Understanding the role of floral development in the evolution of angiosperm flowers: clarifications from a historical and physico-dynamic perspective

Abstract

Flower morphology results from the interaction of an established genetic program, the influence of external forces induced by pollination systems, and physical forces acting before, during and after initiation. Floral ontogeny, as the process of development from a meristem to a fully developed flower, can be approached either from a historical perspective, as a "recapitulation of the phylogeny" mainly explained as a process of genetic mutations through time, or from a physico-dynamic perspective, where time, spatial pressures, and growth processes are determining factors in creating the floral morphospace. The first (historical) perspective clarifies how flower morphology is the result of development over time, where evolutionary changes are only possible using building blocks that are available at a certain stage in the developmental history. Flowers are regulated by genetically determined constraints and development clarifies specific transitions between different floral morphs. These constraints are the result of inherent mutations or are induced by the interaction of flowers with pollinators. The second (physico-dynamic) perspective explains how changes in the physical environment of apical meristems create shifts in ontogeny and this is reflected in the morphospace of flowers. Changes in morphology are mainly induced by shifts in space, caused by the time of initiation (heterochrony), pressure of organs, and alterations of the size of the floral meristem, and these operate independently or in parallel with genetic factors. A number of examples demonstrate this interaction and its importance in the establishment of different floral forms. Both perspectives are complementary and should be considered in the understanding of factors regulating floral development. It is suggested that floral evolution is the result of alternating bursts of physical constraints and genetic stabilization processes following each other in succession. Future research needs to combine these different perspectives in understanding the evolution of floral systems and their diversification.

A temporal shift in resource allocation facilitates flowering before leaf out and spring vessel maturation in precocious species

“…). The observed pattern is suggestive of heterochronic evolution, where changes in the timing of different developmental stages led to diversification of floral type and phenology (Li and Johnston, ; Box and Glover, ; Ronse De Craene, ). However, more research is needed to understand the development of these flowers, their genetic control and the ancestral form of the different congeners.…”

Implications of region‐specific gene expression for development of the partially fused petunia corolla

“…Consistent with this, we found higher expression of two PaxBLH8 / PNF genes (Peaxi162Scf00115g01316 and Peaxi162Scf00014g01719) in the UCT relative to the lobes, with at least the former being expressed most strongly in the floral receptacle compared with other floral and leaf tissues profiled. In A. thaliana the single BLH8 / PNF gene is expressed in the shoot apical meristem (SAM) where it initiates and maintains meristematic cells and restricts the expression of lateral organ boundary genes (Khan et al ., ; Smith et al ., ). In blh8 / pnf mutants, the SAM terminates prematurely prior to flowering, and thus is critical for the eventual development of floral meristems.…”

“…This is suggestive of CUP reducing cell proliferation in these regions, and is consistent with the cup phenotype of increased organs within and between whorls (Weir et al ., ). However, since the ring primordium that gives rise to a congenitally fused (or single receptacle‐borne, sensu Ronse De Craene, ) corolla tube develops despite this proximal zone of CUP expression, a model of petal growth either needs to turn down CUP expression specifically within intra‐whorl petal boundaries or express an additional factor that nullifies/switches the function of CUP away from the negative control of cell division. Following establishment of the corolla tube, evidence suggests that CUP transcription is maintained, or reactivated, between petal lobes at least until a fairly late developmental stage marked by differentiation of stamen thecae.…”

Patterns of Diversity of Floral Symmetry in Angiosperms: A Case Study of the Order Apiales

“…The fixed orientation of the flower with respect to the inflorescence axis (with transversal broader side) can be provisionally explained by the interaction between the developing flower and its subtending bract by the means of mutual pressure [88]. At the establishment of disymmetry, the flower probably enlarges and broadens according to the available space, which is the space with the lowest pressure from both the bract and the axis.…”