Mistletoe Seeds Research Articles

The pattern of germination of teak mistletoe seeds in relation with parasitism

The objectives of this research are to test the initial viability and germination pattern of teak mistletoe seeds, namely Dendrophthoe pentandra and Macrosolen tetragonus in associated with parasitism. Research results showed that the pattern of D. pentandra germination was different from that of M. tetragonus. The germination process of D. pentandra showed incomplete development of germination morphology, marked by an absence of development or growth of hypocotyl and cotyledon. Parasitism was more dominant in D. pentandra, supported by the fact that the sum of mean germination day (RH) in the stem of teak seedling was faster (17.54±2.77 days), as compared with that of M. tetragonus (35.13±1.76 days), although mortality of M. tetragonus seedlings was very low (±3%), whereas that of D. pentandra was fairly high (±34%). It was related to the life cycle of M. tetragonus, which was longer than that of D. pentandra. In the attachment of seeds in the standing tree stems, it was proven that the germination percentage (%K) of D. pentandra (38.5%) was higher than that of M. tetragonus (11.1%). Seed germination of D. pentandra from bird feces also showed that %K (46.4%) was higher than seed attachment in tree stem, including M. tetragonus. It was supposed that there was a correlation between the easiness of germination and parasitism character in teak mistletoes. It was considered that parasitism of D. pentandra was more prominent than that of M. tetragonus in a teak host.

Freezing tolerance of seeds can explain differences in the distribution of two widespread mistletoe subspecies in Europe

The ability of plants to tolerate freezing limits their geographical distribution. Therefore, winter warming may shift a species’ occurrence northwards and/or to higher altitudes. In Europe, the hemiparasitic vascular plant Viscum album (mistletoe) has two common and widespread subspecies: V. a. ssp. album and V. a. ssp. austriacum. The former has a more northern geographic distribution than the latter. Therefore we hypothesised that seeds of V. a. ssp. album are more tolerant to freezing than those of V. a. ssp. austriacum. From these two mistletoe subspecies V. a. ssp. austriacum is, in some managed forest areas, considered a novel threat to tree growth and forest health.Berries of V. a. ssp. album were collected from Sweden, Poland and Switzerland and berries of V. a. ssp. austriacum were collected from Poland, Switzerland and Spain. After storage at −3 °C, seeds were extracted from the pulp of berries and exposed to eight different temperatures between −8 °C and −30 °C, with the storage temperature serving as the control. After freezing treatments, germination of seeds was monitored. In addition, differential thermal analysis was used to measure freeze tolerance of seeds.The seeds of V. a. ssp. album tolerated lower temperatures than seeds of V. a. ssp. austriacum. The temperature at which 50% of seeds lost their ability to germinate (LT50) was −15 °C in V. a. ssp. austriacum and between −15 °C and −19 °C in V. a. ssp. album. The results of differential thermal analysis to determine the freezing point of seeds supported these findings.The freezing tolerance of mistletoe seeds was relatively well coupled with the winter climate at the edge of their current geographic distribution. Based on our results, the warming of winters may eliminate the abiotic barrier that has thus far limited mistletoes’ expansion, opening a window of opportunity for these parasites to increase their abundance and shift their distribution range towards higher latitudes and altitudes.Although mistletoes play important ecological roles in forest ecosystems, their recent increase has raised concern among forest managers, because they may cause a substantial reduction in tree growth in single species dominated stands. Increasing tree species diversity might be an effective method for limiting future mass infestations in homogeneous managed forests.

