Dimorphic enantiostyly and its function for pollination by carpenter bees in a pollen-rewarding Caribbean bloodwort

Flowers that present their anthers and stigma in close proximity can achieve precise animal-mediated pollen transfer, but risk self-pollination. One evolutionary solution is reciprocal herkogamy. Reciprocity of anther and style positions among different plants (i.e., a genetic dimorphism) is common in distylous plants, but very rare in enantiostylous plants. We investigated the pollination and reproductive system of the enantiostylous Caribbean plant Cubanicula xanthorrhizos (Haemodoraceae).

We assessed stylar orientation of flowers and conducted controlled pollination experiments. We used videography of flower visitors and pollen load analysis to determine the pollination mechanism. We also measured floral morphology, pollen production, spectral reflectance, and volatile emissions.

Cubanicula xanthorrhizos exhibits dimorphic enantiostyly with c. 50:50 left- to right-styled morphs. Plants are self-compatible, but pollinator dependent for seed production. Intra- and intermorph crosses are equally fertile. The nectarless flowers are pollinated by female carpenter bees (Xylocopa cubaecola) that collect pollen, often by sonication, from two centrally positioned yellow feeding anthers. An inconspicuous deflected pollinating anther deposits pollen on the side of the bee thorax, which contacts the stigma of the mirror-image morph. A yellow-orange "guide" on the white tepals appears to be a visual attractant. Flowers emit methoxy benzenoid volatiles that may also attract bees.

Reciprocity of the style with a single pollinating stamen in C. xanthorrhizos appears to promote intermorph pollen export via "safe sites" on pollen-collecting bees. This novel case of dimorphic enantiostyly contributes to understanding of the evolution of floral polymorphisms.