Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
e ISSN 2277 - 3290
Print ISSN 2277 - 3282
Journal of Science
Plant Biotechnology
www.journalofscience.net
HORIZONTAL GENE TRANSFER IN PLANTS
M. Amaranatha Reddy*1, T. Rajeswar reddy2 and T.L. Dheemanth3
1
Ph.D Scholar, Dept. of Plant Breeding and Genetics, COH, KAU, Vellanikkara, Thrissur, Kerala, India.
2
Ph.D Scholar, Dept. of Plant Pathology, COH, KAU, Vellanikkara, Thrissur, Kerala, India.
3
M.Sc. (Ag), Dept. of Plant Biotechnology, COH, KAU, Vellanikkara, Thrissur, Kerala, India.
ABSTRACT
The transfer of genetic material from parent to offspring via reproduction is called vertical gene transfer. Horizontal
gene transfer (HGT) is also referred as lateral gene transfer (LGT), relates to the process in which transfer of genetic material
between cells or genomes belonging to unrelated species, occur by processes other than usual reproduction. Since HGT is an
asexual process, it is not restricted by species boundaries and organisms as diverse as prokaryote/eukaryote can engage in the
horizontal exchange of genetic information. In many prokaryotes, HGT has contributed 10 to 20 percent of the genes. It has
often critically influenced prokaryotic evolution, leading to acquisition or modification of important traits such as antibiotic
resistance, virulence, photosynthesis and nitrogen fixation. Though HGT occurs much less frequently in eukaryotes it is not
merely a rare evolutionary accident or curiosity, but rather a highly significant process in eukaryotic genome evolution as
evidenced in chloroplast genome of plants. Among the eukaryotes, unicellular ones generally experience the HGT.In plants,
the first reported eukaryote to eukaryote nuclear HGT is related to transposons.Another case of plant HGT was detected in
the parasitic plant Strigahermonthica that infests monocots such as sorghum and rice. During pathogenesis, Agrobacterium
transforms its host with several plasmid-encoded genes, with HGT as a natural consequence. Other examples include the
acquisition of aquaglyceroporins from a Eubacterium 1200 million years ago and glutathione biosynthesis genes from a
Proteobacterium. Plant mitochondrial genomes experience the most frequent and evolutionarily widespread horizontal
transfer of genes acquired from other eukaryotes. Horizontal transfer of genes among tobacco species has been demonstrated
through in vitro graft experiments. The mechanism of HGT is not well understood. However, it can increase genetic diversity
and promote the novel adaptations in living organism.
Keywords: HGT, Striga hermonthica, Agrobacterium, Eubacterium.
INTRODUCTION
Life! It's everywhere on Earth. We can find
living organisms from the poles to the equator, from the
bottom of the sea to several miles in the air, from freezing
waters to dry valleys, from undersea thermal vents to
groundwater thousands of feet below the Earth's surface.
Over the last 3.7 billion years, living organisms on the
Earth have diversified and adapted to almost every
environment imaginable. The diversity of life is truly
amazing, but all living organisms do share certain
similarities. All living organisms can replicate and the
replicator molecule is DNA. Until comparatively recently,
living organisms were divided into two kingdoms: animal
and vegetable or the Animalia and the Plantae. In the
19thcentury, evidence began to accumulate that these were
insufficient to express the diversity of life, and various
schemes were proposed with three, four or more
kingdoms. The scheme most often used currently divides
all living organisms into five kingdoms: Monera
(bacteria), Protista, Fungi, Plantae, and Animalia. This
coexisted with a scheme dividing life into two main
divisions: the Prokaryotae (bacteria etc.) and the
Eukaryotae (animals, plants, fungi, and protists).
Recent work, however, has shown that what
were once called "prokaryotes" are far more diverse than
anyone had suspected. The Prokaryotae are now divided
into two domains, the Bacteria and the Archaeaas
Corresponding Author:-M. Amaranatha Reddy Email:[email protected]
591
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
different from each other as either is from the Eukaryota
or eukaryotes. This tree, like all phylogenetic trees, is
a hypothesis about the relationships among organisms. It
illustrates the idea that all of life is related and can be
divided into three major clades, often referred to as the
three domains: Archaea, Bacteria, and Eukaryota.
Archaea
are
very
ancient prokaryotic microbes.
Eubacteria are more advanced prokaryotic microbes.
Eukaryota are all life forms with eukaryotic
cells including plants and animals.
All living organisms do share certain similarities.
