Mistletoes are very fascinating parasitic plants. For centuries, people have been kissing under mistletoes during Christmas celebrations. Unlike most common plants, mistletoes grow on the branches of other plants and rely on these "host" plants for water and nutrients. Scientists have been trying to understand why parasitic plants differ in the number of host species parasitised. Like many parasitic plants, mistletoes can parasitise several plant species or only use one or far fewer species. This review contributes to the current broad understanding of parasite-host interactions using mistletoe as a case study. Future research needs are also highlighted.

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We documented avian consumers of the mistletoe Agelanthus natalitius in the field and quantified their effects on seed germination in captive studies. Seven bird species were frequently observed to feed on mistletoe fruits in the field, namely Cape Batis (Batis capensis), Cape White-eye (Zosterops virens), Red-fronted Tinkerbird (Pogoniulus pusillus), Red-winged Starling (Onychognathus morio), Dark-capped Bulbul (Pycnonotus tricolor), Speckled Mousebird (Colius striatus) and Village Weaver (Ploceus cucullatus). Of the four bird species used in captive feeding trials (Red-winged Starling, Speckled Mousebird, Village Weaver and Cape White-eye), most of the birds removed the pulp-cover of the mistletoe fruits and left the exposed seeds in potentially germinable condition. Red-winged Starlings ingested but then regurgitated the seeds and wiped their bills on a twig to dislodge the sticky viscin-covered seeds individually. Mistletoe seeds obtained from cages of Red-winged Starlings had the highest germination success of all the seeds obtained from the captive bird study. The handling of mistletoe fruits applied by all species of birds both in the field and in captivity showed that mistletoe dispersal is likely to occur over short distances, although the seeds are likely to be dispersed to safe sites due to directed dispersal on parental host trees. Such autoinfection and local aggregation increase the interaction with already parasitised hosts, which through time may enhance local specialisation on the most frequently parasitised host species. Zusammenfassung Wir haben Vogelkonsumenten der Mistel Agelanthus natalitius im Freiland beobachtet und ihre Effekte auf die Samenkeimung in Studien in Gefangenschaft quantifiziert. Sieben Vogelarten wurden häufig dabei beobachtet, wie sie im Freiland Mistelfrüchte fraßen, nämlich Kapbatis (Batis capensis), Kap-Brillenvogel (Zosterops virens), Feuerstirn-Bartvogel (Pogoniulus pusillus), Rotschwingenstar (Onychognathus morio), Graubülbül (Pycnonotus tricolor), Braunflügel-Mausvogel (Colius striatus) und Textor (Ploceus cucullatus). Von den vier Vogelarten, die für Futterversuche in Gefangenschaft verwendet wurden (Rotschwingenstar, Braunflügel-Mausvogel, Textor und Kap-Brilenvogel), entfernten die meisten Vögel die Fruchthülle der Mistelfrüchte und ließen die freiliegenden Samen in potentiell keimfähigem Zustand zurück. Rotschwingenstare nahmen die Samen auf, würg-ten sie dann wieder hervor und wischten ihren Schnabel an einem Zweig ab, um die klebrigen von einer Viscinschicht umschlossenen Samen einzeln zu entfernen. Mistelsamen, die aus den Käfigen von Rotschwingenstaren geholt wurden, hatten den höchsten Keimungserfolg von allem Samen, die aus der Untersuchung der Vögel in Gefangenschaft gewonnen wurden. Die von allen Vögeln im Freiland und in Gefangenschaft angewendete Behandlung der Mistelfrüchte zeigte, dass die Verbreitung von Misteln wahrscheinlich über kurze Entfernungen geschieht, obwohl es wahrscheinlich ist, dass die Samen aufgrund von gerichteter Verbreitung auf den e...

Abstract:We surveyed the community composition of trees that host the mistletoe Agelanthus natalitius (Loranthaceae) at two sites (Highover and Mtontwane) in South Africa. We recorded a total of 1464 trees (Acacia karroo and A. caffra) hosting 1202 mistletoes in the 64 surveyed plots (20 × 50 m). There were almost four times as many A. karroo as A. caffra at Highover and three times as many A. karroo as A. caffra at Mtontwane. There was no significant difference in prevalence (percentage of infected trees) at Highover (A. karroo = 22% and A. caffra = 26%), but a significantly greater percentage of A. caffra trees were parasitized at Mtontwane (A. karroo = 25% and A. caffra = 34%). Intensity of infection (number of mistletoe infections per tree) was higher for A. karroo (0.73 ± 0.04 and 1.03 ± 0.64) than for A. caffra (0.66 ± 0.01 and 0.89 ± 0.035) at Highover and Mtontwane, respectively. Prevalence and intensity of infection showed a significant positive relationship with tree size for both host species at both sites. We tested the genotype-by-environment interaction effects in this mistletoe by conducting reciprocal transplant experiments (64 individual trees each received 20 seeds). Initial germination was not site-, substrate- or host-sensitive. However, a general pattern was found that hypocotyls of the germinated seeds grew longer when seeds were placed on the same host species as the parent plant within their own source locality. Consistent with this observation, mistletoes placed on their source host species generally had higher survival than those transferred to non-source host species after 6 mo. Overall, mistletoe seeds from parent plants on A. karroo and mistletoe seeds placed on A. karroo had the highest survival. This could be the result of an adaptation of the mistletoe to the most frequently encountered host species.