Flower-like heads from flower-like meristems: pseudanthium development in Davidia involucrata (Nyssaceae)

JPR Symposium Floral development –Re-evaluation of its importance–
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Abstract

Flower-like inflorescences (pseudanthia) have fascinated botanists for a long time. They are explained as condensed inflorescences implying that the pseudanthium develops from an inflorescence meristem (IM). However, recent developmental studies identified a new form of reproductive meristem, the floral unit meristem (FUM). It differs from IMs by lacking acropetal growth and shares fractionation, expansion and autonomous space filling with flower meristems (FM). The similarity among FUMs and FMs raises the question how far flower-like heads originate from flower-like meristems. In the present paper, pseudanthium development in Davidia involucrata is investigated using scanning electron microscopy. D. involucrata has pincushion-shaped heads composed of densely aggregated, perianthless flowers and associated with two large showy bracts. Early developmental stages show a huge naked FUM. The FMs appear almost simultaneously and lack subtending bracts. With ongoing FUM expansion new space is generated which is immediately used by further FM fractionation. The heads have only staminate flowers or are andromonoecious with staminate and a single perfect flower in oblique position. All FMs lack perianth structures and fractionate a variable number of stamen primordia. The perfect FM is much larger than the staminate FMs and forms a syncarpous gynoecium with inferior ovary. Pseudanthium development in D. involucrata confirms the morphogenetic similarity to FMs as to acropetal growth limitation, meristem expansion and fractionation. It thus should not be interpreted as a condensed inflorescence, but as a flower equivalent. Furthermore as the FUM develops inside a bud, its development is considered to be influenced by mechanical pressure. The oblique position of the perfect flower, the developmental delay of the proximal flowers, and the variable number of stamens which were observed in the pseudanthium development, can be caused by mechanical pressure. Next to the Asteraceae, D. involucrata offers a further example of a pseudanthium originating from a FUM. More knowledge on FUMs is still needed to understand diversification and evolution of flower-like inflorescences.

Keywords

Extrafloral bracts Floral unit meristem (FUM) Flower meristem (FM) Inflorescence meristem (IM) Mechanical pressure Utilization of space 

Notes

Acknowledgements

We thank Akitoshi Iwamoto (Tokyo) and Kester Bull-Hereñu (Santiago de Chile) for initiating this special issue on flowers. The first author is very grateful to the Botanical Society of Japan for the invitation and financial support to participate at its annual congress at Okinawa 2016. We thank Bernd Mengel (BG Mainz) for collecting Davidia buds throughout the year, Madeleine Junginger (Mainz) for measuring the primordia at the SEM pictures and Maria Geyer (Mainz) for working over the illustrations. Special thanks go to Margerita Remizowa (Moscow) and an unknown reviewer for stimulating discussion.

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Copyright information

© The Botanical Society of Japan and Springer Japan KK, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Institute of Organismic and Molecular Evolution (iomE)Johannes Gutenberg-University MainzMainzGermany

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