rarest among the four mushroom genera, known only from some punctual records in North and Central America, Asia, and Australasia. Species of Lactarius, Lactifluus, and Russula have repeatedly been introduced with trees outside their native range: An overview article lists introductions in Chile, Argentina, Uruguay, Brazil, the US, Great Britain, the Faroe Islands, South Africa, China, Thailand, and New Zealand. Among the corticioid genera, Pseudoxenasma is only known from Europe. In contrast, Boidinia species have been found in Europe, Taiwan, and Japan, and Gloeopeniophorella species in North America, South America, Europe, West Africa, Taiwan, Australia, and New Zealand. Ecology Ectomycorrhizal symbiosis The genera Lactarius, Lactifluus, Multifurca and Russula form a mutualistic ectomycorrhizal root symbiosis with trees and shrubs, exchanging mineral nutrients for photosynthetic sugar. They are one of several fungal lineages that have evolved such a lifestyle and are sometimes referred to as the "/russula-lactarius" clade in the scientific literature. Worldwide, they are one of the most frequently encountered lineages on ectomycorrhizal roots. While some tropical species were initially believed to be parasitic, the observation that species fruiting on tree trunks do form ectomycorrhiza in tropical Guyana supports the view of an exclusively symbiotic lineage. Associations are known with several plant families. In the Northern Hemisphere, these are essentially the well-known ectomycorrhizal trees and shrubs in the Betulaceae, Fagaceae, Pinaceae and Salicaceae, but in arctic and alpine habitats, Russulaceae also associate with Bistorta vivipara (Polygonaceae), Kobresia (Cyperaceae), and Dryas octopetala (Rosaceae), ectomycorrhizal plants untypic in their respective families. In the tropics, known plant partners include Dipterocarpaceae, Fabaceae, Nyctaginaceae, Phyllanthaceae, Polygonaceae (Coccoloba), Sarcolaenaceae, and the gymnosperm Gnetum gnemon, and in the Southern Hemisphere, Nothofagaceae, Myrtaceae (Eucalyptus and Leptospermum), and Rhamnaceae (Pomaderris). Some Russulaceae are quite specialised in their ectomycorrhizal symbiosis, such as Lactarius and Russula species that only grow with Cistus shrubs in the Mediterranean basin. The different plant partners are reflected in the wide variety of habitats worldwide. Ectomycorrhizal Russulaceae have been observed in arctic and alpine tundra, boreal and alpine forest, north temperate forest, mires, mediterranean forests and scrub (maquis), miombo woodland, tropical lowland rainforest, tropical cloud forest, tropical dry forest, Australian eucalypt woodlands, and south temperate forests. Where they are introduced, they typically grow in plantations of their native host species, e.g. with pine in South Africa, Eucalyptus in Thailand, or birch in New Zealand. Other types of mycorrhiza Some of the ectomycorrhizal Russulaceae are also involved in