There are 17 species of mycoheterotrophic Ericaceae in 12 genera, most of which are monotypic. Cheilotheca has 3 species, Monotropa, Monotropsis and Monotropastrum contain 2 species each, Monotropastrum with one additional variety. Pyrola contains 42 species, but only a variant of Pyrola picta is mycoheterotrophic.
"Pyrola aphylla" was firstly described by James Edward Smith 1814 in Abraham Rees' Cyclopaedia (Vol. 29, No. 7) calling it "leafless". Other authors as well, like De Candolle (1838) and Hooker (1840), described this species as having no leaves. However, Nuttall (1843, cited after Holm 1898) firstly detected leaves at this species, and Holm (1889) ascertained subterranean connecting rhizomes of appearantly leafless specimen with rosettes of green leaves, proving that they belong to the same individum. The same observation was made by Camp (1940), who concluded on a very close relationship, if not identity, of Pyrola aphylla, Pyrola picta and Pyrola dentata. More recently Haber (1987) eventually merged the three species to the highly variable Pyrola picta Sm. Genetically, however, there seem to be differences (Jolles and Wolfe 2012). A morphometric study of Jolles (2015) concludes that the Pyrola picta complex (P. picta, P. aphylla, P. dentata and P. crypta) is highly polymorphic, shows considerable overlap in floral characteristics and can only distinguished statistically by anther pore shapes and floral bract length. Thats why P. crypta, of which only 5 specimen were available, was taken out of the analysis. Of the remaining three species (?) P. dentata was more distant to P. picta and P. aphylla. Possibly, phenological differences account for reproductive isolation (Jolles 2015).
The criterium to be included in our list of mycoheterotrophic plants is optical achlorophylly. The extreme variant of P. picta, P. aphylla s.str., (as such accepted by POWO and WFO) seemingly lacks leaves and the remaining content of chlorophyll in its flower bearing stems (Holm 1898) is optically concealed. Moreover, since Hynson et al. (2009) found characteristic stabel isotope signatures of mycoheterotrophic plants in Pyrola aphylla specimen, we include this species in our checklist. Interestingly, Pyrola picta s.str. did not show signs of mycoheterotrophy according to carbon stable isotope signatures (Hynson et al. 2009). Possibly, dependency on the fungal carbon is not determined by the species in the taxonomical sense but on the actual ability for assimilation in a specimen.
We are aware of intergrading dependences on a mycorrhizal fungus, and even green plants can receive more carbon from its endophyte than via photosynthesis. This is the case in several orchids and pyroloids (e.g. Tedersoo et al. 2007, Zimmer et al. 2008, Cameron et al. 2009) and certainly in P. picta, too (Zimmer et al. 2007, Hynson et al. 2009).
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