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Slideshow: Examples of Eukaryotic Horizontal Gene Transfer

Horizontally transferred genes play significant roles in eukaryotic genomes

christie wilcox buehler
Christie Wilcox

Christie joined The Scientist's team as newsletter editor in 2021, after more than a decade of science writing. She has a PhD in cell and molecular biology, and her debut book Venomous: How Earth’s Deadliest Creatures Mastered Biochemistry, received widespread acclaim.

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Viper and cow photos
© istock.com, DanVostok; © istock.com, Milan Simonovic

The BovB retrotransposon was first nearly one-fifth of the cow genome and is also abundant in most ruminants and other mammals. Before the turn of the 21st century, it was also spotted in vipers, leading scientists to suggest that it moved between the mammals and reptiles via horizontal gene transfer (HGT). More recently, BovB sequences have been identified in ticks, leeches, and other nonspecific parasites, providing scientists with potential vectors for such inter-animal transfers.


Image of bacteria
Rachel Vannette; © istock.com, libre de droit

A clade of nectar-dwelling yeast species have obtained the ability to produce the iron-binding compound enterobactin from bacteria, researchers reported in a 2019 Cell paper. Most intriguingly, the putative bacterial donors dwell in the guts of insects, suggesting that the yeast obtained the enterobactin-encoding DNA after being consumed by a hungry arthropod.


Switchgrass and corn photos
© inaturalist.org, aspidoscelis; © istock.com, bruev

Evidence for ecologically relevant horizontal gene transfers between grasses has been accumulating over the past 15 years. So for a 2021 New Phytologist paper, researchers searched for HGT events among 17 grass genomes from five groups—and found plenty. Some species, such as the switchgrass Panicum virgatum, appeared to have dozens of transferred genes—81 percent of which came from the Andropogoneae, a group which includes corn (Zea mays)—and none of the groups were found to lack HGT events, suggesting gene flow between grasses is commonplace.


Fish photos
Sean Landsman/Engbretson Underwater Photography; inaturalist.org, ciaodb; © alamy.com, blickwinkel

Fishes in icy waters don’t freeze solid thanks to special antifreeze proteins (AFPs). One kind in particular, known as type II AFPs, can lower the freezing point of fish blood by a whole degree Celsius. Likely because of HGT events, these proteins are found in three distantly related fish lineages: Atlantic herring (Clupea harengus), smelts (Osmerus mordax and Hypomesus nipponensis), and sea ravens (including Hemitripterus americanus). Genetic analyses suggest that the ancestor of the smelts received an AFP gene from herring more than 150 million years after their ancestors diverged; the herring, in turn, may have obtained theirs from sea ravens before that. Researchers suspect the fishes’ exuberant and overlapping spawning may create a soup of DNA that can stick to sperm, which then drag the foreign DNA into the egg during fertilization, facilitating transfers.


Stink bug and phospholipase photos
© istock.com, ibunt; Jennifer Heppert; alex grossman

Brown marmorated stink bugs (Halyomorpha halys) consume a great many plant species, an ability aided by a suite of salivary phospholipases that likely let the insects interfere with immune signaling in the plants while feeding. It turns out the insects probably got these helpful proteins from Xenorhabdus bacteria (shown in green in a nematode, and also in zoom) that live mutualistically in Steinernema nematodes. These pathogenic worms infect and kill stink bugs and other insects by releasing their bacterial partners into hemolymph. The researchers suspect that this may have created an opportunity for gene transfer in a stink bug that luckily survived its nematode infection.


Centipede and venom toxin photos
© inaturalist.org, alexis_orion; © wikimedia, Mogana Das Murtey and Patchamuthu Ramasamy

Centipedes wield complex venoms that can contain hundreds of toxic compounds. Some of these appear to have come from other organisms, including bacteria and fungi. In particular, researchers have found evidence that bacterial β-pore-forming toxins, which punch holes in cell membranes, were transferred into centipedes at least twice—once into the ancestor of all centipedes, and again into the ancestor of stone centipedes (order Lithobiomorpha). Scientists suspect these molecules may help predigest the animals’ meals or act as paralytic agents to immobilize prey, as similar toxins do in other venomous species.


Nematode and cellulases photo
Ralf J. Sommer and Max Planck Institute for Biology;  © wikimedia, Usman Bashir

The miniscule soil-dwelling nematode Pristionchus pacificus waits in the folds of a scarab beetle’s cuticle until the insect’s demise, so it can feast on the microbes that decompose the insect’s body. The nematodes also produce cellulose-digesting enzymes encoded by genes that appear to have originated in a gene transfer from a slime mold. Researchers suspect that even though the target microbes don’t produce much cellulose, there’s just enough supporting their biofilms so that the enzymes let the worms get a little bit more sustenance from each meal.

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Greenhouse whitefly - Trialeurodes vaporariorum on the underside of leaves. It is a currently important agricultural pest.

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