Evolution__3rd_Edition

562 PART 5 / Macroevolution
Superfluous DNA can be deleted ...
. . . but the efficiency of deletions
varies
feature: massive gene loss and genome reduction. For example, the bacterium
Buchnera lives symbiotically in the cells of aphids. Buchnera is descended from the
group of enteric bacteria that includes Escherichia coli. E. coli has over 4,000 genes, but
the common ancestor of the enteric bacteria probably had slightly less than 3,000 genes.
Buchnera has only 590 genes: it has lost about 80% of its ancestor’s genome. A gene loss
originates as a deletion mutation, which may then spread by drift or selection. Many of
the deletions in intracellular bacteria could have spread by drift. The environment
inside the host cell contains many of the nutrients and defense systems that a bacterial
cell needs. The resources are provided by the host, and natural selection on some of the
genes in intracellular bacteria will be relaxed. Genes that are needed in a free-living
bacterium to provide the resources that are present in the host cell are not needed in an
intracellular bacterium. Alternatively, the gene loss may be positively advantageous. In
general, a cell with less DNA can reproduce faster. Natural selection may favor gene
reduction for this reason. (Box 19.1 discusses a medically interesting example. Yet
another dramatic example of gene loss in an intracellular bacterium is provided by
mitochondria. We look at mitochondria in the next section.)
Stretches of DNA are lost by deletion events in all species at a certain rate. Some of
the non-coding DNA, for instance, is deleted from time to time, perhaps because
copying it is burdensome. Differences between species in the rate of deletion of noncoding
DNA may help to explain why some species have smaller genomes than others.
Crickets in the genus Laupala have a genome size over 10 times larger than the fruitfly
(Drosophila). Petrov et al. (2000) estimated the evolutionary rate of deletions in noncoding
DNA from the two taxa. They found that DNA is deleted 40 times faster from
fruitflies over evolutionary time than from crickets. Part of the explanation must be
that when non-coding DNA arises in fruitflies it is more likely to be deleted. Natural
selection discriminates more against fruitflies with non-coding DNA than against
crickets with non-coding DNA. Why this should be is a question for the future. But in
Box 19.1
Genome Reduction in Human Pathogens
Parasites tend to have reduced genomes,
and intracellular parasites have
particularly reduced genomes. Shigella
is closely related to Escherichia coli.
Indeed Shigella strains appear to evolve
repeatedly and convergently from
E. coli ancestors. Therefore, Shigella may
not be a proper taxonomic term and we
shoud refer to “shigella” strains within
the species E. coli (Pupo et al. 2000).
Escherichia coli is a normally benign
inhabitant of our guts, but certain
strains of Shigella cause dysentery.
During the evolution of Shigella, certain
genes have been lost. For instance,
the strains of Shigella that cause dysentery
lack a gene (called ompT) that is
present in benign strains of E. coli. The
gene ompT can be experimentally
introduced into Shigella, and has the
effect of reducing the rate at which
Shigella spreads between host cells. The
experimental results suggests that natural
selection positively favored the loss
of ompT in the origin of these Shigella
strains. The loss of ompT was advantageous
not simply because it made the
DNA more economic, but because the
gene somehow increased the efficiency
of cell infection. A knowledge of genome
evolution in these bacteria provides
useful clues for understanding their
pathogenicity.
Further reading: Ochman & Moran
(2001).
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