Culpepper, E.E., E.R. Treadaway, and Conley K. McMullen. 2005. Pollen studies of pin versus thrum flowers in Cordia lutea (Boraginaceae).

Poster Abstract: Cordia lutea Lam. (Boraginaceae) is a Galápagos Islands native that is also known from mainland Ecuador and northwestern Peru. This small tree or shrub (to 8 m tall) occupies arid lowlands throughout the archipelago, and provides an attractive floral display with its bright yellow flowers. Early research on the pollination biology of this species indicated that its flowers are a major source of nectar and pollen for resident insects. It was assumed that all of the flowers of this species were perfect and monomorphic. However, our studies indicate that this is not the case. Instead, the species is heterostylous and produces two types of flowers known as pins and thrums. Pin flowers have styles that are longer than the stamens, while thrum flowers have stamens that are longer than the styles. Since two types of flowers are produced, this type of heterostyly is referred to specifically as distyly. Each individual plant possesses either pin flowers or thrum flowers, and as a rule, pollen from a pin can function only on the stigma of a thrum, while pollen from a thrum functions only on the stigma of a pin, thus promoting outcrossing. It has been hypothesized that a "supergene" controls this type of heterostyly, and that pin flowers and thrum flowers differ not only in stamen and stylar length, but also in pollen number, pollen size, corolla size, corolla shape, stigma size, and stigma surface. Results of our studies on pollen number and pollen size are discussed.