Why does an obligate autogamous orchid produce insect attractants in nectar? – a case study on Epipactis albensis (Orchidaceae)

Abstract

Background The flowers of some species of orchids produce nectar as a reward for pollination, the process of transferring pollen from flower to flower. Epipactis albensis is an obligatory autogamous species, does not require the presence of insects for pollination, nevertheless, it has not lost the ability to produce nectar, the chemical composition of which we examined by gas chromatography-mass spectrometry (GC–MS) method for identification of potential insect attractants. Results During five years of field research, we did not observe any true pollinating insects visiting the flowers of this species, only accidental insects as ants and aphids. As a result of our studies, we find that this self-pollinating orchid produces in nectar inter alia aliphatic saturated and unsaturated aldehydes such as nonanal (pelargonal) and 2-pentenal as well as aromatic ones (i.e., syringaldehyde, hyacinthin). The nectar is low in alkenes, which may explain the absence of pollinating insects. Moreover, vanillin and eugenol derivatives, well-known as important scent compounds were also identified, but the list of chemical compounds is much poorer compared with a closely related species, insect-pollinating E. helleborine. Conclusion Autogamy is a reproductive mechanism employed by many flowering plants, including the orchid genus Epipactis, as an adaptation to growing in habitats where pollinating insects are rarely observed due to the lack of nectar-producing plants they feed on. The production of numerous chemical attractants by self-pollinated E. albensis confirms the evolutionary secondary process, i.e., transition from ancestral insect-pollinating species to obligatory autogamous.

Epipactis bucegensis—A Separate Autogamous Species within the E. helleborine Alliance

“…We found only minute droplets of nectar that accumulated inside the hypochile of the one to two topmost, young flowers (Figure 2E). Minute nectar production was reported several times in other autogams, such as Epipactis albensis Nováková and Rydlo [17,33,35], Epipactis muelleri Godfery [11] and Epipactis leptochila (Godfery) Godfery [30]. These obligate autogams are relatively young species that recently diverged from within the evolutionarily active Epipactis helleborine alliance [36].…”

“…), can change their mode of pollination from allogamy towards autogamy [54]. In temperate regions, they are allogamous and well-visited by insects [35,70]. However, in xerophilous regions, they may become facultative autogams even before anthesis [54][55][56][57].…”

Nature’s Most Fruitful Threesome: The Relationship between Yeasts, Insects, and Angiosperms

“…Floral nectar can also contain glycoconjugates formed by monosaccharides bound (especially through a ß-glucosidic linkage) to aromatic compounds (aglycones), such as geraniol [ 83 , 84 ], α-terpineol [ 85 , 86 ], methyl salicylate [ 85 , 87 ], 1-hexanol [ 88 , 89 ], eugenol [ 59 , 90 ], vanillin, and vanillyl alcohol [ 91 , 92 , 93 ]. When these glycoconjugates are hydrolyzed by yeast glucosidases, their aglycones volatilize [ 94 , 95 , 96 , 97 , 98 ], working as attractive or repelling agents for insects (see Section 3 ).…”