T’is the season here at the BioPhiles and this week’s species extraordinaire is the European Mistletoe of Christmas carol fame, Viscum album. Its dark green foliage and shiny white berries make it perfect for decking the halls and creating holiday romance, but from a biologist’s point of view, this Christmas standby is no slouch, either. Like all mistletoes, it is a parasite of other tree species and its roots have been modified into specialized structures called haustoria that penetrate the host’s bark, allowing it to divert water and nutrients from its host. Despite being a perfectly adapted thief, the European Mistletoe is considered a hemiparasite because it still retains the ability to photosynthesize and is not dependent on its hosts for all of its nutrition. Nevertheless, heavy mistletoe infections can drastically alter the physiology of their hosts, decreasing water use efficiency, and increasing nitrogen demands. This can ultimately change total biomass production of the tree as well as the relative proportions of leaves, wood, and roots it produces.
The white berries produced by mistletoe are another highly adaptive feature of this plant. The seeds inside the berry are surrounded by sticky flesh called viscin, which contains sugars and long, tightly coiled fibres called cellulose microfibrils. This combination of molecules allows viscin to both stick and stretch. Viscin is a sort of botanical superglue: it doesn’t dissolve in water and isn’t easily broken down by heat, light, or even the digestive tract of birds. When birds eat the mistletoe berries, the seeds that are defecated out are as sticky as when they went in, allowing them to adhere to whatever branch the bird was sitting on when it answered the call of nature. Not all of the seeds make it that far either. The sticky viscin coating often leaves mistletoe seeds stuck to the beaks of birds, which induces preening behaviour. The bird will wipe its face against branches until the offending seed comes off, and the viscin will glue the seed to the branch where it can germinate and start a new infection. This is where bird behaviour becomes important in shaping mistletoe populations. The plants largely depend on birds to disperse their seeds. Because birds prefer to visit trees with tasty mistletoe bushes growing on them, a positive feedback loop initiates, whereby mistletoe populations become concentrated in small patches, and trees have multiple infections. From a biologist’s point of view, the magic in the mistletoe has nothing to do with Christmas cheer, it’s parasitism and adaptation.