Tree diversity reduces pine infestation by mistletoe

The pattern that a given tree species suffers less damage when growing with heterospecific neighbors than amongst conspecific plants, i.e. associational resistance, is common for insect herbivores and many fungal pathogens. However, associational resistance to parasitic plants has never been tested in a replicated study. Using paired forest plots, we investigated whether tree diversity triggered associational resistance to a tree parasite, the European mistletoe Viscum album ssp. austriacum, by comparing pure stands of Scots pine (Pinus sylvestris) with mixtures of Scots pine and Maritime pine (Pinus pinaster) in northern Spain. Maritime pine, with 1.2% of trees being infested, was considered a non-host species in the study area. The infestation level of Scots pines was significantly higher in pure plots (45.1%) than in mixed plots of Scots pines and Maritime pines (25.4%). Our study is the first to quantify associational resistance to a plant parasite in mixed vs. pure forest stands and suggests that mechanisms proposed to explain associational resistance to insects and pathogens also apply to plant parasites. Scots pine trees that were taller than the surrounding trees had a higher infestation probability, in both pure and mixed stands. Scots pine trees growing in mixtures were slightly lower than Maritime pines, suggesting that associational resistance was partly driven by reduced relative tree height. However, the effect of plot type (pure vs. mixed) remained significant after the effect of tree height was accounted for, thus indicating that other factors also contributed to lower mistletoe infestation in mixed plots. In particular, the behavior of birds dispersing mistletoe seeds might differ in mixed vs. pure stands.

Estudio de susceptibilidad de variedades de aguacate al ataque de muérdagos en Michoacán

Las infestaciones ocasionadas por plantas parásitas de muérdago, que atacan a plantas perennes silvestres y cultivadas de México, representan uno de los principales factores limitantes de la producción y son causantes de altas tasas de mortalidad en plantas perennes. El objetivo fue estudiar la susceptibilidad de los diferentes materiales de aguacate al ataque por especies de muérdago, e identificar los géneros y especies de plantas de muérdago que crecen sobre la diversidad de árboles de aguacate en la región aguacatera de Michoacán. Se muestreó una fracción de esta región, y se colectó material botánico de plantas parásitas que estaban afectando árboles de aguacate. En vivero en plantas de aguacate ‘Hass’, ‘Fuerte’ y “criollo”, se indujeron infestaciones con semillas de las diferentes especies de muérdago colectadas en árboles de aguacate in situ. Se colocaron tres semillas por planta y mediante un diseño experimental de bloques completos al azar, con seis repeticiones; se observó la respuesta durante un año, en cuanto a la tasa de germinación y desarrollo de tejido endofítico del hemiparásito sobre las plantas infestadas. Se registraron 3 géneros y 4 especies parasitando árboles de aguacate (Persea americana Mill. var. drymifolia). Los resultados de la colonización inducida del muérdago, en Psittacanthus calyculatus, se encontró diferencia estadística altamente significativa (Pr>F=0.0024) para el desarrollo de tejido endofítico de la planta parásita sobre los diferentes materiales de aguacate utilizados. La mayor incidencia fue en aguacate “criollo”, con el 83.3% y en aguacate ‘Fuerte’ fue del 66.7 % de plantas infestadas; el aguacate ‘Hass’ en ninguna de las plantas desarrolló la infección.

Consequences of disperser behaviour for seedling establishment of a mistletoe species

AbstractThe dispersal process in plants links adults and their offspring. For frugivore‐dispersed plants, animal behaviour can have a strong effect on plant fitness. Many mistletoes are totally dependent on animals that deposit seeds on suitable hosts and particular branch diameters. We characterised the seed dispersal and seedling establishment of the mistletoe Tristerix corymbosus, which at our study site, is exclusively dispersed by the marsupial Dromiciops gliroides. Mistletoes’ fruits have a viscous pulp that remains in the seed even after dispersal. This substance adheres the seed to the host branch. We estimated host branch availability in the forest and seed deposition (faeces) by the marsupial in the study area. Specifically, the branch suitability factors we assessed were host identity, branch status (alive or dead), branch diameter, height, and canopy cover. Lodged faeces were individually marked and the number of seed deposited within these droppings was counted, and we recorded the number of seedlings with true leaves that had established after 1 year to estimate the probability of seedling establishment. Branch diameter and canopy cover had a significant positive effect on seed deposition probability. Seedling establishment probability decreased with the number of seeds deposited per faeces and with canopy cover. In general, the marsupial deposited mistletoe seeds in microsites that increase the chance of seedling establishment. Thus, the movement behaviour of the marsupial has a positive effect on the regeneration process of this mistletoe species.