All living organisms can replicate, and the replicator
molecule is DNA. DNA the genetic material of all living
organism, can be transferred from one organism to
another and the process of transfer of genetic material
from one organism to another organism is referred as
Vertical gene transfer. Thus, vertical gene transfer is
the normal mode in which DNA is shared among
individuals and passed on to the following
generations.
DNA can
however,
also
more
infrequently spread to unrelated species through a
process called Horizontal gene transfer (HGT).
Horizontal gene transfer
HGT, sometimes also called lateral gene
transfer, occurs independently of normal sexual
reproduction and is more common among single-celled
organisms such as bacteria. HGT is a one-way transfer
of a limited amount of DNA from a donor
cell/organism into single recipient cells. Horizontal gene
transfer (HGT), the transfer of genes between non-mating
species, is remarkably common and important in
prokaryotes.
HGT has contributed 10–20% of the genes in
Prokaryotes [1].
HGT often critically influences
prokaryotic evolution, leading to acquisition or
modification of such important traits as antibiotic
resistance, virulence, photosynthesis, and nitrogen
fixation. Some authors suggest that HGT „may be the
dominant force [in prokaryotic evolution], affecting most
genes in most prokaryotes‟.
With very rare exception, HGT occurs much less
frequently in eukaryotes than in bacteria, although the
process may have been more common early in eukaryotic
evolution. Several groups have inferred that the
eukaryotic nuclear genome derives from HGT through the
fusion of archaebacterial and eubacterial genomes but this
interpretation has been called into question. Following the
endosymbiotic origin of mitochondria and chloroplasts,
many genes of eubacterial origin migrated to the nucleus
from these organelles via intracellular gene transfer
(IGT). Functional IGT from the mitochondrial genome
has, based on current evidence, entirely ceased in animals
and virtually ceased in fungi. In contrast, it occurs
relatively frequently in flowering plants.
Among the eukaryotes, unicellular eukaryotes
generally experience the most HGT, because they lack a
sequestered germline and because they often engulf their
prey releasing DNA near the nucleus. Most of the foreign
genes detected in these protists were acquired from
bacterial donors. Although frequent in eukaryotic terms
the amount of HGT in unicellular eukaryotes ranges from
a single gene to several dozen accounting for <1% of the
genome. Nuclear HGT is rare in multicellular eukaryotes
(animals, fungi, and plants). Nearly all known cases
involve bacteria as donors.
Discovery of HGT
The discovery of horizontal gene transfer (HGT)
can be traced back to 1928 when Fred Griffith reported
the transfer of genetic material from heat-killed virulent
Streptococcus pneumoniae to an avirulent form of the
bacterium by a process he described as transformation. It
wasn‟t until 1946 that other forms of non-reproductive
gene transfer between organisms were identified and
variously described as conjugation, transduction,
recombination, rearrangement etc. Since the 1980s these
different examples of gene transfer have become known
collectively as either horizontal or lateral gene transfer.
Process of HGT
A donor cell (of any origin) can release DNA
which can persist in the environment. The subsequent
uptake of DNA fragments by exposed recipient cells
is called HGT. Such HGT can occur deliberately e.g.
by gene therapy in humans or genetic engineering of
plants. Bacteria have several processes that can
facilitate HGT, including transformation, conjugation
and transduction. Examples of HGT are the spread of
antibiotic resistance among bacterial species, gene
therapy in humans, and Agrobacterium-infection in
plants. HGT of recombinant DNA from genetically
modified organisms (GMOs) to bacteria is a potential
bio-safety concern.
Endosymbiotic Evolution and the Tree of Genomes
The host that acquired plastids probably
possessed two flagella. The nature of the host cell that
acquired the mitochondrion (lower right) is fiercely
debated among cell evolutionists. The host is generally
accepted by most to have an affinity to Archaebacteria but
beyond that, biologists cannot agree as to the nature of its
intracellular organization (prokaryotic, eukaryotic or
intermediate), its age, its biochemical lifestyle or how
many and what kind of genes it possessed. The host is
usually assumed to have been unicellular and to have
lacked mitochondria.
Endosymbiotic Theory
Symbiosis occurs when two different species
benefit from living and working together. When one
592
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
organism actually lives inside the other it's called
endosymbiosis [2]. The endosymbiosis theory postulates
that the mitochondria of eukaryotes evolved from aerobic
bacteria (probably related to the rickettsia‟s) living within
their host cell. The chloroplasts of red algae, green algae
and plants evolved from endosymbiotic cyanobacteria.
Both mitochondria and chloroplasts have their
own genome, and it resembles that of bacteria not that of
the nuclear genome. Both genomes consist of a single
circular
molecule
of
DNA.
