Perianth evolution in the sandalwood order Santalales

Abstract

Flowers of Santalales remain largely unexplored with several questions of homology unanswered despite the large size of the order. Morphological and ontogenetic floral studies have the potential to identify new informative characters. We studied floral development in species of Loranthaceae, Santalaceae, Opiliaceae, and "Olacaceae" with scanning electron microscopy to clarify the origin and evolution of the perianth in Santalales. The perianth is either dichlamydeous or when monochlamydeous interpreted as sepals, petals, or tepals. A girdling calyculus of debatable origin is found in some clades. We show that species of Diogoa and Heisteria in "Olacaceae" have a dichlamydeous perianth, unlike Olax in which the calyx is replaced by a calyculus. The calyculus arises by development of two lateral primordia, supporting the hypothesis of bracteole origin. A calyculus with similar development is present in species of Loranthaceae and possibly of Opiliaceae, suggesting a position of Olax closer to these families than to traditional genera of "Olacaceae". The monochlamydeous perianth in Santalaceae is shown to correspond to petals of other members of Santalales. Flower ontogenetic evidence suggests a repeated loss of the calyx, replacement by a calyculus, and further loss, leading to monochlamydeous perianths in Santalaceae.

Prophyll, calyculus, and perianth in <I>Santalales</I>

“…While I cannot judge the data and illustrations provided by Wanntorp & Ronse De Craene (2009) for 'Olacaceae' (Diogoa, Heis teria and Olax), there are convincing indications that in nonmistletoe families the prophyllar hypothesis must also be rejected. In those flowers where undeniable calyculi develop (sometimes after anthesis), non-articulated pedicels support them, thus making the idea of any morphogenetic contribution by even very small or unnoticed prophylls a very questionable and remote proposition.…”

“…The loranthaceous material used in Wanntorp & Ronse De Craene's (2009) study consists of one species each of Passovia ('Phthirusa') and Struthanthus. Each of these genera is characterized by inflorescences bearing various numbers of paired, lateral triads.…”

“…Most of these studies have dealt with the phylogenetic history of the order and its multiple components, especially the work of Nickrent et al (2010). However, one of these contributions has instead focused on the structural elements related to flowers (Wanntorp & Ronse De Craene 2009), and has resulted in some novel interpretations. It is the purpose of the present contribution to subject these interpretations to scrutiny.…”

“…It is the purpose of the present contribution to subject these interpretations to scrutiny. The significant questions raised by Wanntorp & Ronse De Craene (2009) revolve around three apparently distinct structural issues. There is, first of all, the status and occurrence of prophylls as defined below.…”

“…The calyculus in Loranthaceae has been generally interpreted as a reduced calyx, a concept recently challenged by Wanntorp & Ronse De Craene (2009), who concluded that it represents a fusion of prophylls. This view, as well as the accompanying biseriate corolla notion, is rejected.…”