Rainey, "No sooner do the mats arise, than they fail: the same process that ensures their success - natural selection - ensures their demise."

To find a workaround, his team set up differing life cycles between two bacterial colonies. The first colony experienced a two-phase life cycle, with cheater cells functioning as germ line cells - specialized for reproduction. Such two-phase life cycles are similar to those among most modern multicellular organisms.

The second group was purged of cheater cells, and the mats grew through fragmentation.

Interestingly, both populations succeeded and increased when they were allowed to compete with one another in the same petri dishes.

What's more, cheaters in the two-phase populations grew to sacrifice their individual fitness in the interest of the whole, reproducing at a lower rate. This meant natural selection had begun favoring collective cells over individual ones, which had begun "working for the common good".

This means the two-phase population had evolved into a new multicellular entity whose hardiness was no longer due to the fitness of individual cells within the collective.

In addition to solving a long-standing evolutionary mystery, these experiments may eventually help reveal the origins of soma (diploid, standard cells) vs. germ (haploid, reproductive egg and sperm) cells, according to Dr. Rainey.

Pseudomonas fluorescens are a group of common, non-pathogenic (non-disease-causing) saprophytes (organisms like fungi which feed upon dead organic matter) which colonize soil, water and plant surfaces. The name "fluorescens" refers to the greenish glow the bacterium produces. It moves about by means of multiple whip-like "flagella" and subsists upon mineral salts and carbon.

Because many varieties of P. fluorescens protect plant seeds and roots against fungal infection,  it is being extensively investigated for agricultural uses. Experiments are also underway to exploit its ability to partially or completely degrade environmental pollutants such as styrene, TNT and PAHs (polycyclic aromatic hydrocarbons), fossil fuel pollutants known to be carcinogenic.

Sources: "From single cells to multicellular life: Researchers capture the emergence of multicellular life in real-time experiments, press release, Max-Planck-Gesellschaft, November 5, 2014.;

"Pseudomonas fluorescens", MicrobeWiki, 2014

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