There
are no histones associated with the DNA. A number of
antibiotics (e.g., streptomycin) that act by blocking
protein synthesis in bacteria also block protein synthesis
within mitochondria and chloroplasts. They do not
interfere with protein synthesis in the cytoplasm of the
eukaryotes.
The antibiotic rifamicin, which inhibits the RNA
polymerase of bacteria, also inhibits the RNA polymerase
within mitochondria. Over millions of years of evolution,
mitochondria and chloroplasts have become more
specialized and today they cannot live outside the cell
duplicative HGT and differential gene conversion (DHDC)-in which intra and inter-organellar gene transfer and
recombination are creative forces in the generation of
mitochondrial genetic diversity.
HGT in Eukaryotes
With very rare exception, HGT occurs much less
frequently in eukaryotes than in bacteria, although the
process may have been more common early in eukaryotic
evolution. Several groups have inferred that the
eukaryotic nuclear genome derives from HGT through the
fusion of archaebacterial and eubacterial genomes but this
interpretation has been called into question [3]. Following
the endosymbiotic origin of mitochondria and
chloroplasts, many genes of eubacterial origin migrated to
the nucleus from these organelles via intracellular gene
transfer (IGT). Functional IGT from the mitochondrial
genome has based on current evidence, entirely ceased in
animals and virtually ceased in fungi. In contrast, it
occurs relatively frequently in flowering plants.
HGT in different domains
During evolution, plants have served as both
sources and targets of horizontally transferred genes
(Andersson et al., 2003).
HGT from prokaryote to eukaryote
The examples of HGTs from prokaryotes to
multicellular eukaryotes are sparse. The best studied case
is Agrobacterium. This genus of bacteria is well known to
cause tumors in plants by genetically transforming the
genomes of targeted plants.
Not all strains of
Agrobacterium are virulent. The virulence comes from the
possession of tumor-inducing (Ti) or root-inducing (Ri)
plasmid. Ti plasmid contains a segment called transfer
DNA (T-DNA) which encodes the genes for transforming
the infected plants [4]. Ti plasmid also encodes virulence
factor genes (vir genes) whose products facilitate the
transfer and integration of T-DNA into plant genome. The
process of T-DNA transfer is to some extent similar to
conjugation. However only the single-stranded T-DNA
not the whole Ti plasmid is transferred into plant cells.
The transferred T-DNA is most likely integrated into
plant chromosomes through illegitimate recombination or
non-homologous end-joining. T-DNA genes are then
expressed in plant cells to produce enzymes synthesizing
plant hormones auxin and cytokinin for plant tumor
induction, and opines used by the Agrobacterium as
carbon and nitrogen sources HGTs between prokaryotes
and eukaryotes favor the direction from prokaryotes to
eukaryotes [5]. Eukaryote-to-prokaryote HGTs are rare.
On one hand, this phenomenon could be deceptive due to
the observation bias caused by the uneven distribution of
the amount of sequenced genomes between prokaryotes
and eukaryotes.
On the other hand, this phenomenon could also
be biologically true: introns are barriers for transferred
eukaryotic genes to be correctly expressed in prokaryotes;
eukaryotic genomes are less diverse to offer prokaryotes
fitness advantages there is a far larger population of
prokaryotes than the around eukaryotes in nature which
statistically favors prokaryotes as the potential donors of
the transferred genes [6].
HGT from eukaryote to eukaryote
Natural movement of genes between different
plant species and from other kingdoms (Monera, Fungi)
into plants occurs by gene transfer mediated by natural
agents such as microorganisms, parasites, epiphytes,
viruses or mites and by direct cell to cell transfer. Such
transfers occur at a frequency that is low compared with
the hybridization that occurs during pollination, but can
be frequent enough to be a significant factor in genetic
change of a chromosome on evolutionary time scales.
In plants, the first reported eukaryote-toeukaryote nuclear HGT is related to transposons. Some
Mu-like elements (MULEs) from genus Setaria are found
highly similar to a small family of MULEs from rice
genus Oryza. Setaria (Subfamily Panicoideae) and Oryza
(Subfamily Bambusoideae) are from family Poaceae [78].
Another case of plant HGT was detected in the
parasitic plant Strigahermonthica [9]. A gene named
ShContig9483 (function unknown) from the eudicot S.
hermonthica shows high similarity to genes in the
monocots sorghum and rice but has no homologs in other
eudicots. S. hermonthica is known to infest monocots
such as sorghum and rice by forming an invasive organ
haustorium, which interconnects their vasculature with
that of their hosts in order to extract nutrients, water, and
even sometimes accidently host mRNAs. The authors
593
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
thus suggested that: one possibility is that ShContig9483
was originally transferred to S. hermonthica as mRNA or
cDNA.
