Encyclopedia of Evolution.pdf - Online Reading Center

horizontal gene transfer
out to be very important indeed: Antibiotic-resistant bacteria
can make other bacteria, even of a different species, resistant
to the antibiotic (see resistance, evolution of). This is how
antibiotic resistance can spread so rapidly, even from one species
of bacterium to another. Recent research suggests some
pathogenic bacteria have obtained resistance genes from bacteria
that live in the soil. Horizontal gene transfer has turned
out to be one of the central facts in public health research
(see evolutionary medicine). It is widespread enough that
one prominent evolutionary biologist (see Margulis, Lynn)
has claimed that there is really only one species of bacterium:
According to the biological species concept, potentially interbreeding
organisms can, if they produce fertile offspring, be
considered members of the same species.
When biologists began to determine the nucleotide
sequences of different species of bacteria, they were surprised
by a further discovery: Horizontal gene transfer has affected
a large portion of the DNA of many bacteria. In the gut bacterium
Escherichia coli, one of the most common bacteria
used in education and research, 755 of the 4,288 genes (18
percent) have resulted from 234 horizontal gene transfers
during the past 200 million years. This fact was not known
until the exact sequence of nucleotides could be determined
and compared with those of other bacterial species. Horizontal
gene transfer occurs mostly between bacteria that are
closely related to one another (see tree of life). However, it
can even occur between bacteria and archaebacteria. Horizontal
gene transfer allows rapid evolutionary response to
new environmental conditions. For example, some species of
bacteria are able to metabolize and destroy naphthalene, the
main ingredient of mothballs, and other species of bacteria
cannot. When both kinds of bacteria are mixed and exposed
to naphthalene, within 24 hours the first species confers this
genetic ability to the second species. It does not need to happen
very often; just a single horizontal gene transfer creates a
new variant of the second species, the populations of which
can then explode.
Viruses can also participate in horizontal gene transfer.
Sometimes, when two kinds of virus inhabit the same host
cell, they exchange genetic material. This is how human
influenza viruses can obtain new genes from the influenza
viruses of other species, and why new influenza vaccines are
needed each year. Sometimes a virus, when leaving a host
chromosome to travel elsewhere, can take some host DNA
with it.
Horizontal gene transfer is not limited to viruses and
bacteria, although microbes frequently aid in the process. As
noted above, viruses can take some of their host DNA with
them as they leave, and put it into the chromosome of a new
host. Since some viruses (like influenza viruses) have more
than one species of host, they may transfer genes from one
host species to another. One species of bacterium, Agrobacterium
tumifaciens, infects plants and causes a swelling known
as crown gall on the stems of several kinds of plants. It has a
plasmid that inserts into the chromosomes of its host plant. If
the plasmid carries the gene from one species of plant host, it
can insert this gene into another species of plant host.
There is even some evidence that RNA molecules can
travel through the phloem tissue of plants, which contains
the vessels that carry sugar solution in plant stems. It is conceivable
that RNA molecules from one species of plant can
transfer directly to the cells of another species of plant, so
long as the two plants are in direct phloem contact. This is
the situation with some parasitic plants (see coevolution).
Perhaps the ultimate plant parasite is Rafflesia, a Sumatran
plant that has lost its leaves, stems, roots, and chlorophyll
and lives inside the stems of Tetrastigma, a tropical grape
vine. Rafflesia looks like strands of fungus until it produces
a flower: the biggest flower in the world, nearly three feet (a
meter) across, and smelling like rotten flesh, which attracts
flies as pollinators. For most of its life, Rafflesia soaks up
whatever organic molecules are in the phloem of its host
plant, and this apparently includes RNA. If retroviruses
are present (as they commonly are in cells), the RNA can
be reverse transcribed into DNA which can insert into the
host chromosomes. This is what HIV, a retrovirus, does to
human hosts (see AIDS, evolution of). In this situation,
it might not surprise scientists that Rafflesia might get new
genes, not by evolving them itself but by acquiring them
from its host.
New research indicates that when the mitochondrial
matR gene, the 18S ribosomal gene, or the PHYC gene is
used in a phylogenetic analysis (see cladistics), Rafflesia is
found to be closely related to other plants in the order Malphigiales.
However, if the mitochondrial nad1B-C gene is
used, Rafflesia is found to be closely related to grapevines.
The anomalous result of the cladistic analysis using nad1B-C
is best explained by saying that the nad1B-C gene, perhaps
in the form of an RNA transcript, transferred from the Tetrastigma
grapevine host to the Rafflesia parasite through the
phloem. The host gene can then spread from the first parasite
to many other parasites in the population, through normal
sexual reproduction. Other research suggests that certain
ferns obtained some of their mitochondrial DNA from flowering
plants parasitic upon them.
Since Rafflesia lives only in Tetrastigma, the host genes
will never go further than those two species. If a similar phenomenon
occurs in a multi-host plant parasite like Cuscuta,
might the parasite facilitate the transfer, however rarely,
from one host species to another? Might parasites that act
as disease vectors transfer genes from one species of animal
to another? This is unlikely, because if a mosquito injected
nucleic acid from another mammal species into a person’s
blood, the nucleic acid would probably be destroyed by the
host’s immune system.
There are some anomalies that have always puzzled
geneticists. For example, there are some animals that produce
cellulose. Cellulose is a major component of plant cell walls
(and of cotton) and is assumed to be found only in plant and
bacterial cells. But it is also the major component of the outer
layer (the tunic) of tunicates, which are among the closest
invertebrate relatives of the vertebrates (see invertebrates,
evolution of). An ancestor of these animals may have
obtained the gene or genes for making cellulose by horizontal