Prophyll in Monocots: The Starting Point of Lateral Shoot Phyllotaxis

The interaction between heterochrony and mechanical forces as main driver of floral evolution

Heterochrony acts as a fundamental process affecting the early development of organisms in creating a subtle shift in the timing of initiation or the duration of a developmental process. In flowers this process is linked with mechanical forces that cause changes in the interaction of neighbouring floral organs by altering the timing and rate of initiation of organs. Heterochrony leads to a delay or acceleration of the development of neighbouring primordia, inducing a change in the morphospace of the flowers. As changes in the timing of development may affect organs differently at different stages of development, these shifts eventually lead to major morphological changes such as altered organ positions, fusions, or organ reductions with profound consequences for floral evolution and the diversification of flowers. By concentrating on early developmental stages in flowers it is possible to understand how heterochrony is responsible for shifts in organ position and the establishment of a novel floral Bauplan. However, it remains difficult to separate heterochrony as a process from pattern, as both are intimately linked. Therefore it is essential to connect different patterns in flowers through the process of developmental change.Examples illustrating the importance of heterochronic shifts affecting different organs of the flower are presented and discussed. These cover the transition from inflorescence to flower through the interaction of bracts and bracteoles, the pressure exercised by the perianth on the androecium and gynoecium, the inversed influence of stamens on petals, and the centrifugal influence of carpels on the androecium. Different processes are explored, including the occurrence of obdiplostemony, the onset of common primordia, variable carpel positions, and organ reduction and loss.

Reproductive development of common buckwheat (Fagopyrum esculentum Moench) and its wild relatives provides insights into their evolutionary biology

INTRODUCTION

Understanding the complex inflorescence architecture and developmental morphology of common buckwheat (Fagopyrum esculentum) is crucial for crop yield. However, most published descriptions of early flower and inflorescence development in Polygonaceae are based on light microscopy and often documented by line drawings. In Fagopyrum and many other Polygonaceae, an important inflorescence module is the thyrse, in which the primary axis never terminates in a flower and lateral cymes (monochasia) produce successively developing flowers of several orders. Each flower of a cyme is enclosed together with the next-order flower by a bilobed sheathing bract-like structure of controversial morphological nature.

METHODS

We explored patterns of flower structure and arrangement in buckwheat and its wild relatives, using comparative morphology, scanning electron microscopy and X-ray microtomography.

RESULTS

Our data support interpretation of the sheathing bract as two congenitally fused phyllomes (prophylls), one of which subtends a next-order flower. In tepal-like bract, a homeotic mutant of F. esculentum, the bilobed sheathing bract-like organ acquires tepal-like features and is sometimes replaced by two distinct phyllomes. Wild representatives of F. esculentum (ssp. ancestrale) and most cultivars of common buckwheat possess an indeterminate growth type with lateral thyrses produced successively on the primary inflorescence axis until cessation of growth. In contrast, determinate cultivars of F. esculentum develop a terminal thyrse after producing lateral thyrses. In contrast to F. esculentum, the occurrence of a terminal thyrse does not guarantee a determinate growth pattern in F. tataricum. The number of lateral thyrses produced before the terminal thyrse on the main axis of F. tataricum varies from zero to c. 19.

DISCUSSION

The nine stages of early flower development formally recognized here and our outline of basic terminology will facilitate more standardized and readily comparable descriptions in subsequent research on buckwheat biology. Non-trivial relative arrangements of tepals and bracteoles in Fagopyrum and some other Polygonaceae require investigation using refined approaches to mathematical modelling of flower development. Our data on inflorescence morphology and development suggest contrasting evolutionary patterns in the two main cultivated species of buckwheat, F. esculentum and F. tataricum. The genus Fagopyrum offers an excellent opportunity for evo-devo studies related to inflorescence architecture.