Understanding the development, evolution, and function of bullseye pigmentation patterns in Hibiscus trionum

Abstract

Colourful spot, stripe and ring patterns decorate the corolla of many flowering plants and fulfil important biotic and abiotic functions. These petal patterns are created by spatial differences in pigmentation, cell shape and texture of the adaxial petal epidermis. The mechanisms controlling formation and evolution of these patterns, and their exact role in plant-animal communication are not well understood. My PhD adds to current understanding of petal patterns by investigating the development, evolution, and function of the bullseye pigmentation pattern on Hibiscus trionum petals.

The bi-coloured bullseye pattern of H.trionum is created from contrast in cell shape, cuticle texture, and pigmentation between the bottom and top of the petal. The bottom petal appears shiny and purple because cells in this region are elongated with a striated cuticle, and are pigmented with anthocyanins. A minimal regulatory network restricting anthocyanin pigment production to the bottom of Hibiscus trionum petal was identified. The network relies on two MYB regulators: HtCREAM1 which represses anthocyanin production in the top of the petal; HtBERRY1 which promotes anthocyanin biosynthesis in the bottom of the petal. This minimal network is a starting point to understand the molecular mechanisms both creating, and creating diversity, in petal patterns.

Natural variation in the bullseye pattern in close relatives of H.trionum from Australia and New Zealand is in part due to four independent restrictions of anthocyanin pigmentation in the petal bottom. Preliminary results suggest restriction of pigmentation in H.richardsonii, sister-species to H.trionum, results from mutation in the regulatory region and the coding sequence of the BERRY1 homolog of H. richardsonii.

Natural variation in the bullseye pigmentation pattern could reflect a function in plant-pollinator communication. Buff-tailed bumblebees were found to discriminate between, and prefer, artificial flowers with H.trionum-like bullseye patterns to H.richardsonii-like bullseye patterns.