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Natural GMOs Part 284. Now, about Oskar. Oskar is a chimeric gene: Half of it is of bacterial origin, and the other half is of insect origin

19:08 EDT 12 Jul 2019 | GMO pundit



...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


Original Article: Natural GMOs Part 284. Now, about Oskar. Oskar is a chimeric gene: Half of it is of bacterial origin, and the other half is of insect origin

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