Dissecting the contributions of dispersal and host properties to the local abundance of a tropical mistletoe

Summary The interplay between dispersal and adaptation to local environments ultimately determines the distribution of plant species, but their relative contribution remains little understood. Tropical mistletoes provide the opportunity to dissect these contributions of dispersal and adaptation, because as hemiparasitic plants, they are typically adapted to grow on a handful of species within diverse tropical communities and are non‐randomly dispersed by mutualistic frugivorous birds. Here we hypothesized that the primary determinant of the abundance of a tropical mistletoe (Dendropemon caribaeus, Loranthaceae) in Puerto Rico will be the compatibility between the mistletoe and plant species in a community. Alternatively, the mistletoe's abundance could be primarily shaped by other factors such as the availability of potential host plants, or factors that determine how mistletoe seeds are dispersed by avian frugivores. We conducted surveys and experiments to assess the capacity of this mistletoe to grow on trees available in the local community, and measured the monthly phenology and seed dispersal patterns of the mistletoe and other bird‐dispersed plants in the community over a period of 4 years. A path model was used to evaluate how the abundance of the mistletoe was shaped by host abundance, fruiting phenology, bird dispersal and compatibilities with host plants. Our analyses show that the compatibility between mistletoe and host tree species, measured by mistletoe survival and growth rate, was the most important factor for mistletoe abundance. The next most important factor was the phenological characteristics of the hosts; this outcome likely arose because frugivory and seed dispersal services for mistletoes and hosts are performed by the same birds. Synthesis. Mistletoes often parasitize only a subset of the suitable plant species that are available in a given community. Our results indicate that such patterns are not only determined by host quality and abundance but also by the phenological patterns of trees that influence the probabilities of mistletoe seeds being deposited on them by shared avian seed dispersers.

Uncorrelated mistletoe infection patterns and mating success with local host specialization in Psittacanthus calyculatus (Loranthaceae)

Mistletoe infection between conspecific and interspecific hosts can be restricted by seed dispersal, host-mistletoe compatibility and abiotic factors, yet no studies have linked mistletoe infection patterns and pollination together for understanding mistletoe distribution at a local scale. Psittacanthus calyculatus (Loranthaceae) is a hemiparasitic plant with a broad host range across its geographic distribution. The potential for local host adaptation has been shown using cross-inoculation experiments, in which plants of mistletoe seeds collected from a given host are more likely to survive when they are inoculated on conspecific host trees compared with those inoculated on other host provenances. Here we evaluate host adaptation by describing the local patterns of infection (prevalence and intensity) of P. calyculatus mistletoes on three native host tree species (Alnus acuminata, Quercus crassipes, Salix bonplandiana) and one introduced species (Populus alba) and carried out cross-pollination experiments to examine how pollination affects infection patterns of different host species. Mistletoe infection prevalence (proportion of infection) and infection intensity (mean number of mistletoes per tree) were in general disproportional with respect to the availability of native host tree species but higher to that of non-native host tree species. Cross-pollination experiments showed higher mating success on the native host tree species, suggesting higher local adaptation to specially Q. crassipes. The observed spatial distribution of host tree species and mistletoe infection along with the non-random mating could contribute to local genetic structuring of mistletoe populations.

The Impact of Environmental and Host Specificity in Seed Germination and Survival of Korean Mistletoe [Viscum album var. coloratum (Kom.) Ohwi

Humankind has been searching for medicinal materials from various plant sources in an attempt to treat disease. Mistletoe is one indubitable plant source for these materials due to its effectiveness in treating various diseases, but it has almost disappeared from the mountainous areas of Korea due to excessive harvesting. In this study, in order to select host tree species for Korean mistletoe [Viscum album var. coloratum (Kom.) Ohwi] by seed inoculation and to clarify the effect of host specificity among various tree species were conducted for the purpose of gaining basic information for the artificial cultivation of Korean mistletoe. Almost all the seeds of Korean mistletoe germinated in vitro at the temperature of 15℃. Among host trees used in this study, Prunus mume showed the highest parasitic affinity with inoculated Korean mistletoe, compared with any other host plants. However, treatment of hormones could not increase the low survival rate of Korean mistletoe on the host trees.

