From the Brazilian lowlands to the Andes: specialist fungus gnat pollination and self-incompatibility in two Malaxis species (Malaxidinae: Orchidaceae)

Background and aims – Malaxis is a cosmopolitan genus comprising approximately 300 species and is one of the most diverse within the subtribe Malaxidinae. However, to date, no detailed studies have addressed its reproductive biology in the Neotropics. This study aimed to document the floral traits, pollination mechanisms, breeding system, and fruiting success of two native Neotropical species. Material and methods – Plants of Malaxis parthoni were studied in Porto Alegre, southern Brazil (79 m a.s.l.), while individuals of M. excavata were examined at 3,500 m a.s.l. in Ayacucho, in the Peruvian Andes. To evaluate the breeding system, we tested for autonomous pollination and self-compatibility using flowers isolated from pollinators through bagging. Nectar production was assessed through qualitative tests to detect the presence of sugars in floral secretions. The pollination process, under natural conditions, was recorded in the field through video and photographs, and pollination efficiency and natural fruiting success were documented. Key results – Both Malaxis species possess nectar-producing flowers and were found to be pollinator-dependent and self-incompatible. Fungus gnats of the genus Mycomya (Mycetophilidae) acted as pollinators. In both species, pollinia were attached to the ventral part of the prothorax. In both species, a high percentage of flowers with pollinia removed was observed, indicating high pollination efficiency. On average, fruiting success was 11.24% in M. parthoni and 36.01% in M. excavata , the latter showing a statistically higher percentage. Conclusion – Our findings reveal that both species require cross-pollination to achieve fruit set, with fungus gnats acting as effective pollinators. The relatively high fruiting success compared to other congeners and self-incompatible orchids may result from a combination of factors, including the presence of floral rewards and high pollinator efficiency. This study provides the first comprehensive account of the reproductive biology of Neotropical Malaxis species.