Morphological fit promotes pollen segregation and disassortative pollination in an open-flowered, nontubular heterostylous species.

Heterostyly is a classic Darwinian adaptation promoting disassortative pollination. The polymorphism usually depends on floral tubes to enforce spatial segregation of pollen deposited on pollinators. How this is achieved in species with bowl-shaped flowers and generalised pollinators remains less well-understood. We investigated the mechanistic basis of Darwin's cross-promotion hypothesis in distylous Persicaria jucunda, which lacks a floral tube and is fly-pollinated. We integrated morphological measurements of flowers and flies with high-definition video analysis of foraging behaviour. Exploiting pollen-size dimorphism, we quantified morph-specific pollen placement on pollinator body parts and the composition of naturally pollinated stigmatic pollen loads. A 'morphological fit' was evident with fly proboscis length matching lower-level sex organs and upper-level organs matching the position of the head. This morphological congruence caused L-morph pollen to be deposited primarily on proboscides (76.6%) and S-morph pollen on heads (85.7%), resulting in a significantly higher proficiency of intermorph than intramorph pollen transfer. Despite the absence of a floral tube in P. jucunda, our findings of a morphological fit between flower and pollinator structures and consistent foraging behaviour by flies provide mechanistic support for Darwin's cross-promotion hypothesis and resolve the puzzle of how disassortative pollination occurs in open-flowered heterostylous systems.