For inter-genomic MGEs, phylogenetic methods
are nowadays the most promising ways to identify HGTs.
The key of this type of approach is to find the
phylogenetic incongruence between the (mostly) rRNAbased taxonomic phylogeny and the phylogeny of
interested gene. This is the most robust method for
ancient HGT detection. Bias will appear in this type of
approach due to data incompleteness, when comparing to
the relative abundance of rRNA databases, of interested
genes, or due to incorrect construction of phylogenetic
tree. Like for transposon detection, nucleotide
composition patterns are also used to identify HGTs
because of the existence of the pattern differences
between donor and receipt sequences of HGTs. These
patterns include base composition, G+C content, purineto-pyrimidine ratio, oligonucleotide frequency and codon
usage. However nucleotide composition intra-genome
regional variation and pattern difference reduction of
ancient HGTs will cause difficulties in detection.
Homology search suchas BLAST is the simplest
and fastest way to detect HGTs. The drawback of this
approach is being crude and inaccurate. Sequence
homologies suggest that mitochondria and chloroplasts
evolved separately, from lineages that are common with
eubacteria, with mitochondria sharing an origin with apurple bacteria, and chloroplasts sharing an origin with
cyanobacteria. The closest known relative of
mitochondria among the bacteria is Rickettsia (the
causative agent of typhus), which is an obligate
intracellular parasite that is probably descended from
free-living bacteria.
Horizontal
gene
transfer
(HGT) plays an important role in genome evolution [10].
In plants, the majority of reported cases of HGT have
been limited to exchanges between plants and microbes,
mitochondrial transfer, or the translocation of mobile
elements among related species. Parasitic plants are
known to be vectors of mitochondrial HGT, but it has
been unclear whether they also mediate nuclear HGT.
Possible routes for HGT between plants
(a) Plant–plant parasitism: Phylogenetic evidence
suggests that the intimate contact between parasitic plants
and their hosts facilitates HGT [11]. Shown here is a
mountain ash infested by mistletoes. Mistletoes belong to
the (largely parasitic) angiosperm order Santanales, which
has been implicated in plant–plant HGT.
(b) Natural grafting: Experimental evidence suggests that
genetic material travels between plant cells when they
contact each other in grafts. Shown here is a natural graft
between a maple tree (left) and a poplar tree (right) [12].
Modes of HGT with plants
HGT from Plant to Bacteria
Unexpectedly close sequence homologies
between plant and bacterial genomes have been detected
in different cases [13]: glyceraldehyde-3-phosphate
dehydrogenase (GAPDH), glucose-6-phosphate isomerase
(GPI), glutamine synthetase II (GSII), hemoglobin and TDNA discuss the degree of sequence homology and the
probability of a potential HGT for the first three
examples. While the transfer of genes coding for the
GAPDH and the GPI is seen to be likely, it seems to be
unlikely for the GSII gene.
Glyceraldehyde-3-phosphate
dehydrogenase
(GAPDH): The bacterium Escherichia coli and also
other enteric bacteria have two GAPDHs. GAPDH A
is similar to the eukaryotic form of this enzyme while
GAPDH B is typical for bacteria. The phylogenetic trees
indicate that transfer of the GAPDH A gene occurred
before the divergence of animals and plants.
Glucose-6-phosphate isomerase (GPI): The GPI
of Escherichia coli is highly homologous to the GPI from
the chloroplasts of the wild flower Clarkia sp. In the gut
of its animal host this enteric bacterial species might
have taken up the plant DNA [10].
Glutamine synthetase II (GSII): Of the two
glutamine synthetases, GSI is characteristic for
prokaryotes, while GSII is present in eukaryotes. The
discovery of GSII in plant symbiotic bacteria of the
Rhizobiaceae family was originally seen as an example
for HGT. But the detection of both GS types also in nonsymbiotic bacteria belonging to Streptomytaceae argues
against HGT and the arrangement of both GS genes
in tandem position in the plant symbiotic bacterium
Frankia indicates that these two genes could be derived
from gene duplication and subsequent divergent
development during evolution.
