The

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomencla ...

''Arceuthobium'', commonly called dwarf

mistletoe Mistletoe is the common name for obligate hemiparasitic plants in the order Santalales. They are attached to their host tree or shrub by a structure called the haustorium, through which they extract water and nutrients from the host plant. ...

s, is a genus of 26 species of

parasitic plant A parasitic plant is a plant that derives some or all of its nutritional requirements from another living plant. They make up about 1% of angiosperms and are found in almost every biome. All parasitic plants develop a specialized organ called the ...

s that parasitize members of

Pinaceae The Pinaceae, or pine family, are conifer trees or shrubs, including many of the well-known conifers of commercial importance such as cedars, firs, hemlocks, larches, pines and spruces. The family is included in the order Pinales, formerly know ...

and

Cupressaceae Cupressaceae is a conifer family, the cypress family, with worldwide distribution. The family includes 27–30 genera (17 monotypic), which include the junipers and redwoods, with about 130–140 species in total. They are monoecious, subdioeci ...

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the C ...

Central America Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. C ...

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...

, Europe, and

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. Of the 42 species that have been recognized, 39 and 21 of these are endemic to North America and the United States, respectively. They all have very reduced shoots and

leaves A leaf ( : leaves) is any of the principal appendages of a vascular plant stem, usually borne laterally aboveground and specialized for photosynthesis. Leaves are collectively called foliage, as in "autumn foliage", while the leaves, ste ...

(mostly reduced to scales) with the bulk of the plant living under the host's bark. Recently the number of species within the genus has been reduced to 26 as a result of more detailed genetic analysis.

Description

They are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

, individual plants being either male or female. The

fruit In botany, a fruit is the seed-bearing structure in flowering plants that is formed from the ovary after flowering. Fruits are the means by which flowering plants (also known as angiosperms) disseminate their seeds. Edible fruits in particul ...

is unusual in that it builds up hydrostatic pressure internally when ripe and shoots the single sticky

seed A seed is an Plant embryogenesis, embryonic plant enclosed in a testa (botany), protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, includ ...

up to speeds nearly , an example of

rapid plant movement Rapid plant movement encompasses movement in plant structures occurring over a very short period, usually under one second. For example, the Venus flytrap closes its trap in about 100 milliseconds. The traps of Utricularia are much faster, closi ...

. The lodgepole pine dwarf mistletoe,'' Arceuthobium americanum, has been found to explosively-disperse its seeds through

thermogenesis Thermogenesis is the process of heat production in organisms. It occurs in all warm-blooded animals, and also in a few species of thermogenic plants such as the Eastern skunk cabbage, the Voodoo lily ('' Sauromatum venosum''), and the giant w ...

.
Rolena A.J. deBruyn, Mark Paetkau, Kelly A. Ross, David V. Godfrey &

Cynthia Ross Friedman Cynthia "Cindy" Ross Friedman (1970 or 1971 – 24 December 2018) was a professor in biological sciences at Thompson Rivers University in Kamloops, British Columbia, Canada. She was inducted into the inaugural cohort of the Royal Society of Cana ...

. Thermogenesis-triggered seed dispersal in dwarf mistletoe. Dwarf mistletoe seeds are enveloped in a

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substance ...

, glue-like substance called viscin. Many fail to land on a suitable host's shoot, but some succeed, and in this way they are spread through the forests as a pest front. The spread of dwarf mistletoes in forest stands is greatest from the overstory to the understory, owing to gravity. Advantageous stand conditions for the spread of the parasite include an uneven-aged stand structure with severely infected hosts in dominant and codominant crown classes, species composition dominated by the primary host, and tree densities between 175 and 500 trees/ha. There are also several species from Europe and

including one of the smallest in the genus, ''A. minutissimum'' that lives on its host, ''

Pinus wallichiana ''Pinus wallichiana'' is a coniferous evergreen tree native to the Himalaya, Karakoram and Hindu Kush mountains, from eastern Afghanistan east across northern Pakistan and north west India to Yunnan in southwest China. It grows in mountain valle ...

'' in the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 100 ...

Effects of parasitism

In western forest ecosystems of

, numerous dwarf mistletoe species are considered to be serious forest-borne disease agents. Severe dwarf mistletoe infection can result in a reduction in tree growth, premature tree mortality, reduced seed and cone development, and reduced wood quality, and increases the susceptibility of the host tree to pathogen and/or insect attack. Most of the commercially important conifers in western North America are parasitized by one or more dwarf mistletoes. The interaction between dwarf mistletoes and their host can be generalized as a source-to-sink relationship. Dwarf mistletoes derive the majority of their nutrition from the host’s

vascular tissue Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There ...

s. Dwarf mistletoes have a root-like

endophytic An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease. Endophytes are ubiquitous and have been found in all species of plants studied to date; h ...

system, composed of primary and secondary

haustoria In botany and mycology, a haustorium (plural haustoria) is a rootlike structure that grows into or around another structure to absorb water or nutrients. For example, in mistletoe or members of the broomrape family, the structure penetrates ...

