Mistletoes are parasitic or semi-parasitic plants with flowers that attach to the stem of their host absorbing water, mineral salts, and nutrients from the branches and trunk, which by itself cannot obtain. They are grouped into three families: Lorantaceae, Viscaceae, and Santalaceae; with approximately 200 species that are distributed from natural ecosystems such as forests, jungles, and green areas of the urban zone. Their endophytic system provides food to the aerial region provided with leaves, flowers, and fruits. It is in the fruit where its life as a parasite begins, because it is consumed by birds, which are its main dispersers, later the seed passes intact through the digestive tract and upon falling on a branch on another tree it can continue its biological cycle. The provision of food that they give to birds are examples of the intimate relationship they have with each other. Likewise, the nectar of their flowers is taken by various pollinators: hummingbirds, passerine birds, butterflies, wasps, bees, and bumblebees.

Distribution in Mexico

In Mexico City five genera of mistletoe are frequent and abundant: Arceuthobium, Phorandendron, Psittacanthus, Cladocolea, and Struthanthus. These plants are adapted to make the most of what their host provides, causing a decrease in photosynthetic reactions.

The factors that favor the presence of mistletoe in both natural and urban ecosystems are the birds of the area, as they are the main vectors for the dispersion of the seeds, temperature is another key element, since mistletoes are adapted to temperate climates, although it has been reported that in cold areas they grow when in summer the temperature is higher. In Mexico, the families Loranthaceae and Santalaceae are present in almost all the natural ecosystems of the country. With a direct affectation of 40% of the urban trees, which means that about one million trees of the few that today exist in the urban sprawl are in danger of dying.

Non-anthropic ecosystems vs urban green areas

In a Bulletin of The Botanical Society of Mexico in 1963, a list was made with the species that make up the Lorantaceae family, which is represented by eight genera distributed as follows: Phoradendron with approximately 80 species, Struthanthus with 12 species, both throughout the country. Psittacanthus with 7 species, in the central, south, and southeast of the Republic (Guanajuato, Guerrero, Jalisco, México, Michoacán, Morelos, Nayarit, Oaxaca, Puebla, Querétaro, Quintana Roo, Sinaloa, Sonora, Tabasco, Tamaulipas, Tlaxcala, Veracruz y Yucatán) Phrygilanthus, with two species, in Baja California, Sonora, Chihuahua, Jalisco, and Puebla. Arceuthobium, with one species in Baja California and one in the central region. Oryctanthus, with one species in Chiapas. Razoumofskya in Sonora, Coahuila, and Oaxaca with one species. Antidaphne with one specie in Chiapas. (Fig. 1)

Fig. 1 Distribution of mistletoe in Mexico

In the southern region of the country, Fomento ecológico y social A.C. reported in 2017 the presence of two species of mistletoe, Psittacanthus calyculatus and Struthanthus sp. in mangroves of the Ostion Lagoon in Chiapas. They are species of greater concern because invasive species cause the loss of biodiversity of the reserve. In the case of P. calyculatus (Fig. 2) that has a wide range of distribution, has a significant impact on the local economy because it grows and develops on several species of fruit and forest trees in the center and south of Mexico, such as avocado trees, tejocote, capulin, and peach.

Fig 2. P. calyculatus on a poplar belonging to the urban trees.

Urban vegetation is a remnant of biota and a result of the floristic composition created by humans, studying what encompasses that structure is of utmost scientific interest. Since the trees that live there withstand adverse environmental changes, the main cause of susceptibility by mistletoe. Among which the absence of a space for the proper growth of its roots and the volume of its crown stands out. Mistletoes generally parasitize very tall trees in natural habitats, as well as in urban areas.

There is a direct correlation between the degree of infestation by mistletoe and air humidity, as well as the predominance of infection in trees close to bodies of water due to the water balance and the content of moisture since it is the perfect requirement for the germination of mistletoe seeds and thus have a successful development.

Characteristics of mistletoe

They are considered pathogens of trees due to their harmful effects, particularly in the reduction of growth, the distortion of branches, and the decrease in longevity. The seeds of mistletoes are surrounded by a sticky substance, which is vital for adherence to the host and is called viscin. They have the particularity of germinating on the same tree where they fall or on another, this depends on the vector.

Once the seed falls on the branch of the tree, it begins generating the haustorium. In mistletoes, there are two types of haustorium structures, a primary one: which is formed from the radicular apex that the seedling establishes directly with the host, and the secondary haustorium that originates as lateral organs generally in the epicortical roots of the parasite.

The relationship with water after infection is made through the host's transpiration system. The deviation of this depends on whether the mistletoe is a holoparasite, that is, it takes advantage of both the xylem and the phloem. However, if it is a hemiparasite like most mistletoes, it will fully exploit the host's xylem, taking advantage of the water and mineral salts from the parasitized tree (Fig. 3)

Fig. 3 Salts and minerals are obtained from the xylem (Xl) and floem (FL) of the host. This images shows a cross section of the vascular bundle of Gaiadendron punctatum.

True Villains?

In the summer, when they bloom, the nectar in their flowers is consumed by several species of hummingbirds, and other species of passerine birds, butterflies, wasps, bees, and bumblebees. Their flowers last several days open. The nectar of these mistletoes is the most critical resource during the summer because there are no other species of flowering plants that are so abundant. There is a coevolution of seed dispersal through birds in neotropical systems. Birds choose a particular species of mistletoe that matches the color and size of the fruit. The hummingbird Sephanoides galeritus was the flower visitor of the mistletoe Tristerix corymbosus on the edge of the Pacific of South America.

There are also plants that have applications in medicine, here we present some examples. Recently in a species of the Loranthaceae family; Tapinanthus isolated a new flavonoid glycoside tapinantoside with anticancer action (Gade S., et al.2022), Ligaria cuneifolia and Jodina rhombifolia are plants that grow in the northwest of Argentina and are used in traditional medicine as antibacterial, hemostatic and vulnerary (Soberón et.al, 2014).; Viscum album is used in the pharmacological industry as it possesses antitumor, immunomodulatory, and antimycobacterial properties (G. Gupta et al., 2012).

Conclusion

In conclusion; knowing about the behavior of mistletoe as a parasite and benefactor of other communities is of vital importance since in Mexico and around the world urban areas are a cornerstone in reducing air pollution, supporting biodiversity, mitigating heat, improving human quality, and enhancing urban forestry. It must be considered that the set of all reserves, parks, and green areas within an urban system and the extent and/or condition of these are important for them to provide main ecological services, so the analysis and response to their presence must be immediate. This also helps the accumulation and generation of new knowledge in the various roles that mistletoe has with other organisms like us.

References

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