Hemoglobin: HGT has been assumed for the
hemoglobin of Vitreoscilla which shows a maximum
sequence homology with the lupine leg hemoglobin
[14]. Sequence homology is observed also for the
untranslated 3‟ends of this bacterial gene and several
plant leghemoglobin genes. As Vitreoscilla is found in
oxygen-poor environments such as decaying vegetable
matter, it might have taken up the hemoglobin gene from
this rotting plant material.
T-DNA: Sequence homology has been detected
between the T(transfer)-DNA of the Ri (root
inducing) plasmid of Agrobacterium rhizogenes and the
genome of Nicotianaglauca [15].
White et al., (1983) suggested two theories for
explaining this homology: (1) The bacterium might
have captured plant genes which became linked to
the bacterial chromosome and now are reinserted as a
part of the T-DNA into plant genomes during infection
by Agrobacterium. (2) The plant may have acquired
bacterial genes during past Agrobacterium infections and
retained these genes.
594
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
Sequencing data from other members of the
genus Nicotiana and the family Solanaceae supported
the second theory. Since homology of T-DNA was
limited to species of the subgenera N. rustica and N.
tabaccum, infection of a common ancestor of both
subgenera is assumed or, alternatively, the infection
of one subgenera followed by lateral spread of the
bacterial sequence through plant inter-specific
hybridization.
If bacteria can absorb free DNA, then in theory
at least, they can also absorb plant DNA. Therefore
studies have been conducted in those areas where foreign
genes from transgenic plants could conceivably spread in
the presence of bacteria, such as the rhizosphere of plants
and the intestine of animals. All the studies indicate that
there are no insurmountable barriers to horizontal gene
transfer between bacteria. However, transfer of plant
DNA to bacteria could not be demonstrated under natural
conditions [15].
Following aspects enable a gene to actually be
transferred from the plant to a bacterium: A) the foreign
gene must be released intact from a dead plant cell. B) It
must survive in the bacteria‟s natural environment. The
DNA often breaks down quickly and is only present in
fragments. C) This gene must then encounter a
“competent” bacterial cell which is capable of absorbing
DNA and penetrate it. D) Finally it must be integrated
into the bacterial genome in order to become effective i.e.
so that it can be expressed and converted to a protein. But
this is extremely unlikely, since foreign DNA generally
undergoes degradation in the bacteria, i.e. it is broken
down further.
Provoking transfer in the laboratory: So far
horizontal gene transfer has been demonstrated under
“optimum laboratory conditions” in two safety research
projects. In the laboratory it is possible to adjust bacterial
concentrations and climatic conditions, use isolated plant
DNA and specifically target bacteria capable of
absorption. Furthermore, in both these projects a bacterial
strain was prepared in such a way that a horizontal gene
transfer was actually provoked: An incomplete gene
for kanamycin resistance was introduced into the bacteria.
The bacteria needed a specific gene segment from the
transgenic plants to repair this gene.
HGT from Plant to Fungi
In the host derived fungus repetitive DNA
sequences have been detected. After infecting a
susceptible variety of Sinapsisalba with P. brassicae, the
new resting spores lacked the B. napus sequences but
instead contained a diverged version specific for S. alba.
It was concluded that P. brassicae takes up host specific
sequences during each infection cycle.
Oliver and
Solomon (2008) have attempted to detect HGT
between transgenic tomato and the fungal pathogen
Cladosporumfulvum. The genetic elements of interest
are retrotransposons (Tnt-1 of plants, CfT-1 of fungi)
which have been marked by integrating an antibiotic
resistance gene under control of a fungal or plant
viral promoter [16].
Co-cultivated transgenic Brassicaceae with the
naturally transformable fungus, Aspergillusniger in
microcosms resistant to hygromycin under sterile
conditions. The results of three experiments dealing
with HGT from plants to fungi have been published.
In one case an unmodified plant acted as gene donor
while in the other two cases GMPs were used.
Horizontal Gene Transfer from Plant to Viruses
The mechanism of HGT from plants to viruses is
recombination. As the genome of most plant viruses
consists of RNA, recombination has to take place between
plant mRNA and the virus genomic RNA. The ability of
RNA viruses to undergo genetic recombination in
general has been demonstrated for a few viruses.
Unlike DNA recombination which usually involves
double stranded DNA, only single stranded RNA has
been shown to undergo RNA recombination. Since
recombination occurs only during RNA synthesis a copy
choice mechanism is assumed which involves a RNA
polymerase jumping from one template to the other.
The three different types of RNA recombinations
are: A) Homologous recombination involves two
homologous RNA molecules with the crossover occurring
at the same site of each molecule.
