Biology Reference
In-Depth Information
to the tree diagram then? The three-domain tree also supposes that eukaryotes arose
from prokaryotes in some way, so we need to ask how this might have happened. Did
some prokaryotes develop eukaryotic features on their own or did some prokaryotic
cells fuse together to produce the larger, more complex type of cell?
Lateral Gene Transfer
Evidence has accumulated since the 1970s for a widespread process by which genes
are transferred between unrelated organisms. This process is called lateral gene
transfer (abbreviated to LGT), also known as horizontal gene transfer (abbreviated
to HGT).
Lateral gene transfer (LGT) is defined as any process in which an organism incor-
porates genetic material from another organism without being the direct descendent
of that organism. All definitions of the term “species” assume that an organism gets
all its genes from one or two parents which are very like that organism, but the occur-
rence of lateral gene transfer makes this assumption false in some cases, especially
in prokaryotes.
LGT was first described by Japanese scientists in 1959, when it was found that
resistance to some antibiotics was transferred between different species of bacteria,
but the significance of this phenomenon was not appreciated by Western scientists
until the 1970s. You will be familiar from media reports with the current prob-
lem of the transfer of antibiotic resistance between harmless bacteria and those
that that cause life-threatening conditions in humans, especially in hospital envi-
ronments. We now know that LGT is common amongst bacteria, including ones
that are only distantly related, and several different mechanisms have been discov-
ered. The simplest mechanism is called
transformation,
where bacteria and archaea
take up foreign DNA from the environment. Most of this DNA is degraded within
the cell, but some becomes incorporated into the host chromosome and may confer
new properties, such as resistance to antibiotics, on the cell that enhance survival
in certain environments. A different mechanism is called
transduction,
and involves
transfer of DNA by viruses that infect bacterial cells but do not destroy them. A third
process involves direct contact between a cell that donates some DNA and a different
cell that receives this DNA, and is called
conjugation
. LTG has also been observed
between some bacteria and some archaea, between some bacteria and some fungi,
and between some bacteria and some unicellular eukaryotes.
In multicellular eukaryotes, the major cause of LGT is the phenomenon of
endosymbiosis
. Endosymbiosis is now defined generally as one type of cell living
inside a cell from an unrelated species without harming it, but was initially defined
as the evolutionary process by which the plastids and mitochondria of eukaryotic
cells originated from free-living bacteria that were taken up by other cells. The
engulfed bacteria then evolved into symbiotic relationships within the host cell to
form plastids and mitochondria that are now totally dependent on their host cell
for their continued existence, because most of the genes for the proteins in these
organelles have been laterally transferred to the nucleus. “Plastid” is a general term
