Chemical composition of floral perfumes in Mormodes (Orchidaceae): implications for selective attraction of euglossine bees.

Background and aims: The genus Mormodes (Catasetinae) comprises ∼80 Neotropical orchid species, predominantly from the Amazon. Like other Catasetinae, these orchids offer floral perfumes as their sole reward to specialized pollinators - male euglossine bees (Apidae). Growing as epiphytes on tall, usually dead trees, Mormodes plants pose challenges for studying their natural history. Consequently, little is known about its pollinator interactions and perfume chemistry. Studies on related genera suggest that species-specific perfume blends are critical for attracting distinct euglossine species. Here, we present the first extensive ecological and chemical study of Mormodes, testing whether perfume variation is associated with differences in pollinator spectra and may function as a prezygotic reproductive barrier.

Methods: Floral perfumes of ten species were collected via dynamic headspace and analysed using gas chromatography-mass spectrometry. Floral visitors were captured on flowers in a region of high Mormodes diversity in the Amazon forest, northern Brazil.

Key results: We identified 139 compounds in the floral perfumes of Mormodes spp., with species-specific profiles. While six species exhibited sesquiterpene-dominated bouquets, three species were dominated by aromatic compounds and one by monoterpenes. In the floral visitor survey, we reported ten species of Euglossa, four of Eulaema and one each of Exaerete, Aglae and Eufriesea. Some Mormodes species had exclusive pollinator spectra, while others shared pollinators.

Conclusions: Unlike most Catasetinae perfumes, typically dominated by monoterpenes or aromatics, Mormodes perfumes are mostly sesquiterpene-based, a trend that may represent a third evolutionary trajectory within the subtribe. Species-specific chemical profiles were associated with distinct pollinator spectra, while similar bouquets resulted in pollinator sharing. These results highlight the key role of floral perfume chemistry in selective pollinator attraction and reproductive isolation. However, in species producing similar perfume chemistry and attracting the same pollinators, additional isolating mechanisms, such as floral morphology and phenology, might be required.