Spatial heterogeneity of a parasitic plant drives the seed‐dispersal pattern of a zoochorous plant community in a generalist dispersal system

Summary Biota plays a central role as sources of spatial heterogeneity, having great potential to define ecological processes and patterns in the landscape. Mistletoes are fleshy‐fruited parasitic plants that dwell in forest canopies showing a strong aggregated spatial distribution. Parasitized trees potentially concentrate frugivore activity on their canopy, where birds find food, places to perch and protection against predators. Thus, seed‐deposition patterns generated from the canopy are expected to reflect the heterogeneity associated with the parasite. This becomes especially important in generalist dispersal systems; however, so far, we do not know the implications of mistletoe spatial heterogeneity on the seed‐dispersal pattern of other plants with which they simultaneously fruit. In a Mediterranean pineland, we analyse the impact of Viscum album subsp. austriacum on the seed‐deposition pattern of a zoochorous plant community, taking into consideration the spatial and temporal variability of environmental factors influencing the frugivore's habitat use, such as fruit availability and forest tree density. For four consecutive years, we studied 55 pairs of trees parasitized and unparasitized by mistletoe, analysing zoochorous fruit availability, frugivore visits and the zoochorous seed rain in selected trees. As expected, frugivorous birds responded to mistletoe heterogeneity by visiting parasitized trees preferentially to unparasitized ones, generating a differential deposition of mistletoe seeds on tree branches while dispersing seeds of co‐fruiting species under the host canopy. Availability of understory fruits remained similar in patches of parasitized and unparasitized trees, but showed strong temporal fluctuations reflected in the seed rain. On the other hand, mistletoe proved more copious in patches of parasitized trees and their fruit crops varied little between years, making mistletoes reliable food resources likely to lead to consistency in fruit‐deposition patterns. In conclusion, mistletoes, by patchily growing on the canopy layer and concentrating zoochorous seeds underneath, can shape the spatial seed‐deposition pattern of fleshy‐fruited plants in the forest. Moreover, as seeds constantly reach the same deposition sites over long periods, the soil beneath the host canopy could become hotspots for community regeneration. In degraded areas, such mistletoe effects might be critical, possibly promoting recolonization and vegetation recovery through the frugivore's activity.

Modeling occupancy of hosts by mistletoe seeds after accounting for imperfect detectability.

The detection of an organism in a given site is widely used as a state variable in many metapopulation and epidemiological studies. However, failure to detect the species does not necessarily mean that it is absent. Assessing detectability is important for occupancy (presence—absence) surveys; and identifying the factors reducing detectability may help improve survey precision and efficiency. A method was used to estimate the occupancy status of host trees colonized by mistletoe seeds of Psittacanthus plagiophyllus as a function of host covariates: host size and presence of mistletoe infections on the same or on the nearest neighboring host (the cashew tree Anacardium occidentale). The technique also evaluated the effect of taking detectability into account for estimating host occupancy by mistletoe seeds. Individual host trees were surveyed for presence of mistletoe seeds with the aid of two or three observers to estimate detectability and occupancy. Detectability was, on average, 17% higher in focal-host trees with infected neighbors, while decreased about 23 to 50% from smallest to largest hosts. The presence of mistletoe plants in the sample tree had negligible effect on detectability. Failure to detect hosts as occupied decreased occupancy by 2.5% on average, with maximum of 10% for large and isolated hosts. The method presented in this study has potential for use with metapopulation studies of mistletoes, especially those focusing on the seed stage, but also as improvement of accuracy in occupancy models estimates often used for metapopulation dynamics of tree-dwelling plants in general.

