The Effects of Seasonal Cues and Differential Gene Expression on the Developmental Switch of a Flower Polyphenism in Mimulus douglasii

Abstract

Angiosperms have evolved multiple mating systems that allow reproductive success under varied conditions. Striking among these are cleistogamous mating systems, where individuals can produce alternative flower types specialized for distinct mating strategies. Cleistogamy is thought to be environmentally-dependent, but little is known about environmental triggers that induce cleistogamous flower or the gene regulatory networks that determine the final floral phenotypes. If production of alternate flowers is environmentally induced, populations may evolve locally adapted responses. Mimulus douglasii, exhibits a cleistogamous mating system, and ranges across temperature and day length gradients, providing an ideal system to investigate environmental parameters that control cleistogamy and the gene regulatory networks responsible for the different floral forms. In these studies, we compared flowering responses across M. douglasii population accessions that produce phenotypically distinct outcrossing, and self-pollinating flower morphs. Under controlled conditions, we determined time to flower, and number and type of flowers produced under different temperatures and day lengths. We also compared gene expression profiles between chasmogamous and cleistogamous flowers using RNA-seq. We find that temperature and day length both effect onset of flowering. Long days shift flower type from predominantly chasmogamous to cleistogamous. The strength of the response to day length varies across accessions whether temperature varies or is held constant. We also find that gene expression patterns differ between the early development chasmogamous and cleistogamous flower buds. Cleistogamy is an environmentally sensitive polyphenism in Mimulus douglasii, allowing transition from one mating strategy to another. Longer days induce flowering and production of cleistogamous flowers. Shorter days induce chasmogamous flowers. Population origin has a small effect on response to environmental cues. Subtle shifts in the expression of cell division, cell expansion, and metabolic process related transcripts lead to the massive size difference observed between chasmogamous and cleistogamous flowers.