Pollinator visitation pattern and differential reproductive compatibilities maintain bract colour polymorphism in a nocturnal ginger from Western Ghats, India

Polymorphism, the presence of discrete floral morphs within a population, can be maintained by positive or negative frequency selection. Pollinators often show a positive frequency-dependent foraging which confers reproductive advantage to common morphs over rare morphs. However, pollinator-mediated selections often act on visual floral displays. Here, we investigate hawkmoth-mediated selection in a nocturnal Curcuma caulina (Zingiberaceae) with bract colour polymorphism, where the role of visual display is minimal.

We used path analyses to explain maintenance of differential morph abundances in C. caulina . For this, we quantified inter-morph trait variations in floral structures, nectar traits, pollinator visitations, physiological compatibilities and reproductive fitness.

We noted that the rarest morph was self-compatible, exhibited higher nectar reward and pollinator visits, and low reproductive fitness, whereas the two common morphs were complete to partially self-incompatible, with lower nectar rewards and pollinator visits, but higher reproductive fitness.

We conclude that while hawkmoths show a positive response to floral rewards, self-compatibility confers lower reproductive fitness to the nectar-rich rare morphs. In contrast, self-incompatibility confers higher fitness to the common morphs despite lower pollinator visits. Thus, colour polymorphism in nocturnal C. caulina is explained by floral traits associated with bract colour, which are under pollinator-mediated selection.