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Alien Genes in Our Genome!!!
Authors: Varshney R
Number of views: 363
Have you ever heard of Horizontal Gene Transfer
(HGT)? In essence, the idea is that genes and DNA
can be transferred between species in the same
environment.[1] It is well known process for the
evolution for unicellular organisms like bacteria in
resistance to antibiotics;[2] nematode worms who
have acquired genes from microorganisms and
plants; some beetles that gained bacterial genes to
produce enzymes for digesting coffee berries and
the genome of wasps was found to contain not just
DNA from their own species, but also from
microbes that live in their guts.
Now a days, the idea that HGT occurs in more
complex animals, such as humans, rather than them
solely gaining genes directly from ancestors, has
been a serious matter of debate and discussion.[3] If
confirmed, this finding would significantly impact
theories about the underlying mechanisms of
evolution. Traditionally, it was believed that genes
were inherited solely from parents, but the process
of DNA evolution could prove far more complex
than once believed. Most, if not all, animals may
have evolved in part due to this transfer of foreign
genes, researchers believe.
Well known author Crisp et al[4] from the
University of Cambridge, UK, said: "This is the
first study to show how widely horizontal gene
transfer (HGT) occurs in animals, including
humans, giving rise to tens or hundreds of active
'foreign' genes. Surprisingly, far from being a rare
occurrence, it appears that HGT has contributed to
the evolution of many, perhaps all, animals and that
the process is ongoing, meaning that we may need
to re-evaluate how we think about evolution."
The researchers studied the genomes of 12 species
of Drosophila or fruit fly, four species of nematode
worm, and 10 species of primate, including
humans. They calculated how well each of their
genes aligns to similar genes in other species to
estimate how likely they were to be foreign in
origin. By comparing with other groups of species,
they were able to estimate how long ago the genes
were likely to have been acquired.[4]
It was a matter of surprise that a number of genes,
including the ABO blood group gene, were
confirmed as having been acquired by vertebrates
through HGT. The majority of the other genes were
related to enzymes involved in metabolism.
In humans, 17 previously-reported genes acquired
from HGT were confirmed, and 128 additional
foreign genes were identified in the human genome
that has not previously been reported.[4] Some of
those genes were involved in lipid metabolism,
including the breakdown of fatty acids and the
formation of glycolipids.[4] Others were involved in
immune responses, including the inflammatory
response, immune cell signalling, and antimicrobial
responses, while further gene categories include
amino-acid metabolism, protein modification and
antioxidant activities.[4]
The team were able to identify the likely class of
organisms the transferred genes came from.
Bacteria and Protists, another class of
microorganisms, were the most common donors in
all species studied.[4] They also identified HGT
from viruses, which was responsible for up to 50
more foreign genes in primates.[4] Some genes
were identified as having originated from fungi.[4]
This explains why some previous studies, which
only focused on bacteria as the source of HGT,
originally rejected the idea that these genes were
'foreign' in origin. The majority of HGT in primates
was found to be ancient, occurring sometime
between the common ancestor of Chordata and the
common ancestor of the primates.
The authors say that their analysis probably
underestimates the true extent of HGT in animals
and that direct HGT between complex multicellular
organisms is also plausible, and already known in
some host-parasite relationships.[4] The study also
has potential impacts on genome sequencing more
generally. Genome projects frequently remove
bacterial sequences from results on the assumption
that they are contamination. While screening for
contamination is necessary, the potential for
bacterial sequences being a genuine part of an
animal's genome originating from HGT should not
be ignored, say the authors.