, which invade, but do not injure, both the

xylem Xylem is one of the two types of transport tissue in vascular plants, the other being phloem. The basic function of xylem is to transport water from roots to stems and leaves, but it also transports nutrients. The word ''xylem'' is derived from ...

and

phloem Phloem (, ) is the living tissue in vascular plants that transports the soluble organic compounds made during photosynthesis and known as ''photosynthates'', in particular the sugar sucrose, to the rest of the plant. This transport process is ...

of the host. Because this root-like endophytic system is not

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

borne, dwarf mistletoes are dependent solely on their host for water. Along a xylem-to-xylem link, dwarf mistletoes draw water from their host using the difference in water potential between parasite and host. The greater transpiration rate of the dwarf mistletoe produces a lower water potential, allowing water to flow from host to parasite. The water gradient or transpiration stream is consistently maintained, even when the host is under moderate water deficit. In addition to host-water dependence, dwarf mistletoes must acquire carbohydrate and mineral nutrition from their hosts. Dwarf mistletoes have both

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to ...

a and chlorophyll b and the necessary mechanisms for photosynthesis, but chlorophyll concentrations in dwarf mistletoes are approximately 1/5 to 1/10 of those found in their host’s foliage, and dwarf mistletoes have low photosynthetic rates as measured by the rate of

carbon fixation Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon (particularly in the form of carbon dioxide) is converted to organic compounds by living organisms. The compounds are then used to store energy and a ...

. The principal carbohydrate transported from the host to dwarf mistletoe is sucrose. Because dwarf mistletoes are phloem-deficient, they draw carbohydrates from their hosts by connections to the host phloem and ray

parenchyma Parenchyma () is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology it is the name for the tissue that fills the interior of flatworms. Etymology The term ''parenchyma'' is New Latin from the word � ...

. The rate of carbohydrate transport varies by season, but dwarf mistletoes continuously draw carbohydrates from their hosts throughout the year.

Dwarf mistletoe rating system

A standardized system called the Hawksworth 6-class dwarf mistletoe rating (DMR) system has been devised to determine how much dwarf mistletoe has infected a tree. To use this system, the living part of the

tree crown The crown of a plant refers to the total of an individual plant's aboveground parts, including stems, leaves, and reproductive structures. A plant community canopy consists of one or more plant crowns growing in a given area. The crown of a wo ...

needs to be broken up into 3 sections, (top, middle, bottom). Each section is then rated either 0, 1, or 2 with a score of 0 being assigned for a uninfected section, 1 for light infection with fewer than half of the branches infected, or 2 for heavy infection with over half of the branches infected. The 3 numbers are then added together to give the total rating for the tree.

Species

Arceuthobium occidentale on Pinus sabiniana

* '' Arceuthobium abietinum'' Engelm. ex Munz * ''

Arceuthobium americanum ''Arceuthobium americanum'' is a species of dwarf mistletoe known as American dwarf mistletoe and lodgepole-pine dwarf mistletoe. It is a common plant of western North America where it lives in high elevation pine forests. It is a parasitic plan ...

'' Nutt. ex Engelm. * '' Arceuthobium apachecum'' Hawksworth & Wiens * '' Arceuthobium azoricum'' Hawksw. & Wiens * '' Arceuthobium blumeri'' A. Nels. * '' Arceuthobium californicum'' Hawksworth & Wiens * '' Arceuthobium campylopodum'' Engelm. * '' Arceuthobium cyanocarpum'' (A. Nels. ex Rydb.) A. Nels. * '' Arceuthobium divaricatum'' Engelm. * '' Arceuthobium douglasii'' Engelm. * '' Arceuthobium gillii'' Hawksworth & Wiens * '' Arceuthobium globosum'' Hawksw. & Wiens * '' Arceuthobium laricis'' (Piper) St. John * '' Arceuthobium littorum'' Hawksworth, Wiens & Nickrent * '' Arceuthobium microcarpum'' (Engelm.) Hawksworth & Wiens * '' Arceuthobium minutissimum'' Hook. f. * '' Arceuthobium monticola'' Hawksworth, Wiens & Nickrent * '' Arceuthobium occidentale'' Engelm. * '' Arceuthobium oxycedri'' (DC.) M.Bieb. * '' Arceuthobium pusillum'' Peck * '' Arceuthobium siskiyouense'' Hawksworth, Wiens & Nickrent * '' Arceuthobium tsugense'' (Rosendahl) G.N. Jones * '' Arceuthobium vaginatum'' (Humb. & Bonpl. ex Willd.) J. Presl (Syn. ''Viscum vaginatum'' Humb. & Bonpl. ex Willd.)

References

External links