Combining pollination ecology and fine-scale spatial genetic structure analysis to unravel the reproductive strategy of an insular threatened orchid

• Vanilla humblotii bioecology and fine-scale genetic structure are integrated.
• Although self-compatible, two putative pollinators ensure allogamy.
• Phalanx growth influences the autocorrelations only at short distances.
• Seed dispersal and genotypic diversity support the geitonogamy avoidance hypothesis.

In Vanilla species (Orchidaceae), the influence of sexual and asexual mating systems on the spatial structuring of population genetic diversity is understudied. These elements are crucial in restoration program to limit inbreeding depression and to maintain the genetic diversity of natural populations. In the remnant fragments of tropical dry forest in Mayotte (Comoros Archipelago, Indian Ocean), the clarification of the reproductive strategies of the orphan leafless Vanilla humblotii Rchb. f. will provide a better understanding of its fine-scale spatial genetic structure. Approaches combining reproductive biology and fine-scale spatial genetic structure analyses using ten microsatellite markers in 49 individuals sampled in the only remaining large population of V. humblotii were employed to unravel the reproductive strategies of this species. The results showed that V. humblotii displays unscented flowers and is allogamous and pollinator-dependent although also self-compatible. A total absence of pollen movements and a low level of natural fruit set (~ 1%) are reported, although a wild bee (Allodape obscuripennis Strand, Xylocopinae) and a bird (Nectarinia coquerelli, Nectarinidae) visited the flowers. A high genotypic diversity (G/N = 0.88) and a phalanx clonal growth are detected, and seed dispersal is higher than pollen dispersal. The phalanx distribution of the repeated genotypes (ramets arisen from the same genet) is responsible for significant autocorrelations detected at small distances. Limited inbreeding was detected although geitonogamy could have been enhanced by vegetative reproduction. This study highlights the need to perform interdisciplinary studies to unravel the reproductive strategy of clonal plant species with a deceptive pollination system.