Post n. 46 English
English Etichetta Zoa
Lateral or
horizontal transmission
DNA (deoxyribonucleic acid) is the molecule that
contains the genetic information. It is written in DNA whether an organism will
be a human being, a tree or a microorganism.
In the DNA of all organisms, tens of thousands of
segments called genes have been identified.
The DNA genes are transcribed into messenger RNA and expressed in
proteins. It is genes, or groups of genes, that determine skin colour, the
number of fingers on a hand and so on. Every living organism always transmits its genetic heritage to its
descendants. Often in the descendants the inheritance undergoes mutations, i.e.
random changes, which can change the expression of the gene. As we wrote in the
previous article, as a result of these discoveries Darwin's theory was extended
to genetics under the name of neo-Darwinism, now known as 'synthetic theory'.
The theory states that natural selection works on genes, that the variations
Darwin is talking about are random mutations that appear continuously in genes
and are passed on to descendants, and this transmission is called vertical.
During his voyage around the world, Darwin had
already realised that species were not fixed entities, as was then believed,
and that there was a link between living and extinct species. He represented
this idea with a drawing, a sketch of a possible family tree, which later
became Darwin's tree.
After the publication of the Origin of Species
there was, by scientists who accepted Darwin's theory of evolution, a
flourishing of family trees whose branches, representing the species, converged
into a trunk. The branches of these trees of life, through the bond of kinship,
descended from the most complex organisms to the least complex organisms along
the trunk of the tree. Since microbes were already known at the time, some
scientists placed them at the base of the trunk, under the name of Monera, giving
the idea that life began with microbes.
The
discovery of genes confirmed the above idea, and the fact that they are passed
on from one organism to its descendants by establishing a kinship link,
Darwin's family tree, which concerned higher organisms, was definitively
extended to microorganisms. Carl Woese's research finally showed that all
microorganisms could be subdivided into: Bacteria, Archaea and Eukaryotes, and
that all living or extinct organisms, through vertical transmission descend
from these microorganisms. The representation of evolution by means of family
trees was a powerful metaphor that helped to spread Darwin's theory and which
Woese did not shy away from either. His tree naturally starts with bacteria,
archaea and eukaryotes but places a hypothetical 'Universal Progenitor' at the
base of the trunk. What this 'universal progenitor' actually was is not known.
In the second half of the last century, vertical
transmission was the only one accepted by scientists. The term germline was coined, which could not
possibly be invaded by the germline of other species.
Everything seemed clear enough when, in the
mid-1990s, lateral transmission, also known as horizontal transmission, was
finally accepted in the bacterial world and among single-celled eukaryotes: genes are not only passed on from one
organism to its descendants, but also between cells that are not related at all.
This discovery was like a bombshell: the family tree based on vertical
transfers, i.e. descent and kinship, suddenly lost its trunk base.
The story of lateral gene transmission begins at
the beginning of the last century. At that time, a doctor named S. Griffith
noticed that by putting together live
Streptococcus Pneumoniae, called R, which did not cause serious diseases
with dead Streptococcus Pneumoniae
type S, which created fatal diseases, the harmless live pneumococci became
fatal. No one at the time could explain why, because bacterial species were
thought to be fixed and stable entities.
It was not until the middle of the last century that Griffith's studies
were taken up by Oswald Avery, a doctor at the Rockefeller Institute, and his
team. Avery discovered that it was the genome, DNA, or pieces of it that were
transferred into living bacterial cells to be passed on to future generations,
and he called this type of gene transfer transformation.
In the following decade, two other types of transfer were discovered by Joshua
Lederberg of Yale University: conjugation,
which occurs through contact between bacteria, a kind of bacterial sex, a
quickie as David Quammen dubbed it, and
transduction due to viruses that often take genetic material from one
bacterium and transfer it to another bacterium. The problem of antibiotic
resistance by pathogenic bacteria also arose around the 1960s. Such a rapid
emergence of drug resistance by different bacterial strains could not be
explained as a slow Darwinian process of mutations, but could only be explained
by the lateral transmission of parts of genetic information between bacteria.
By the mid-1970s, it was realised that horizontal gene transfer was not just a
medical issue, but affected the entire bacterial world, from the origin of life
to the present day and influenced its evolution. Towards the end of the last century,
lateral gene transfer now universally accepted, definitively changed the shape
of the tree and for W. J. Doolittle it became a reticulated tree.
However, the discovery of drug resistance also raises
general considerations. If a bacterial strain succeeds in developing drug
resistance it should, according to natural selection, be the best adapted in
the new environment and the less adapted evolutionary lines should slowly die
out. What happens instead is that the colony that has developed drug resistance
transfers the information to its competitors through lateral gene transfer,
i.e. instead of competing, it cooperates. In the previous article, it was
pointed out that, according to Lynn Margulis, the real evolutionary novelty
comes from symbiosis and that life on earth has followed the path of
cooperation and not of the struggle for survival. However, while symbiosis
originated 1.5 billion years ago, bacteria have been practising cooperation
since their origin 3.5 billion years ago. Since life originated with bacteria,
then life on earth did not follow the path of cooperation but is based on
cooperation.