B) Aberrant
homologous recombination involves two RNAs with
homologous or similar sequences, but the crossovers
occur at nearby sites of the molecules resulting in
duplication or deletion of sequence areas. C) Nonhomologous or illegitimate recombination occurs between
two RNA sequences which do not have any sequence
homology.
HGT through Mobile genetic elements
Transposition was first discovered in corn by
Barbara McClintock. Ever since geneticists have found
various genetic events at which some genetic elements are
capable of moving around the genome actively or
passively. These genetic elements now referred to as
mobile genetic elements (MGEs). MGEs are categorized
into two main groups whether they move within a genome
(intra-genomic mobility) or cross genomes (inter-genomic
mobility). Plasmids, found in all three domains: Archaea,
Bacteria and Eukaryota, are extra-chromosomal DNA
molecules which carry the genes enabling them to selfreplicate. Plasmids can be transferred between cells
through transformation or conjugation, which is
considered as one of the major forms of horizontal gene
transfer (HGT).Intra-genomic MGEs: Intra-genomic
MGEs are defined as movement of DNA fragments
(transposable elements/ transposons) within the genome
using non-sequence-homologous mechanisms [17]. The
595
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
main form of intra-genomic mobility is transposition
Transposons are found ubiquitously in both prokaryotic
and eukaryotic organisms. Transposon-derived sequences
contribute to 1-5% fraction of prokaryote and lower
eukaryote genomes. In higher eukaryotes, this proportion
can even reach 40% or more. One traditional way to
categorize different transposons is based on the
transposition intermediate. Under such classification,
transposons can be assigned to Class I (retrotransposons)
or Class II (DNA transposons) groups depending on
whether a RNA intermediate is involved during
transposition. However this is a rough way to classify
transposons without considering the detailed transposition
mechanisms. A more informative categorizing way is
based on the transposase encoded by the transposon,
which characterizes the main molecular nature of each
different transposition mechanism.
Inter- genomic MGEs: Process of inter-genomic
movement of genetic material - HGT/LGT (Archibald,
2003). HGTs can occur within the same species or
between different species. It is more diverse than intragenomic MGEs.
Artificial vectors enhance horizontal gene transfer:
A) They are derived from natural genetic parasites that
mediate horizontal gene transfer most effectively. Their
highly chimaeric nature means that they have sequence
homologies (similarities) to DNA from viral pathogens,
plasmids and transposons of multiple species across
Kingdoms. This will facilitate widespread horizontal gene
transfer and recombination.
B) They routinely contain antibiotic resistance marker
genes that will enhance horizontal transfer in the presence
of antibiotics, either intentionally applied, or present as
pollutant in the environment. Antibiotics are known to
enhance horizontal gene transfer between 10- and 10000fold.
C) They often have „origins of replication‟ and „transfer
sequences‟ signals that facilitate horizontal gene transfer
and maintenance in cells to which they are transferred.
D) Chimaeric vectors are well known to be structurally
unstable, that is, they have a tendency to break and rejoin
incorrectly or join up with other DNA, and this will
increase the propensity for horizontal gene transfer and
recombination.
E) They are designed to invade genomes and to overcome
mechanisms that break down or disable foreign DNA thus
increasing the probability of horizontal transfer.
HGT from Genetically Modified Organisms
Studies on genetically-modified (GM)
plants have gone on for three decades, and this year is the
17th year of the first commercialization of a GM crop.
Genetic modification is not only used to produce elite
crop varieties, which may help enhance and stabilize
agriculture production, but is also a powerful tool for
scientists who are interested in studying gene function.
Potential hazards of horizontal gene transfer from
genetic engineering
1.Generation of new cross-species viruses that
cause disease(Kuiper and Davies, 2010). 2. Generation of
new bacteria that cause diseases. 3.Spreading drug and
antibiotic resistance genes among the viral and bacterial
pathogens making infections untreatable. 4.Random
insertion into genomes of cells resulting in harmful effects
including cancer. 5.Reactivation of dormant viruses,
present in all cells and genomes which may cause
diseases. 6.Spreading new genes and gene-constructs that
have never existed. 7.Multiplication of ecological
impacts.
Potential Risks from GM plants due to HGT
The application of genetic modification allows
genetic material to be transfer red from any species into
plants or other organisms. The introduction of a gene into
different cells can result in different outcomes and the
overall pattern of gene expression can be altered by the
introduction of a single gene [18].
The report prepared by the Law Centre of IUCN,
the World Conservation Union ( 2004), enlists numerous
environmental risks likely to occur by the use of GMOs in
the field. Each gene may control several different traits in
a single organism. Even the insertion of a single gene can
impact the entire genome of the host resulting in
unintended side effects, all of which may not be
recognizable at the same time. It is difficult to predict this
type of risk.
