other types of root symbioses with plants. A mutualistic association similar to ectomycorrhiza but with some hyphae penetrating into the plant root cells, termed arbutoid mycorrhiza, is formed by Russulaceae with shrubs of the genera Arbutus and Arctostaphylos, both in subfamily Arbutoideae of the Ericaceae. Some Russulaceae are associated with myco-heterotrophic plants of the Ericaceae subfamily Monotropoideae, forming monotropoid mycorrhiza. This is an epiparasitic relationship, where the heterotrophic plant ultimately derives its carbon from the primary, ectomycorrhizal plant partner of the fungus. The association is often very specific, with the heterotrophic plants only associating with selected fungus partners, including Russulaceae. Russulaceae are also an important group of orchid mycorrhizal fungi. This symbiosis is mutualistic in the case of green orchids, but a partly or fully epiparasitic relationship in the case of myco-heterotrophic and mixotrophic orchids, respectively. In some cases, the association with Russulaceae is, as in monotropoid mycorrhiza, very specific: the Mediterranean orchid Limodorum abortivum predominantly associates with Russula delica and closely related species; in Corallorhiza maculata, different genotypes of the same species have distinct Russula partners. Wood decay species The corticioid species in Boidinia, Gloeopeniophorella, and Pseudoxenasma are saprotrophic, wood-degrading fungi that develop on dead wood. Their early-branching positions in the phylogeny suggests this has been the ancestral trophic mode of the Russulaceae, and that the mycorrhizal lifestyle (see above) evolved later. The saprotrophic nature of these species has been questioned, based on the observation that other inconspicuous, crust-forming fungi are ectomycorrhizal; a subsequent author reaffirms nevertheless that "[n]one of the corticioid species in the family shows any sign of mycorrhizal activity." Hypogeous fruiting Hypogeous fruitbodies, or fruitbodies developing below ground, occur in Lactarius and Russula and have previously been considered as distinct genera (see Systematics and taxonomy: Internal systematics). As such species are especially diverse in some warm and dry regions, e.g. in Spain, California, or Australia, below-ground fruiting has been interpreted as an adaptation to drought. However, hypogeous Russulaceae are also known from cold temperate regions and tropical rainforest. The fact that hypogeous species in the Russulaceae do not form their own lineages but are scattered in Russula or Lactarius shows that this type of fruiting evolved several times. It is believed that these changes are evolutionarily quite recent. Parasites Russulaceae fruitbodies are subject to parasitisation by other fungi. The genus Asterophora develops on old fruitbodies of the mushroom species in the family, as does Dendrocollybia racemosa