Dipterocome, an addition to the second-order headed genera in Compositae?

Abstract

The genus Dipterocome Fisch. & C. A. Mey. has a quite complicated history. It was escribed as Dipterocome and as Jaubertia, and even included in Koelpinia. All the attempts of establishing firmly its tribal ascription in the Compositae failed for 150 years: it was placed tentatively in tribe Calenduleae by Bentham, Boissier and Hoffmann, but it was clear that the similarities to Calendula were superficial. De Candolle was so frustrated that he ranked Dipterocome among the unclassified genera of the Compositae acknowledging that it was an impossible task. Thanks to the use of molecular markers, Anderberg finally placed the genus in the Cardueae. Using next-generation sequencing, we have confirmed that Dipterocome belongs to the Cardueae and constitutes a monotypic subtribe Dipterocominae with affinities to the subtribe Xerantheminae. In our presentation, we will discuss the morphology and structure of the capitulum. Based on the excellent drawings by Jaubert & Spach and new histological evidence, we will challenge the traditional interpretation as achenes of the peculiar, spiny structures in the heads. Rather, they should be interpreted as diaspores; the achene is enclosed in a sheat of fused spiny bracts that would be hard to derive from receptacular scales. Our hypothesis is that the heads of Dipterocome are syncephalies or second-order inflorescences, and the “florets” are actually individual heads; the female ones have horned bracts enclosing the achenes, and the male ones show only a tiny basal sheat. It is worth mentioning that at least four of the genera with second-order inflorescences in the Compositae, Dipterocome, Echinops, Gundelia, and Hecastocleis, are plants from extremely dry habitats.