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Go to Brilliant.org/SciShow to learn about  their Knowledge and Uncertainty course.   Every winter, people hang mistletoe in doorways,  and couples kiss under the leaves and berries.   It’s a time-honored romantic tradition. And that makes sense, because mistletoe is   all about forming strong bonds… with  the host it sucks the life out of.   That’s right, even though it doesn't typically  kill its host, mistletoe is a parasite— but that’s not even the strangest thing about it.   It also just so happens to lack the genes needed to make energy.

You know, the ones that you and me and all  other multi-celled lifeforms have because they allow us to convert food into cellular fuel.   So it technically shouldn’t exist! Oh, and did we mention it’s poisonous  but might also save people’s lives? Mistletoe: It’s complicated, but it can teach us a lot about alternative lifestyles.

Really, when I say “mistletoe”, I should be saying “mistletoes,”  because there are more than 1,300 species worldwide.   They belong to the order Santalales—which, for  the record, is not named after Santa Claus.   It’s named after sandalwood. Now, most mistletoes have green leaves and can photosynthesize. But they also use their roots to tap   into the branches of trees like oak, apple,  and maple to steal water and nutrients.   In fact, their parasitism is one of the two  major reasons species like the European mistletoe have come to symbolize winter romance.    See, because they siphon nutrients off a host all winter, they’re able to be green year-round.   

And people have long associated evergreen plants  with feelings of lasting tenderness.

The other reason we associate many  mistletoes with passion has to do with the sticky way they reproduce.   Many species produce white berries  filled with a viscous seedy substance,   and this substance resembles semen. What can I say? People love being  literal with their fertility symbols.

This sticky whitish goo evolved because mistletoes hang out in the tree canopy.   That gives them easy access to sunlight,  especially in winter if their host’s leaves have fallen.   But living in the sticks presents a challenge. In order to be fruitful and multiply,  the plant needs to disperse its seeds onto other branches without them falling to the ground.   The solution? Sticky seeds.

When birds eat the goo-filled berries, their mouths get covered with the syrupy white stuff.   This often prompts efforts to remove the goo  by rubbing their beaks on a tree  branch… which spreads the seeds.   Even if the birds digest the seeds and poop them  out, the gluey poo sticks to their bottoms.   So they have to sit on a branch and wipe their butts.   In fact, the name mistletoe comes from  Anglo-Saxon words that mean “dung on a twig.”   Romantic, eh? Anyhow, once a mistletoe seed is glued to its future host, it germinates and starts to grow.    This baby plant drills its roots into the tree’s cambium – the inner tissue  that’s filled with stem cells.   That’s how it steals the  tree’s nutrients and water.   And, on an evolutionary scale, we know  mistletoes have also stolen their hosts’ genes,   probably to allow them to hack  into the cambium undetected.   But genetics is where mistletoes  get really weird, because somehow,   they’re able to survive without the  genes needed to turn sugar into energy.   In most plants and animals, this  process, dubbed “respiration,” mainly happens in the cells’ mitochondria,

which contain four molecular machines called respiratory complexes. These complexes form an assembly line called the electron transport chain, which essentially siphons energy from electrons to create the cell’s molecular fuel.  Mistletoes can’t do that.   They can’t fully make any of the parts  scientists consider essential for respiration.   Most importantly, at some point,  they lost the genes for complex 1:   the first stop in that electron transport chain.   That means mistletoes are lacking something that all other forms of multicellular life depend on for survival.

That should be fatal, and yet mistletoes cling on. Why would they ditch something that works  well for every other multicellular organism? Scientists aren’t totally sure, though they  think it could be a form of reductive evolution.

That’s when organisms get rid  of genes that don’t spark joy  and opt for a minimalist lifestyle based on theft. Even though the proteins in question are used  to produce energy, they’re not free to make. So, maybe mistletoes steal the  finished proteins from their hosts  and save on those manufacturing costs.

Or maybe they figured out how to rely  on other ways of making cellular fuel. Trouble is, right now, researchers aren’t  sure how much energy mistletoes make or what exactly they steal from their host trees. And no one’s really sure how the baby plants get the energy to  grow before they’ve tapped into a host.

So more research will help us understand how  these parasites work at a fundamental level   and teach us more about plants overall. Now, I wouldn’t want you to walk away from  this thinking mistletoe is just a leafy,   sticky vampire. There are actually  a lot of ways this plant gives back.

Like, the berries are essential to the  diets of many birds, mammals and insects, especially in winter when food is scarce.  In fact, some places, like Australia, have launched mistletoe-planting  programs to boost biodiversity.   A word of warning, though:  they’re toxic to humans. Still, mistletoe may end up saving people’s lives, too. Several studies have suggested that mistletoe   injections reduce cancer symptoms and increase  survival of people with various types of tumors.   And that’s just one of several potential  ways it could prove useful medically.   Clinical trials are underway to dig deeper  into the medical uses of this strange plant.   And hey, if mistletoe ends up helping  us heal—that’d be an even better reason   to hang up this parasitic, poop-reproducing  weirdo and give someone you care about a kiss.

Of course, you could also show  your affection in other ways— like by giving them an annual  subscription to Brilliant! It’s the perfect gift for  anyone with a curious mind.   And with Brilliant’s dozens of STEM  courses, there’s something for everyone. One of my favorites is their Knowledge  and Uncertainty course course.

It teaches you how to wrap your brain  around the math behind uncertainty   so you can feel more confident about your  understanding of the world around you. And right now, you can save 20%  on an annual subscription if you’re one of the first 200 people to sign up   at Brilliant.org/SciShow! So head on over there  to check out what Brilliant has to offer.