Sepal Identity of the Pappus and Floral Organ Development in the Common Dandelion (<i>Taraxacum officinale</i>; Asteraceae)
2021
Kitty Vijverberg | Monique Welten | Marjan Kraaij | Bertie Joan van Heuven | Erik Smets | Barbara Gravendeel
The dry one-seeded fruits (<i>cypselae</i>) of the Asteraceae are often crowned with a <i>pappus</i>, an appendage of hairs or scales that assists in dispersal. It is generally assumed, but little investigated, that the pappus represents the outer floral whorl where the <i>sepals</i> are usually located. We analysed pappus–sepal homology in dandelions using micromorphological and floral gene expression analyses. We show that the pappus initiates from a ring primordium at the base of the corolla, heterochronic to the petals. Pappus parts form from this ring, with those in the <i>alternipetalaous</i> position usually being ahead in growth, referring to sepal identity. <i>Tof-APETALLA</i>1 expression increased during floret development and was highest in mature pappus. <i>Tof-PISTILLATA</i> expression was high and confined to the floral tissues containing the petals and stamens, consistent with expectations for sepals. Apart from the pappus, the dispersal structure of dandelion consists of the upper part of the fruit, the <i>beak</i>, which originates from the inner floral whorl. Thus, our results support the homology of the pappus with the sepals, but show that it is highly derived. Together with our floral stage definitions and verified qPCR reference genes, they provide a basis for evolution and development studies in dandelions and possibly other Asteraceae.
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