Host specificity and experimental assessment of the early establishment of the mistletoe Phoradendron crassifolium (Pohl ex DC.) Eichler (Santalaceae) in a fragment of Atlantic Forest in southeast Brazil

Mistletoe establishment relies heavily on a seed reaching a proper host plant. Small frugivorous birds usually disperse large numbers of mistletoe seeds. However, in the field, mistletoes are absent from some potential available hosts. We investigated whether the mistletoe Phoradendron crassifolium has some preferences for specific host trees in a fragment of Atlantic Forest in southeast Brazil. We surveyed 397 tree individuals of 50 species within 25 families. Seven of those species (14%) bore P. crassifolium infections. Although prevalence at the individual level was low (11.6%), there were marked deviations in infection levels among species and families. Most (87%) of the infections (40 of 46) occurred in species belonging to the families Anacardiaceae (Lithraea molleoides and Tapirira guianensis) and Siparunaceae (Siparuna guianensis), which nevertheless accounted for only 26% of the potential individual hosts (103 of 397). We also performed an experiment simulating bird behavior. We inoculated 480 mistletoe seeds to the bark of four potential hosts in field, following the fate of the seeds for five months. No differences in host preference were observed. The low specificity detected at the local level was confirmed by a survey of exsiccata collected over the geographical distribution of the mistletoe, suggesting that P. crassifolium prevalence is more dependent on dispersal limitation than on mistletoe-host compatibility.

Transcriptome analysis of mistletoe (Viscum album) haustorium development

The haustorium is a specific organ for penetration of host tissue in parasitic plants. To better understand the molecular mechanisms of the establishment of host-parasite relationships using gene expression profiles, early stage haustorium germination from mature mistletoe seeds was examined. We have generated a cDNA library that is an excellent source of expressed sequence tags (ESTs) specifically related to haustorium development and host penetration. We analyzed 4,771 ESTs derived from a mistletoe (Viscum album) haustorium cDNA library. Cluster analysis identified 727 contigs (23.88%) and 2,317 singletons (76.12%), yielding a total of 3,044 unique genes. Among these genes, 1,760 clones (57.8%) showed significant similarity to known genes; the remaining 1,284 clones were novel. Annotation of genes with Gene Ontology indicated that the most abundant ESTs reveals that defense against biotic / abiotic stresses, primary metabolic processes, cell wall loosening and modification is critical for haustorium development and establishment of the host-parasite connection. Especially, it is likely that xyloglucan endotransglycosylases (XETs), glucanase, expansins and other cell wall hydrolases cooperate in cell wall modification during the stages of host-parasite connection. Functional expression of full-length forms corresponding to target ESTs was applied to characterize haustorium genes. This EST information will help gene discovery and characterization and provide promising targets for genetic engineering. The EST analysis presented here represents the first reported transcriptome data for V. album var. coloratum. Although the number of ESTs analyzed may not be sufficient to completely elucidate the mechanism of the host-parasite connection, the integrated approaches reported here represent an essential step toward understanding haustorium development in parasitic plants and provide a valuable resource for defining molecular mechanisms in the host-parasite connection.

Generalist birds govern the seed dispersal of a parasitic plant with strong recruitment constraints.

Mistletoes constitute instructive study cases with which to address the role of generalist consumers in the study of plant-animal interactions. Their ranges of safe sites for recruitment are among the most restricted of any plant; therefore, frugivores specializing in mistletoe have been considered almost indispensable for the seed dispersal of these parasitic plants. However, the absence of such specialists in numerous regions inhabited by many mistletoe species raises the question of whether unspecialized vectors may successfully disperse mistletoe seeds to narrowly defined safe sites. Using the European mistletoe Viscum album subsp. austriacum as a study case, we recorded a broad range of 11 bird species that disperse mistletoe seeds. For these species, we studied the mistletoe-visitation rate and feeding behavior to estimate the quantity component of dispersal effectiveness, and the post-foraging microhabitat use, seed handling, and recruitment probabilities of different microhabitats as a measure of the quality component of effectiveness. Both endozoochory and ectozoochory are valid dispersal mechanisms, as the seeds do not need to be ingested to germinate, increasing seed-dispersal versatility. Thrushes were the most effective dispersers, although they were rather inefficient, whereas small birds (both frugivores and non-frugivores) offered low-quantity but high-quality services for depositing seeds directly upon safe sites. As birds behave similarly on parasitized and non-parasitized hosts, and vectors have broad home ranges, reinfection within patches and the colonization of new patches are ensured by an ample assemblage of generalist birds. Thus, a parasitic plant requiring precision in seed dispersal can rely on unspecialized dispersers.

