Analysis of late stage flower development in Primula vulgaris reveals novel differences in cell morphology and temporal aspects of floral heteromorphy.

Heterostyly in Primula is characterized by the development of long-styled pin and short-styled thrum flowers, with anthers midway down the corolla tube in pin flowers, and at its mouth in thrum flowers. Other differences include pollen size and stigmatic papillae length. Several linked genes at the S locus control these differences. In this study we have analyzed pin and thrum flowers through the temporal development of heteromorphy.These studies indicate that the S locus linked genes that orchestrate heteromorphic flower development act in coordination, but with different temporal and spatial dynamics. Style length is differentiated by longer style cells in pin than thrum. However, our studies on cell shape and size within the corolla tube show that a different mechanism mediates the dissimilar elevation of anthers between pin and thrum types. These studies have also revealed that upper corolla tube cells in thrum flowers are wider than those in pin flowers. This results in a larger corolla tube mouth in thrum flowers and represents a new and previously undocumented heteromorphic variation between pin and thrum flowers.