Plant Vampires Have Hosts They Won’t Bite

Mistletoes are parasitic plants, but how do they choose their hosts? Jeremy Midgley of the University of Cape Town has published a study in the African Journal of Ecology that suggests some plants have immunity from Mistletoes. The research is based on epi-parasites, plants that can parasitise a parasite. The study has implications for understanding how parasitic plants interact with their hosts.

What are mistletoes?

Mistletoes are parasitic plants that attach to the branches of trees and shrubs and steal water and nutrients from their hosts. Unlike most plants, mistletoes don’t have true roots in the ground. Instead, they tap into the host’s vascular system using structures called haustoria.

There are over 1,500 species of mistletoes globally. Mistletoes thrive in temperate and tropical regions around the world. Some mistletoe species are specialists that only parasitise a single species or genus of host plants. Others are generalists that can infect multiple unrelated hosts. In general, mistletoes are constrained by their ability to penetrate host tissues and access water.

Success for a mistletoe is down to it having a higher transpiration rate than its host. It draws liquid in and then loses it through its leaves as it photosynthesises. Because water passes through the mistletoe faster, it’s constantly drawing in fresh nutrients to grow.

Why epi-parasitic mistletoes matter

In rare cases, one mistletoe will attach to and parasitise another mistletoe plant. This phenomenon is called epi-parasitism. There are only about 27 known obligate epi-parasitic mistletoe species. These are plants that have to parasitise another mistletoe to survive. Examples of epi-parasitic mistletoes include Viscum goetzei and Viscum loranthicola.

If these epi-parasites are to be successful, then they need transpiration rates higher than the host mistletoe, which in turn is higher than the original host plant. So – if the reason for a mistletoe attacking a plant is physiological, then an epi-parasite should also be happy attacking its host’s host. If the epi-parasite cannot attack the host’s host, the base host is immune to a direct attack from the epi-parasite.

So this is what Jeremy Midgley tried to find out. Did any epi-parasites attack a host’s hosts? This isn’t a simple survey, as some mistletoes can live as parasites, or epi-parasites, or even parasitise themselves.

Studying mistletoes in South Africa

To test the immunity hypothesis, Midgley conducted field research in South African thicket vegetation in the West Coast National Park. This area contains a diversity of woody shrubs and trees suitable as mistletoe hosts.

Midgley focused on three common mistletoe species: Viscum rotundifolium, Viscum capense, and Septulina glauca. Viscum rotundifolium is a specialist that taps a single host species, while Viscum capense is a generalist. Septulina glauca also specializes on one or two host plants. These three mistletoes allowed comparisons of parasitism strategies and host preferences within the same location.

In total, the three mistletoe species parasitized 15 different woody plant species at the site. However, they did not infect many other potential host species that were present. Midgley also surveyed epiparasitic interactions and found no cases where the mistletoes parasitized the host species of other mistletoes.

Evidence for the immunity hypothesis

Midgley’s field observations provided evidence in support of the immunity hypothesis for mistletoe host specificity.

For example, Midgely found that the mistletoe Viscum capense parasitised Viscum rotundifolium at the study site. However, Viscum capense did not infect any of the common host species that Viscum rotundifolium typically relies on, even though those hosts were abundant.

This avoidance of direct parasitism of a mistletoe host’s own hosts contradicts the idea that mistletoes only choose compatible hosts based on environmental needs. It also does not fit with the fact that many mistletoes are specialised on a narrow range of host plants.

Globally, obligate epi-parasitism, where a mistletoe can only parasitize other mistletoes, is very rare – occurring in about 1% of mistletoe species. The immunity hypothesis can better explain this rarity.

Midgley’s findings point to chemical or physical barriers preventing mistletoes from directly parasitizing relatives of their typical hosts. The results support the immunity hypothesis where host specificity is mediated by incompatibility more than environmental factors.

READ THE ARTICLE
Midgley, J. (2023) “Epi‐parasitic mistletoes don’t parasitise their host’s host and this supports the immunity hypothesis for host choice,” African Journal of Ecology. Available at: https://doi.org/10.1111/aje.13212.

Cover: Viscum rotundifolium. Image: JMK / Wikimedia Commons