Morphology, ultrastructure and typology of orbicules in Loganiaceae s.l. and related genera, in relation to systematics

Abstract

Minute granules of sporopollenin, called orbicules or Ubisch bodies, can be observed on the innermost tangential and/or radial walls of secretory tapetum cells. Orbicules were investigated with light microscopy as well as scanning electron, and transmission electron microscopy in 19 species of Loganiaceae s.l. and 32 related species. The species investigated cover all the recently recognised families segregated from Loganiaceae s.l.: Loganiaceae s.str., Gelsemiaceae, Geniostomaceae, Strychnaceae and Loganiaceae tribe Potalieae which is now included in the Gentianaceae. Orbicules were found in all the species of Loganiaceae s.l. that were investigated with the exception of Buddleja and Retzia. From the related species that were studied, 20 species (18 genera, seven families) possess orbicules. On the basis of observed morphological and ultrastructural variations, five orbicule types are described. The types are based on the orbicule typology previously described for the rubiaceous subfamilies Cinchonoideae and Ixoroideae. The systematic value of this typology is tested by comparison with the most recent classifications within Loganiaceae s.l. The heterogeneity of Loganiaceae s.l. is reflected in the variation of orbicule morphology. The two tribes Antonieae and Strychneae (Strychnaceae) are characterised by the presence of two very distinct types of orbicules (I or IIIa). In the most recent classifications of the Loganiaceae s.l. a relationship between Strychneae and the Loganiaceae s.str. (including Geniostomaceae) is suggested; this is confirmed by the presence of the same type of orbicules in all of these taxa. Gelsemiaceae species have one or other of two orbicule types (III or IV). The inclusion of the Loganiaceous tribe Potalieae in Gentianaceae is supported by the presence of the same orbicule type (V) as found in the genus Exacum and the former Loganiaceous genera Anthocleista and Fagraea now also included in Gentianaceae. In Apocynaceae s.l. orbicules do not occur in all tribes. Orbicules are present in the tribes Plumerioideae, Apocynoideae, and Periplocoideae (Asclepiadaceae). In the more derived genera of Asclepiadaceae (except Periplocoideae) orbicules do not occur. Results show that the systematic application of orbicule characteristics can be of value at the tribal level in the Loganiaceae s.l.