Genetic
Contamination/Interbreeding
Introduced GMOs may interbreed with the wild-type or
sexually compatible relatives. The novel trait may
disappear in wild types unless it confers a selective
advantage to the recipient. However, tolerance abilities of
wild types may also develop, thus altering the native
species‟ ecological relationship and behaviour.
Competition with Natural Species - Faster growth of
GMOs can enable them to have a competitive advantage
over the native organisms. This may allow them to
become invasive to spread into new habitats and cause
ecological and economic damage.
Increased Selection Pressure on Target and
Nontarget Organisms - Pressure may increase on target
and nontarget species to adapt to the introduced changes
as if to a geological change or a natural selection pressure
causing them to evolve distinct resistant populations.
Ecosystem Impacts - The effects of changes in a single
species may extend well beyond to the ecosystem. Single
impacts are always joined by the risk of ecosystem
damage and destruction.
596
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
Impossibility of Follow-up- Once the GMOs
have been introduced into the environment and some
problems arise it is impossible to eliminate them. Many of
these risks are identical to those incurred with regards to
the introduction of naturally or conventionally bred
species. But still this does not suggest that GMOs are safe
or beneficial or that the y should be less scrutinized.
Reducing the risk of HGT from transgene to virus
Various transgene modifications and
safeguards include: Avoidance of certain proteins known
to interact with other viruses, Mutation or disablement of
protein coding sequences, Removal of replicationassociated sequences, Use of mild and endemic strains
and Use of short viral sequences.
Importance of HGT
The impact of HGT remains more
controversial. In the short term, HGT can increase genetic
diversity and promote the spread of novel adaptations
between organisms. HGT has been a major contributory
factor to the rapid spread of antibiotic resistance amongst
pathogenic bacteria in the last 50 years and the emergence
of in-creased virulence in bacteria, eukaryotes and
viruses. In the long term it has been proposed that HGT
has contributed to the major transitions in evolution.
Rapidly growing library of complete genome
sequences reveals that HGT is a major factor in shaping
the genomes of all organisms.
Detection of HGT
Experimental evidence - Genetic marker is monitored
for gene transfer to a recipient organism. Laboratory
studies - provided valuable insights of mechanisms. More
recently field studies on HGT support many of the
laboratory findings.
Figure 1. A schematic representation of horizontal gene
transfer (HGT)
HGT - recorded in a number of environmental situations
such as soil, seawater, freshwater, animal and industrial
waste products, plant surfaces, animal intestines, human
saliva and food products.
Codon- based approaches - Based on the principle that
particular organisms have biases in G+C content and
codon usage. Detected by looking for genes or groups of
genes that have a G+C content and or codon biases that
differs from that of the host.Advantages - Able to detect
HGT events in a genome without needing other genomes
for comparison. Limitations - Approach is valid when
dealing with relatively recent HGT events. Hence this
approach is unlikely to detect HGT events that occurred
as a result of endosymbiotic events in eukaryotic
evolution. Even if HGTs are identified accurately, there is
no indication of the origin of the gene. Overcome Further phylogenetic analysis is needed. Furthermore
when such phylogenetic analysis has been carried out, it
has revealed that the results of the initial G+C or codoncontent-based analysis contained many false positives and
negatives
BLAST- based approaches - Rapid and simple way of
identifying which proteins in a database are most similar
to a query protein.
Gene-distribution- based approaches - Look to identify
genes that are unevenly distributed between related
species. Limitations: 1. Insufficient and biased taxon
sampling may lead to the misidentification of HGT
events. 2. Any events that are identified could also have
been brought about by gene loss or rapid divergence in
the related species. 3. Also it can only be used to identify
the HGT of novel genes from genomes that are
completely sequenced
Figure 2. Endosymbiotic Evolution and the Tree of
Genomes
597
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
Figure 3. Endosymbiotic Theory
Figure 4. HGT in different domains
Figure 5. Two possible routes for HGT between plants
Figure 6. Modes of HGT
598
Amarantha Reddy M et al. / Journal of Science / Vol 4 / Issue 9 / 2014 / 591-599.
CONCLUSION
unknown.
