Abstract
Diversification in flower shape and function is triggered by the high plasticity of flower meristems. Minute changes in space and time can profoundly affect the formation of adult structures. Dipsacoideae provides an excellent model system to investigate the evolutionary aspects of temporal and spatial changes in flower development due to its small size, the resolved phylogenetic framework, and significant diversity of perianth form and merosity. In the present study, we investigated the sequence of floral organ initiation and quantified the interactions between flower meristem expansion and petal primordium size in eight species representing two major clades of Dipsacoideae. Our quantitative study indicates the plasticity of the flower meristem for the regulation of pentamery either due to a decrease in petal primordium size (Scabiosa) or an increase in flower meristem size (Pterocephalus and Lomelosia) compared to tetramerous flowers. According to our results, temporal shifts of organ initiation during flower evolution contribute to the morphological diversity of perianth. Sepal reduction in members of the Dipknautids is paralleled by a delay in sepal initiation. The multiplication of sepals in Lomelosia and Pterocephalus is correlated with an extension of initiation time. Some heterochronies in early development do not affect adult morphology. The effects of a temporal change in early development can be enhanced, reduced, or eliminated by later changes of the growth rate during development. Our results confirm the hypothesis that the interaction between timing and space plays an important role for evolutionary diversification of flowers.
Highlights
Understanding the evolution of flower diversity has been a main challenge for evolutionary developmental biologists over the past 20 years (Becker et al, 2011)
We investigated the sequence of floral organ initiation and the interaction between flower meristem expansion and petal primordia size in eight species representing two major clades of Dipsacoideae
Buds of inflorescences and flowers of the following species were collected in May–August 2015 at the Botanical Garden of the Johannes Gutenberg University Mainz (Germany): Succisa pratensis Moench., Succisella inflexa (Kluk) Beck., Cephalaria transsylvanica (L.) Schrad., Dipsacus fullonum L. and Knautia arvensis (L.) Coult., Lomelosia palaestina (L.) Raf., Pterocephalus papposus (L.) Coult., and Scabiosa ochroleuca L
Summary
Understanding the evolution of flower diversity has been a main challenge for evolutionary developmental (evo–devo) biologists over the past 20 years (Becker et al, 2011). Heterochrony, i.e., changes in the timing and rate of developmental events, provides a plausible explanation for the diversification of characters in the course of flower evolution. The heterochronic changes have been the subject of several plant evolutionary studies through the twentieth century (reviewed in Li and Johnston, 2000). Most previous cases of heterochrony have been documented through the quantitative analysis of organ shape and size during flower development (Li and Johnston, 2000) which fall into the definition of “growth heterochrony” (Smith, 2001). The comparative developmental studies of flowers within a robust phylogenetic framework can reveal changes in the start or end point, growth rate, or sequence of early developmental events which promote the morphological diversity of flowers at maturity
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