Palynological characters and their systematic significance in Naucleeae (Cinchonoideae, Rubiaceae)

Abstract

Phylogenetic studies have improved Naucleeae classification, but the relationships among the subtribes remain largely unresolved. This can be explained by the inadequate number of synapomorphies shared among these lineages. Of the 49 morphological characters used in phylogenetic analyses, none were from pollen. It has been proposed that H-shaped endoapertures form a synapomorphy of the Naucleeae. Further study of Naucleeae pollen is needed to test this hypothesis as the endoapertures of many Naucleeae genera are unknown. Pollen morphology of 24 species was examined using scanning electron and light microscopy. Naucleeae pollen is very small to small, with a spheroidal to subprolate shape in equatorial view. Three compound apertures are present, each comprised of a long ectocolpus, a lolongate to (sub)circular mesoporus, and an often H-shaped endoaperture. The sexine ornamentation is microreticulate to striate, rugulate, or perforate. Pollen wall ultrastructure of five species was studied with transmission electron microscopy. The exine is composed of a perforated tectum, short columellae, and a thick nexine. The nexine is often differentiated into a foot layer and an endexine, and thickened into costae towards the aperture. The intine often protrudes from the aperture forming a protruding oncus. Our observations support the phylogenetic delimitation of the Naucleeae sensu Razafimandimbison and Bremer, but pollen morphology is of little value in distinguishing the subtribes and genera of the Naucleeae. Ancestral state reconstruction using MacClade is unambiguous in showing that the possession of an H-shaped endoaperture and protruding onci (a new character for the tribe) form morphological synapomorphies of the clade Hymenodictyon+ Naucleeae.