Although PCR-based studies have
revealed much about HGT clearly the next major step is
to sequence whole mitochondrial genomes of plants
known to have experienced HGT. Genome sequencing
will uncover transfers that are too short or from such
evolutionarily distant donors that they fail to amplify by
PCR with flowering plant primers. Another clear need is
to uncover very recent transfers that could provide further
insight into the transfer process and answer several
outstanding questions: how long are tracts of HGT; does
DNA move back and forth between donors and recipients
or is transfer unidirectional. Although HGT in plant
mitochondria is unlikely to confer radically new
phenotypes (as it often does in bacteria), the evolutionary
consequence ofthese new genes remains entirely
FUTURE DIRECTIONS
As the amount of eukaryotic sequence
data that is available increases investigation of the levels
of HGT that have occurred throughout eukaryotic
evolution becomes possible. Phylogenetic investigation
can be used to identify HGT although much care must be
taken while doing this. Phylogenetic investigation can be
used to identify HGT. Future studies on the comparative
genomics of eukaryotes. PCR-based studies have revealed
much about HGT clearly the next major step is to
sequence whole mitochondrial genomes of plants known
to have experienced HGT. One challenge in future
research will be to find a suitable model system to explore
all of the stages of HGT process.
REFERENCES
1. Koonin EV, Makarova KS and Aravind L. Horizontal gene transfer in prokaryotes: quantification and classification.
Annu. Rev. Microbiol, 55, 2001, 709-742.
2. Gray MW. Mitochondrial evolution. Science, 283, 1999, 1476–1481.
3. Andersson JO. Lateral gene transfer in eukaryotes. Cell. Mol. Life Sci, 62, 2005, 1182 – 1197.
4. Suzuki K. Tobacco plants were transformed by Agrobacterium rhizogenes infection during their evolution. Plant J, 32,
2002, 775–787.
5. Richardson AO and Palmer JD. Horizontal gene transfer in plants. J. Exp. Bot, 58, 2007, 1–9.
6. Richards TA, Hirt RP, Williams BAP and Embley TM. Horizontal gene transfer and the evolution of parasitic protozoa.
Protist, 154, 2003, 17–32.
7. Andersson JO, Sjogren AM, Davis LAM, Embley TM and Roger AJ. Phylogenetic analyses of diplomonad genes reveal
frequent lateral gene transfers affecting eukaryotes. Curr. Biol, 13, 2003, 94-104.
8. Won H and Renner SS. Horizontal gene transfer from flowering plants to Gnetum.Proc. Natl. Acad. Sci. USA, 100, 2003,
10824-10829.
9. Yoshida S, Maruyama S, Nozaki H and Shirasul K. Horizontal gene transfer by the parasitic plant Strigahermonthica.
Science, 328, 2010, 1126-1128.
10. Keeling PJ and Palmer JD. Horizontal gene transfer in eukaryotic evolution. Nat. Rev. Genet, 9, 2008, 605–618.
11. Davis CC, Anderson WR and Wurdack KJ. Gene transfer from a parasitic flowering plant to a fern. Biological Sciences,
272, 2005, 2237–2242.
12. Stegemann S, Keuthe M, Greiner S and Bock R. Horizontal transfer of chloroplast genomes between plant species. Proc.
of Natl. Acad. of Sci, 10, 2011, 107-115.
13. De Vries J, Meier P and Wackernagel W. The natural transformation of the soil bacteria Pseudomonas stutzeri and
Acinetobacter sp. by transgenic plant DNA strictly depends on homologous sequences in the recipient cells. FEMS
Microbiol. Lett, 195, 2001, 211–215.
14. Deaville ER and Maddison BC. Detection of transgenic and endogenous plant DNA fragments in the blood tissues and
digesta of broilers. J. Agric. Food Chem, 53, 2005, 10268–10275.
15. Archibald JM. Lateral gene transfer and the evolution of plastid-targeted proteins in the secondary plastid-containing
alga Bigelowiellanatans. Proc. Natl. Acad. Sci. U. S. A, 100, 2003, 7678–7683.
16. Oliver RP & Solomon PS. Recent fungal diseases of crop plants: is lateral gene transfer a common theme? Mol. Plant
Microbe Interact, 21, 2008, 287–293.
17. Hall C, Brachat S and Dietrich FS. Contribution of horizontal gene transfer to the evolution of Saccharomyces
cerevisiae. Eukaryotic Cell, 4, 2005, 1102–1115.
18. Kleter GA, Peijnenburg ACM and Aarts HJM. Health considerations regarding horizontal transfer of microbial
transgenes present in genetically modified crops. J. Biomed, 4, 2005, 326–352.
19. Kuiper AH and Davies HV. The Safe foods Risk Analysis Framework suitable for GMOs?. A case study Food Control,
21, 2010, 1662–1676.
599