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Monotropoideae, sometimes referred to as monotropes, are a flowering plant subfamily in the family Ericaceae. Members of this subfamily are notable for their mycoheterotrophic and non-photosynthesizing or achlorophyllous characteristics.
The emerging shoots may be erect or nodding, with erect or pendulous flowers, which may become more erect as the plant matures. The flowers themselves, in common with other members of the Ericaceae, have corollas that are generally bell- or cup-shaped, though the petals themselves may or may not be fused. However, the Monotropoideae lack the poricidal anthers that are characteristic of the majority of the Ericaceae. (The Pyroleae do have poricidal anthers, however.) Pollen grains are released as a monad, in contrast to the majority of the Ericaceae, which release pollen grains in tetrad groups. (The Pyroleae variously release pollen as monads, tetrads, or polyads.) Fruits are dry loculicidal dehiscent (or sometimes indehiscent) capsule or a berry. Seeds are highly reduced dust seeds. The shoot may or may not be persistent after seed dispersal.
Other classifications have included Monotropoideae as a subfamily of Hypopityaceae, by August W. Eichler (1875), and as a subfamily of Pyrolaceae by Carl Georg Oscar Drude (1889); both classifications united the monotropes with the pyrolids in a single group. Over the next century, authors have variously treated this group as a distinct family or as a subfamily of the Ericaceae, though the trend from Margaret W. Henderson (1919) onward was toward the latter subfamily classification, albeit, the influential Cronquist system and continued to treat the group as the family Monotropaceae, separate from the Ericaceae.
Contemporary molecular phylogenetics has clearly established the Monotropoideae as a group within the larger Ericaceae, though many of the details of relationships between the Monotropoideae and the rest of the Ericaceae are still (as of 2015) a topic of active research, particularly the question of whether or not the Pyroleae and the rest of the Monotropeae form a single monophyletic group.
The morphology of the root and the root-level fungal symbiont is distinctive and referred to as monotropoid mycorrhiza. (Although mycorrhizas are generally considered to be mutualistic relationships, it is generally recognized that mutualism and parasitism exist on a continuum, and that plant-fungus symbioses with a clearly mycorrhizal root anatomy can include exploitative relationships.)
Distribution is through much of the temperate Northern Hemisphere, though ranging into the subarctic and montane tropical regions as well. Distribution is limited by available moisture (Monotropoideae species have limited ability to survive long enough to set seed during seasonal dry periods), and by the distribution of conifer genera that are hosts of the specific host fungi these plants parasitize.
The distribution of Monotropa is responsible for the majority of the range of this subfamily, with other genera not having the same global distribution. The center of biodiversity for this subfamily is found in temperate western North America, along the northern and central California and Pacific Northwest coast and montane areas as far east as the Sierra Nevada-Cascade cordillera. Seven of the 10 genera usually recognized as members of this subfamily (excluding Pyroleae) occur there, with 6 of these occurring only in that region.
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