Effect of Harvesting Time and Storage Duration of Viscum album Seeds on in vitro and ex vitro Germination on the Branch of Prunus mume

Viscum album var. coloratum (mistletoe) is considered as one of the endangered plant species in Korea. Our objective is to restore its population and multiplication of plant by ex situ method. In this research we explored the maximum germination (in vitro) of freshly harvested and stored seeds of mistletoe collected in different time intervals. Ex vitro germination after artificial inoculation on the branches of Prunus mume in different physiological conditions was also monitored. The research revealed that the lately harvested seeds (Feb. and March 2014) were superior over early harvested seeds (Nov. 2013 and Jan. 2014) of mistletoe due to the higher percentage of germination (above 93%). According to the data, it is also revealed that the survival and germination rate of mistletoe seeds decreased with the increase in storage duration. In ex vitro germination, the fluctuated temperature of a glass house in natural condition enhanced (four fold) the rate of germination on the branches of Prunus mume than the constant temperature condition in the glass house.

Effect of Storage Conditions and Scarification on in vitro Seed Germination in Lorathus tanakae Hosok

Loranthus tanakae (Franch. & Sav.) is an endangered species of mistletoe, distributed in Korean peninsula. The objective of our research is to determine the effect of storage duration and conditions [air flow (AF) and air tight (AT)] at different temperatures for survivability and germination of mistletoe seeds, and also to monitor the effect of seed scarification on germination in vitro. The result revealed that the seeds stored in natural conditions (no stratification) showed highest survival rate of 100% and retained up to 93.3% even after two months of storage in natural conditions and showed higher germination percentage (90%) compare to after ripened seeds. However, the seed stored at <TEX>$0^{\circ}C$</TEX> decreased the germination percentage (ranged from 63 to 73%). Therefore, it can be confirmed that mistletoe does not need after ripened treatment to promote germination. Our research also showed that the storage of L. tanaka seeds in freezing temperatures of <TEX>$-20^{\circ}C$</TEX> and in room temperature for long time either in AT or AF conditions caused the loss of survival and germination rate. On the other hand, the chemical scarification (0.01N HCl incubation for 12 hrs. at <TEX>$38^{\circ}C$</TEX>) method was proven more effective to enhance germination percentage of L. tanakae. Regarding the temperature regime, <TEX>$22^{\circ}C$</TEX> showed early germination of mistletoe seeds in vitro.

Linking safe sites for recruitment with host-canopy heterogeneity: The case of a parasitic plant, Viscum album subsp. austriacum (Viscaceae).

• Premise of the study: Canopies are ecologically relevant compartments of forests. Multiple sources of heterogeneity interact within forest canopies due to their structural complexity, which exert major influences on the structure and composition of epiphyte communities. Here, we explore canopy environmental heterogeneity of a Mediterranean pine forest, identifying the key biotic and abiotic factors determining mistletoe (Viscum album subsp. austriacum) recruitment at coarse and fine spatial scales.• Methods: Through field experiments, we assessed the range of suitable host species for V. album subsp. austriacum (hereafter, V. a. austriacum). We characterized the variation in abiotic factors at a fine spatial scale on the host species. Finally, we examined the effects of biotic (predation) and abiotic (light, temperature) factors on the fate of mistletoe seeds and seedlings along host branches.• Key results: We confirmed the tight specificity of V. a. austriacum to pine species, in particular to P. nigra at the local scale. Biotic constraints increased toward the branch interior, with minor effects on apical locations due to the positive effect of pine-needle coverage. Contrarily, abiotic constraints increased toward branch extremities, harming mistletoe seeds by encouraging their desiccation.• Conclusions: Biotic and abiotic variables exert a strong, nonrandom filter on V. album regeneration, resulting in recruitment hotspots at the periphery of the branches and sites with a high probability of recruitment failure at thicker and more exposed locations. The narrow range of suitable host species and the scarcity and spatially restricted recruitment hotspots for V. a. austriacum leads to the clumping of mistletoe populations at the finer spatial scale.

