ONE of the more surprising discoveries made by gene mapping and sequencing
projects has been that most genes seem to be utterly useless. But these
鈥渘onessential鈥� genes do contribute to an organism鈥檚 wellbeing, though in little
ways that may prove difficult to spot, say researchers in Utah.
快猫短视频s usually decide how a gene works by knocking it out or disabling
it, and watching to see what happens. Some mutations will kill the organism
during early development, and others can make it less successful in stressful
situations. But most random mutations do not have perceptible effects at
all.
The most widely accepted explanation is that these genes protect against some
unknown stress not found in the lab, and that their functions would be revealed
if an organism ever encountered that problem. But now researchers have found
that mutant yeast strains that have no obvious defects are actually at a
disadvantage鈥攅ven when they are in stress-free, nutrient-rich
environments.
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Joe Dickinson and his colleagues at the University of Utah in Salt Lake City
took a strain of yeast and generated mutant lines by randomly inserting foreign
bits of DNA into the genome. The organisms died if the mutation was in a gene
essential for life, because the yeast strain carried only one set of chromosomes
and so had no backup set of genes.
The strains that did survive were tested in a variety of stressful
conditions, including various different temperatures, to identify and eliminate
mutants with obvious handicaps. This process left 27 strains that appeared to
function exactly like the original strain in all the lab tests.
The researchers then placed a nutrient-rich medium in test tubes, and put
equal amounts of the parent yeast and a mutant strain into each. During two days
of incubation, spanning approximately 75 to 150 generations, they periodically
checked to see if the ratio of the parent strain to the mutant had changed.
They found that in 19 cases, the parent dominated the mutants (
Proceedings of the National Academy of Sciences, vol 95, p 253). 鈥淲hen we
do evolution in miniature, slowly but persistently the mutant strain loses out,鈥�
says Dickinson.
These results support the idea that many genes persist throughout evolution
because they carry a very slight competitive edge. Dickinson compares the
nonessential genes to a car component that doesn鈥檛 seem vital. 鈥淭here鈥檚 no way
that you can disable the car鈥檚 operation by breaking the petrol gauge, but in a
long endurance test, which is what evolution is, the car with the petrol gauge
will perform a little better,鈥� he says.
Although these results would be expected, it鈥檚 good have confirmation from
experiments, says Michael Snyder, a yeast genome researcher at Yale University
in New Haven, Connecticut. Snyder agrees that 鈥渆ssential is overrated鈥�.
快猫短视频s analysing the masses of data from gene sequencing projects are in for
a tough time, he adds, because most genes make only a minor contribution that鈥檚
very difficult to detect. 鈥淲e still need all the assays that are available and we鈥檒l need a
heck of a lot more.鈥