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032408-1[1] - Evolution Stooping to conquer How to become...

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Unformatted text preview: Evolution Stooping to conquer How to become multioellular IEWED from humanity‘s lofty heights. single-celled creatures are the scum of the earth. In reality. though. almost all liv- ing things are unicellular. and with good reason. Multicelluiarity requires most of the cells in a body to make the supreme Darwinian sacrifice. by giving up re- producing. This helps the few cells special- ised for reproduction to do so more effec- tively. Since the self-sacrificing cells have the same genes as the specialised re- producers they are. in effect, reproducing collaterally. But it is still a hard trick to pull ofl‘. and it has not happened often. One creature that has managed the trick—separately from plants. animals and fungi. who are the real experts in the field—is an alga called Volvox corteri. An adult Volvox consists of around 2,000 body cells, whosejob is to move the organ- ism around using their flagella, and 16 cells capable of reproducing. A perfect example of division of labour Clearly. Volvox evolved from similar al- gae that exist only as single cells, but until now the genetics of the process have been obscure. However. as they report in Molec- ular Biology and Evolution. Aurora Nedelcu of the University of New answick and Richard Michod of the University of Ari- zona think they have worked out what happened. In doing so, they have shed light on the type of process that eventually resulted in human beings. The gene that stops the body cells of Volvox reproducing is called regA. It works by suppressing the production of proteins needed to make new chloroplasts in a cell. Chloroplasts are the structures in which photosynthesis happens. Without an ade- quate supply of them a cell cannot grow big enough to divide. What Dr Nedelcu and Dr Michod did was to look for an antecedent of regA in a single-celled creature, in order to find out what its job was. By searching for genetic sequences similar to regA in the burgeon- ing databases of genes that now exist. they found one in a unicellular alga called Chlamydomonus reinhardtii. In Chlomydo- moans, the gene only gets switched on when environmental conditions are poor-for instance. when sunlight or nutri- ents are scarce. That keeps Chlamydomo- nus from wasting resources during hard times. increasing its likelihood of re- producing over the long haul. Apparently, a mutation in this gene-or. more probably. in the control system that activates it—gave rise to Volvox. When that happened, individual cells could turn their reproductive capacity on or off according to the function they served in Vohvox’s body. Though this often doomed an individual cell‘s own genes. it gave rise to a co-operative organism that successfully passed those genes on indirectly. What genetic change allowed multicel- lular animals to come into existence re- mains obscure. But Volvox shows the sort of thing to look for: a gene that stops re- production in single-celled creatures and has been co-opted to do a new job. I ...
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