A Host Creates an Enemy‐free Space for Mistletoes by Reducing Seed Predation Caused by a Woodboring Beetle: A Hypothesis

AbstractParasitic plants rely on host plants for nutrition. The number of host species varies largely between groups, from single species or genus to hundreds of species. Relative abundance of the host and evolutionary history are the main requisites for parasitic plants to develop specificity to abundant hosts. In the present study, we suggest a novel mechanism by which a hemiparasitic mistletoe can develop local specificity mediated by its host. First, we describe a novel interaction in which a woodboring beetle (Hypothenemus obscurus) preys on mistletoe seeds (Psittacanthus plagiophyllus) attached to tree branches. This beetle preys more frequently on seeds deposited on branches of non‐host species than on branches of its unique local host species (53 percent on average vs. 26 percent respectively). We hypothesize that local host specificity for this mistletoe could be partly mediated by beetle‐host incompatibility, since the host offers a predation‐free space in which mistletoes have better chances to grow. Furthermore, that the exceptional gum exudates produced by this unique host species minimize beetle attacks on branches, thus reducing predation of mistletoe seeds. This novel tritrophic interaction opens an avenue for research on macroscopic host‐specificity mechanisms that occur at the level of the host bark and that have been poorly studied by plant ecologists.

Reciprocal transplant experiment suggests host specificity of the mistletoe Agelanthus natalitius in South Africa

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.

Genetic diversity and population structure of the mistletoe Tristerix corymbosus (Loranthaceae)

The genetic structure of a plant species is influenced by life-history traits, geographical range and ecological interactions that shape gene flow. We examined the genetic structure of the South American mistletoe Tristerix corymbosus using random amplification of polymorphic DNA. This species is found mainly in Chile and inhabits two biomes, the Chilean matorral and the temperate forest. The main pollinator agent, a hummingbird, is the same across the whole range, but the disperser assemblage varies between biomes. We selected 22 populations, eight of which were located in the Chilean matorral, where fruits are yellow and birds act as seed dispersers, and 14 populations in the temperate forest, where fruits are green and a marsupial disperses the mistletoe seeds. A total of ten primers were used to generate amplification products for 121 individuals (ca. six individuals per population) and 91 bands were scored. Results show that this mistletoe species is highly variable with 81 % of the bands polymorphic and a Shannon’s diversity index among populations of 0.634. The temperate forest shows slightly higher diversity indices than the Chilean matorral. The central region of the mistletoe geographic range was more variable than the north and the south regions, suggesting that it is a genetically mixed zone. It is likely that gene flow occurs mainly via hummingbirds moving pollen between biomes and birds moving seeds from north to south during spring migrations.

Mistletoe specialist frugivores: latterday ‘Johnny Appleseeds’ or self-serving market gardeners?

Many plants use birds to disperse their propagules, but mistletoes are especially reliant on their services. As aerial parasites, mistletoe seeds need to be deposited upon branches of suitable hosts, and mistletoe specialist frugivores (from eight different avian families) have long been regarded as their coevolved dispersers. Like the pioneer Johnny 'Appleseed' Chapman who established nurseries that helped open up land for settlement, these birds are considered benevolent dispersers of this keystone resource and often invoked as illustrative examples of mutualistic interactions. We have compared recent research on these specialists with studies of other birds with broader diets (generalists) which also disperse mistletoe seed. Rather than mutualists, we suggest that mistletoe specialist frugivores are better considered exploitative, with multiple lineages evolving independently to capitalize on this reliable, nutritious resource. Although mistletoe specialist frugivores are quantitatively important seed dispersers in some regions, their specialized diet restricts them to areas with high mistletoe densities, resulting in contagious dispersal patterns. By intensifying existing infections, mistletoe specialist frugivores increase their own medium-term food security-akin to market gardeners profiting from intensive cultivation. Exploring the ecological and evolutionary implications of this proposition, we evaluate the consequences of different dispersal patterns on mistletoe fitness and highlight the neglected role of dietary generalists in the stabilization of plant-animal interactions.