A shift between bee and wasp pollinators explains floral divergence in the Duvernoia clade of Justicia (Acanthaceae)

Floral divergence among sister taxa can arise from pollinator shifts and play a role in speciation. We investigate a potential case of pollinator-driven divergence in the Duvernoia clade of Justicia (Acanthaceae), which includes two species with markedly divergent floral morphology. Pollinator observations and pollen load analyses showed that Justicia aconitiflora is pollinated by vespid wasps, while its sister taxon Justicia adhatodoides is confirmed to be pollinated exclusively by large carpenter bees. Reciprocal nectar-feeding experiments show that wasps and bees are not deterred by the taste of nectar of non-preferred species. Floral morphology matches body and tongue morphology of these two insect groups, and this phenotypic matching mediates pollen transfer and physical access to nectar. Controlled pollination experiments showed that both species are self-compatible, yet dependent on pollinator visits for fruit-set. Historical reconstruction of pollination system evolution in Justicia indicates that the direction of the shift in the Duvernoia clade was from bee to wasp pollinators. This study provides a compelling example of a transition between bee and wasp pollination, and highlights the effects of floral architecture on specialization in plant pollination systems.