At the beginning of the new millennium,
horizontal gene transfer was still thought to be restricted to bacteria.
According to most scientists, genes could not be passed from one species to
another; the germ line of higher organisms was protected by an insurmountable
barrier. However, with the technological development of gene sequencing, this
barrier began to break down and eventually collapsed. It began with a specific
group of small animals, the rotifers, in which twenty-two genes were found from
bacteria, fungi and even one from a plant. Later it was also discovered that
bacterial genes had snuck into the genomes of insects and invertebrates. Traces
of bacterial DNA were found in human tumours.
In short, it turns out that lateral gene transfer
affects all organisms.
To conclude, I would like to highlight just a few
of the many publications in recent years.
17 February 2012
Evidence of gene transfer between species in
plants (Red. Le Scienze online)
A new research has documented another way of gene
transmission between plants of different species that share only one ancient
ancestor. The details of the process are not known, but it seems that genetic
material carried in the air by pollen grains on different species is acquired
by host plants during pollination.
08 March 2013
The red alga that survives by stealing genes from
bacteria (Red. Le Scienze online)
The red alga Galdieria
sulphuraria is able to survive in extreme environments, very acidic and
rich in metals, thanks to the genes it has acquired from simpler organisms
through a horizontal transfer. The mechanism, typical of prokaryotic cells but
much rarer among organisms with a complex cell structure, was discovered by
sequencing the alga's genome.
13 March 2015
From microorganisms to the human genome, here are
our foreign genes (Red Le Scienze online)
© Sean Busher/Corbis
Passing genes from microorganisms to more complex
animals is a common mechanism and over the course of evolution has left humans
with a legacy of around 145 'foreign' genes. This is the result of a new
comparative analysis of the genomes of different species, from the fruit fly to
primates, including humans.
10 July 2018
The gene transfer that drove evolution
© iStock/iLexx
Research on several hundred species of fungi,
plants and animals has documented that the transfer of transposable gene
elements between different species has been far more widespread than expected over
the course of evolution, radically changing even the mammalian genome.
30 June 2021
DNA jumps between animal species, no one knows
how often
by Christie Wilcox/Quanta Magazine
Herring bank. Herring and osmerids both produce
antifreeze proteins thanks to the same gene, even though their ancestors split
off over 250 million years ago and the gene is absent in all other related fish
species. (© Humberto Ramirez/Getty Images)
The discovery of a gene shared by two unrelated
fish species is the most recent and striking evidence that horizontal gene
transfers in vertebrates occur with surprising frequency and have a major
evolutionary impact.
Those responsible for such transfer are usually
viruses and bacteria that infect plant and animal cells and transfer segments
of their genes into them. In addition, bacterial pathogens that enter plant and
animal cells can pick up foreign genetic material, transport it into their
cells and thus serve as vectors for horizontal gene transfer. Until a few years
ago, it was thought that viruses and bacteria were the only vectors for
horizontal gene transfer between plants and animals. In other words, there
could be no direct transmission to plant cells, which generally have a
protective cell wall. In recent years, it has been discovered that even
insects, with their hard, sharp parts, can cut through the protective wall and
transfer foreign genetic material to both plants and animals.
It is now believed that there are no barriers to
the entry of foreign genetic material into the cells of any species on earth,
and that in addition to vertical gene transfer, horizontal gene transfer also
affects the entire evolutionary process of living organisms from their origins
to the present day.
Epigenetics
So, returning once again to Darwin, descendants
are not all the same but present variability that accumulates over generations,
and the individual with the most suitable variation in a given environment
survives. And, again recalling Neo-Darwinism, the variations Darwin is talking
about are random mutations that appear continuously in the genes and are
transmitted to the descendants; the individual with the most suitable mutations
in a given environment survives. Ultimately, to paraphrase the concept, the
environment's only task is to choose the individual with the most suitable
genetic mutations.
However, with the choice of the most suitable,
has the environment really completed its task?
As Richard C. Fancis reports in his essay
"L’ultimo mistero dell’ereditarietà" 2011, it was the dark days of
the Second World War. Famine raged throughout Holland, but in the north of the country,
it was particularly severe and persistent. Later, a study was started on the
long-term effects of the famine, particularly on those who were in the womb at
the time. In these people, the researchers found an increase in obesity of
about fifty per cent compared to those who had come into the world before and
after the famine, and psychological problems such as depression. By the late
1990s there was enough evidence to conclude that the foetal environment played
a role in their health.
However,
how could this be produced?
