CYCLOIDEA paralogs function redundantly to specify dorsal flower development in Mimulus lewisii (Phrymaceae)

Abstract

Repeated independent transitions between radial and bilateral flower symmetry have occurred across the angiosperm phylogeny, contributing to the vast diversity we see in floral morphology. The genetic program for bilateral flower symmetry has been documented in the model system Antirrhinum majus where the paralogs CYCLOIDEA (CYC) and DICHOTOMA (DICH) have partially redundant functions in establishing dorsal petal identity. These paralogs resulted from a duplication event in the ECE-CYC2 gene lineage. Within Lamiales, at least 12 additional duplications in the ECE-CYC2 lineage have occurred. The close homologs CYC1 and CYC2 resulted from one of these additional duplication events at the base of the higher core Lamiales (HCL). In this study, we are using the emerging model Mimulus lewisii (Phrymaceae, HCL), to test for conservation of MlCYC1 and MlCYC2 in flower symmetry development, and whether these genes similarly function redundantly compared to AmCYC and AmDICH. Using Agrobacterium-mediated stable transformation techniques and RNA interference (RNAi), we were able to characterize MlCYC1 and MlCYC2 RNAi silenced lines. In addition, by cross-pollinating MlCYC1 and MlCYC2 single lines, we generated and characterized double MlCYC1:MlCYC2 RNAi silenced lines. Our results from RNAi silencing, and consistent with our gene expression analyses, demonstrate that MlCYC1 and MlCYC2 together function to specify dorsal flower identity. Additionally, we find extensive functional redundancy between the paralogs MlCYC1 and MlCYC2, with MlCYC1 playing the dominant role in establishing dorsal petal identity. These results are consistent with what is found in A. majus, nonetheless the paralogs arose from independent gene duplication events in the ECE-CYC2 gene lineage.