...Against All Odds
Despite the great advantages that bacterial genes may confer
on more complex organisms, the odds are still heavily stacked against a
transfer producing a functional gene. Researchers are trying to understand how
this enormous leap occurs.
Before a bacterial gene can even operate in a eukaryotic
cell, it has some major biological hurdles to overcome. It must acquire
introns, a promoter sequence, and a termination sequence. It also has to be
able to interact fruitfully with existing host genes. “How does something that
has been out of a lineage for hundreds of millions of years potentially get
incorporated into a foreign lineage that it has no experience with? It's an
absolutely fascinating question,” says Whiteman.
One successful example is oskar, a gene that is both
necessary and sufficient for germline development in holometabolous insects.
Oskar is a chimeric gene: Half of it is of bacterial origin, and the other half
is of insect origin (12, 13). Because bacteria don't have a germline
development function, the chimeric gene is doing something novel in the
insects. It’s also doing something critically important: Without proper
germline development, an organism is an evolutionary dead end.
So how can horizontally transferred genes acquire central,
critically important roles in their new hosts, even replacing essential host
functions such as embryo sex determination, as in the case of the French
pillbugs?
Malik suspects the answer may lie in the strong evolutionary
pressures that endosymbionts face to alter their host’s physiology and
development to their own advantage. Wolbachia, for instance, lives inside the
germline and reproductive tissues of approximately 50% of the world’s arthropod
species. The bacteria are passed to offspring through their host's eggs and
have evolved multiple strategies to increase the chances of being passed on.
“It’s actually very clear that endosymbiotic bacteria
manipulate the germline of insects,” Malik says. “They do so because they want
to propagate themselves. They are living and getting transmitted through the
oocyte. So there are many bacteria that have manipulated things like the sex
determining mechanisms, or that have feminized a genetic male into a female, as
a means to increase their transmission.”
Hence, it may not have been difficult for a Wolbachia gene
to make the functional leap into the genome of French pillbugs. “Wolbachia was
already interacting with the pillbug’s cells,” explains Cordaux. “Perhaps the
integration was just another way to locate the ‘feminization’ gene.” This may
also shed light on how oskar acquired a novel and essential role in germline
development in holometabolous insects some 450 million years ago. The bacterial
half of oskar might have come from an endosymbiont that was manipulating its host’s
germline development in its favor.
All of this helps explain how genes new to a lineage can
play key roles in old developmental processes. “We used to have this dogmatic
view about genes that are really essential or important—that they are extremely
likely to have been conserved since the dawn of time because, of course,
they're essential so we would have not really had an opportunity to mess around
with them,” says Malik. “More and more work … has really sort of impressed upon
folks the idea that, in fact, new genes can be essential. In fact, young genes
can acquire essential functions.”...
From
Core Concept: Gene transfers from bacteria and viruses may
be shaping complex organisms
Viviane Callier
PNAS July 9, 2019 116 (28) 13714-13716;
https://doi.org/10.1073/pnas.1909030116
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