Today we know that it is genes that create the
special proteins that carry out all the biochemical processes that sustain
living organisms. Gene regulation refers to the cell's control over the
activity of genes, and it was thought that this gene regulation was the only
one of short duration. The Dutch famine highlighted the existence of
long-lasting gene regulation called 'epigenetic regulation'. In 2000 it was
discovered by two researchers, Randy and Waterland, that epigenetic regulation
takes place through markers generally -CH3 groups, a methyl group, which binds
to a gene due to environmental causes and deactivates it, and it is these
methylated genes that were discovered in the subjects who suffered from the
famine. The foetal environment, in this case, had a long-term effect. During
sperm and egg production, the genome is cleansed of markers, but in several cases,
these methylated genes manage to pass on to the next generation, giving rise to
epigenetic inheritance.
Michael K. Skinner in Le Scienze 02 October 2014
in an article entitled 'Un nuovo tipo di ereditarietà' concludes: «The
actions of genes can be regulated by "epigenetic" factors, molecules
that attach to DNA and chromosome proteins and express information
independently of DNA sequences. Most epigenetic changes are cancelled out
shortly after conception. Pollutants, stress, diet and other environmental
factors can cause persistent changes in the mix of epigenetic modifications in
chromosomes, and in this way can alter the behaviour of cells and tissues.
Surprisingly, some acquired changes can be passed on to descendants.
Theoretically, your health and that of your children could be altered by
factors to which your great-grandmother was exposed during pregnancy.
Epigenetic inheritance may play a role in diseases such as obesity and
diabetes, as well as in the evolution of species»
Michael Brooks, too, in “Oltre il limite” 2015,
takes up the problem of the Dutch famine in his book, but expands it in an
evolutionary way. Jean-Baptiste Lamarck was the first to assert that changes in
the organic world were the result of a law and not of miraculous interventions,
and he was, from this point of view, perhaps the first evolutionist. Lamarck
was also convinced that traits acquired during life were passed on to
descendants. He took the example of giraffes, which were initially supposed to
have short necks but which stretched a little to reach the leaves. This
characteristic was passed on to their offspring who in turn in the same attempt
lengthened their necks a little more until all giraffes developed long necks.
Of course, this process was disproved by the facts, the idea of transmission of
acquired traits was buried, Lamarck died blind, poor, thrown into the Paris
catacombs and often mocked by evolutionists.
But Lamarck was not totally wrong; he was just
wrong to take the giraffe as an example.
The ostrich often sits on the ground and at the
points of contact the skin becomes thick and hard, forming calluses where no
feathers grow. It seemed that these calluses formed during the ostrich's life
every time it sat on the ground. In the 1940s, Conrad Waddignton of the University
of Edinburgh discovered that the ostrich's calloused parts are already present
at birth and are therefore formed during the development of the embryo. This
meant that a character acquired from the ostrich's ancestors had been passed on
to its offspring. To confirm his suspicions he subjected fruit flies to heat
shock prior to their metamorphosis and found that the arrangement of veins on
their wings changed and this arrangement was maintained for generations. An
environmental factor had brought about a change in one of the fruit flies'
characters and they passed it on to their offspring. Waddington coined the term
epigenetics (in addition to genetics),
but his ideas remained marginal in the evolutionary context.
After the Dutch famine study, research into how
the environment causes epigenetic changes multiplied. In 2011, researchers at
the Salk Institute in California demonstrated that methyl groups can affect
thousands of genes. The researchers concluded that the influence of epigenetic
effects exceeds that of mutations by hundreds of thousands of times and that
these effects can last up to thirty generations. As Brooks again reports, two
researchers, Kuzawaa and Sweet, analysing data collected from the archives of
the Johns Hopkins Hospital up to the present day, have shown how the effect of
slavery is still reflected today in the weight of babies born to
African-American mothers. In addition, Kuzawa and Sweet conclude: «We
really need to stop our obsession with genetics and start looking at the
longer-lasting effects of the environment on biology and health, especially when
experienced early in life»
Sharon Moalem's essay has the emblematic title
“L’eredità flessibile. Come i nostri geni ci cambiano la vita e come la vita cambia i nostri geni”. I
won't expand on summaries already given, but I will just quote a bee research and a piece of advice from Moalem.
The research: «A
queen and her workers may be born of the same parents, and may have completely
identical DNA. Yet their behavioural, physiological and anatomical differences
are profound, [...]. When the colony decides it is time for a new queen, its
members select a few lucky larvae and dip them in royal jelly. [...] The little
princesses eat and eat and eat until they emerge as elegant blue-blooded
princesses». Their genes are not different but gene expression
has changed, a mechanism called epigenetics. Further on, Moalem reports on the
study by researchers at Oregon State University who found that eating spinach
induces epigenetic changes against genetic mutations produced by carcinogens in
cooked meat.
The advice. Eat spinach.
To conclude, perhaps it is truly desirable, as
Telmo Pievani, (article quoted in part one), to construct an “extended
evolutionary synthesis”, i.e. a theory that is not limited to explaining
evolution through genes and selection alone.
Giovanni Occhipinti
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