Flexibility in the structure of spiral flowers and its underlying mechanisms

Abstract

Spiral flowers usually bear a variable number of organs, suggestive of the flexibility in structure. The mechanisms underlying the flexibility, however, remain unclear. Here we show that in Nigella damascena, a species with spiral flowers, different types of floral organs show different ranges of variation in number. We also show that the total number of organs per flower is largely dependent on the initial size of the floral meristem, whereas the respective numbers of different types of floral organs are determined by the functional domains of corresponding genetic programmes. By conducting extensive expression and functional studies, we further elucidate the genetic programmes that specify the identities of different types of floral organs. Notably, the AGL6-lineage member NdAGL6, rather than the AP1-lineage members NdFL1/2, is an A-function gene, whereas petaloidy of sepals is not controlled by AP3- or PI-lineage members. Moreover, owing to the formation of a regulatory network, some floral organ identity genes also regulate the boundaries between different types of floral organs. On the basis of these results, we propose that the floral organ identity determination programme is highly dynamic and shows considerable flexibility. Transitions from spiral to whorled flowers, therefore, may be explained by evolution of the mechanisms that reduce the flexibility.

Evolutionary diversification of CYC/TB1‐like TCP homologs and their recruitment for the control of branching and floral morphology in Papaveraceae (basal eudicots)

“…It has been proposed that the two STM orthologs in E. californica have evolved specific roles in maintaining the central and ring meristem, respectively. In addition, the initial meristem size was positively correlated with the total organ number per flower in Nigella damascena (Wang et al ., ). Nigella , however, shows considerable variation in the number of all four types of floral organs.…”

Stochastic occurrence of trimery from pentamery in floral phyllotaxis of Anemone (Ranunculaceae)

“…In typical Anemone floral development, the tepal and following stamen primordia arise in a continuous spiral pattern [6], indicating that primordia appearing at the sixth-to-eighth positions stochastically become tepals rather than stamens as in typical flowers. Extension of the ABC model may explain the stochastic change of organ fate from tepal to stamen [8,13,14]. According to the ABC model, primordia consisting of a whorl obey the same organ fate (e.g., black organs in Fig.…”

Bridging evolution and development in plants

“…Similarly, Paula Elomaa (University of Helsinki, Helsinki, Finland) conducted functional analyses on the Gerbera hybrida orthologs of LEAFY ( LFY ) and UNUSUAL FLORAL ORGANS ( UFO ), two genes regulating floral meristem identity of single flowers in many plants, and found that these two genes were recruited for patterning the capitulum, the inflorescence structure in Asteraceae. Hongzhi Kong (Institute of Botany, Chinese Academy of Sciences, Beijing, China) reported that differences in floral meristem sizes and the flexibility in the expression of floral organ identity genes have contributed to the great variation in floral organ numbers in the spirally patterned Nigella damascena (Ranunculaceae) (Wang et al ., ). In fruits, Chaoying He (Institute of Botany, Chinese Academy of Sciences, Beijing, China) showed that the heterotopic expression of MADS‐BOX GENE 2 FROM PHYSALIS FLORIDANA ( MPF2 ) contributes to the formation of the Chinese lantern (a papery calyx) while heterochronic changes in PHYSALIS ORGAN SIZE 1 ( POS1) and POS2 expression are correlated with the variation of berry size among species in Physalis (Solanaceae) (He & Saedler, ; Wang et al ., ).…”