FRO SINGLE CELLS TO MULTICELLULAR LIFE
All multicellular
creatures are descended from single-celled organisms. The leap from
unicellularity to multicellularity is possible only if the originally
independent cells collaborate. So-called cheating cells that exploit the
cooperation of others are considered a major obstacle. Scientists at the Max
Planck Institute for Evolutionary Biology in Plön, Germany, together with
researchers from New Zealand and the USA, have observed in real time the
evolution of simple self-reproducing groups of cells from previously individual
cells. The nascent organisms are comprised of a single tissue dedicated to
acquiring oxygen, but this tissue also generates cells that are the seeds of
future generations: a reproductive division of labour. Intriguingly, the cells
that serve as a germ line were derived from cheating cells whose destructive
effects were tamed by integration into a life cycle that allowed groups to
reproduce. The life cycle turned out to be a spectacular gift to evolution.
Rather than working directly on cells, evolution was able to work on a
developmental programme that eventually merged cells into a single organism.
When this happened groups began to prosper with the once free-living cells
coming to work for the good of the whole.
Single bacterial cells
of Pseudomonas fluorescensusually live independently of each other.
However, some mutations allow cells to produce adhesive glues that cause cells
to remain stuck together after cell division. Under appropriate ecological
conditions, the cellular assemblies can be favoured by natural selection,
despite a cost to individual cells that produce the glues. When Pseudomonas fluorescens is
grown in unshaken test tubes the cellular collectives prosper because they form
mats at the surface of liquids where the cells gain access to oxygen that is
otherwise -- in the liquid -- unavailable.
Given both costs
associated with production of adhesive substances and benefits that accrue to
the collective, natural selection is expected to favour types that no longer
produce costly glues, but take advantage of the mat to support their own rapid
growth. Such types are often referred to as cheats because they take advantage
of the community effort while paying none of the costs. Cheats arise in the
authors' experimental populations and bring about collapse of the mats. The
mats fail when cheats prosper: cheats obtain an abundance of oxygen, but
contribute no glue to keep the mat from disintegrating -- the mats eventually
break and fall to the bottom where they are starved of oxygen.
Paul Rainey, who led
the study at the New Zealand Institute for Advanced Study and the Max Planck
Institute for Evolutionary Biology, explains: "Simple cooperating groups
-- like the mats that interest us -- stand as one possible origin of
multicellular life, but no sooner do the mats arise, than they fail: the same
process that ensures their success -- natural selection -- , ensures their
demise." But even more problematic is that groups, once extant, must have
some means of reproducing themselves, else they are of little evolutionary
consequence.
Pondering this problem
led Rainey to an ingenious solution. What if cheats could act as seeds -- a
germ line -- for the next set of mats: while cheats destroy the mats, what
about the possibility that they might also stand as their saviour? "It's
just a matter of perspective," argues Rainey. The idea is beautifully
simple, but counter-intuitive. Nonetheless, it offers potential solutions to
profound problems such as the origins of reproduction, the soma / germ
distinction -- even the origin of development itself.
In their experiments the
researchers compared how two different life cycles affected group (mat)
evolution. In the first, the mats were allowed to reproduce via a two-phase
life cycle in which mats gave rise to mat offspring via cheater cells that
functioned as a kind of germ line. In the second, cheats were purged and mats
reproduced by fragmentation. "The viability of the resulting bacterial
mats, that is, their biological fitness, improved under both scenarios,
provided we allowed mats to compete with each other," explains Katrin
Hammerschmidt of the New Zealand Institute for Advanced Study.
Surprisingly however,
the researchers found that when cheats were part of the life cycle, the fitness
of cellular collectives decoupled from that of the individual cells: that is,
the most fit mats consisted of cells with relatively low individual fitness.
"The selfish interests of individual cells in these collectives appear to
have been conquered by natural selection working at the level of mats:
individual cells ended up working for the common good. The resulting mats were
thus more than a casual association of multiple cells. Instead, they developed
into a new kind of biological entity -- a multicellular organism whose fitness
can no longer be explained by the fitness of the individual cells that comprise
the collective" says Rainey.
"Life cycles
consisting of two phases are surprisingly similar to the life cycles of most
multicellular organisms that we know today. It is even possible that germ-line
cells, i.e. egg and sperm cells, may have emerged during the course of
evolution from such selfish cheating cells," says Rainey.
Comments
Post a Comment