Mistletoe’s parasitic kiss

Heres a word on mistletoe, for your holiday reading pleasure, from science humorist Alex Reshavov. This post was originally published in 2011. Enjoy!

The quaint holiday decoration you invited into your home and hung over your doorways is a vicious parasite that leeches nutrients from innocent host trees. It is riddled with cytotoxins, and its seeds are dispersed via bird crap. Merry Christmas.

Pretty parasite

Makes for a far nicer centerpiece than a jar of tapeworms. Image Credit: Kenraiz.

Not all parasites are creepy-crawly worms or protozoans. Some are cheerful-looking shrubs with dainty white berries. Viscum album is one species of mistletoe*, a group of parasitic flowering plants in the order Santalales. It is an obligate hemiparasite. This means that while it does not derive all of its sustenance from a host plant, it does need some interaction with the host to reach its mature state. As a hemiparasite, mistletoe need only steal from its host trees xylem, the transport tissue that handles water and water-soluble nutrients. It is gracious enough to eschew the hosts phloem, which transports sugars. This renders it less of a pathogen, as the host loses water but not food to the parasite.

Gracious host

Viscin has worked it magic. Image Credit: Christer Johansson.

Mistletoe bears a fruit that some birds find delicious. The seeds of these berries are covered in a gluey substance called viscin. Birds eat the berries and then fly off to another tree where they eventually expel the digested remains of the fruit, its viscin coating still adhering to the seeds. The sticky seeds cling to the new branch and begin to grow. As it enlarges, the plant forms a peg that drills through the host branch and eventually reaches the xylem. Now the parasite develops its haustorium, a root-like appendage that allows it to siphon nutrients from the host.

Coming to America

Viscum album is native to Europe and parts of Asia. It is the original Christmas mistletoe, a leafy green shrub adorned with white berries. It has a wide host range, infecting over 450 tree species, including both hardwood and coniferous varieties. So, yes, hypothetically your Christmas mistletoe could attack your Christmas tree (were it still planted in the ground, of course).

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Mistletoe’s parasitic kiss