Pre-pollination isolation by pollinator specificity: settling moth versus hawkmoth pollination in two sympatric Habenaria species (Orchidaceae)

Abstract

Background

Habenaria species typically produce green or white flowers, bear nectar spurs, emit crepuscular-nocturnal scents, and are usually pollinated by crepuscular/nocturnal moths. However, the roles of floral traits in pollinator differentiation contributing to reproductive success in sympatric Habenaria species require further definition. In this study, we investigated flowering phenology, floral traits, pollinator behavior, and reproductive success of two co-occurring and co-blooming Habenaria species (H. ciliolaris and H. schindleri) in southeastern China. We also conducted intraspecific and interspecific hand pollinations to determine their respective breeding systems and the extent of interspecific post-pollination isolation.

Results

Both species are self-compatible but rely on pollinators for sexual reproduction. Habenaria ciliolaris was pollinated by two species of nocturnal, settling moths (Thinopteryx nebulosa and Porsica sp.). They foraged for nectar upside down or from one side, carrying pollinaria on their eyes because the curved lateral lobes of the labellum block the movements of settling moths on these flowers. Habenaria schindleri was pollinated by two crepuscular hawkmoths (Eupanacra mydon and Hippotion rafflesi), which carried pollinaria between their palpi while hovering in front of flowers while taking nectar. The proboscis lengths of pollinators of both Habenaria species matched the spur lengths of their corresponding flowers. Habenaria ciliolaris experienced a high level of inbreeding depression. Interspecific pollination by applying pollen grains from H. schindleri to the stigmas of H. ciliolaris, resulted in a low level of seed set.

Conclusions

Differences in floral morphology and nectar volume/concentration appear to contribute to segregating members of the pollinator guild of some Habenaria species. These findings offer new insights to our understanding of the partitioning of pollinators between co-blooming congeners in the Orchidaceae.