Pollen morphology and its phylogenetic significance in tribe Sanguisorbeae (Rosaceae)

Abstract

Pollen grains of tribe Sanguisorbeae (Rosaceae, Rosoideae) were examined using scanning electron microscopy to identify useful characters, test taxonomic and phylogenetic hypotheses among genera, and elucidate pollen character evolution based on a molecular phylogeny. Aperture number, aperture structure, pollen shape, and exine sculpturing were variable within Sanguisorbeae and were used to delineate six pollen types. Four types (I–IV) were observed only in subtribe Sanguisorbinae whereas two types (V–VI) were found only in subtribe Agrimoniinae. Pollen grains of tribe Sanguisorbeae were generally subprolate to spheroidal in shape, had operculate or pontoperculate apertures, and had three apertures, except for Margyricarpus (tetraperturate). Exine sculpturing within Sanguisorbinae represented variations of striate, verrucate, rugulate, and perforate patterns often with microechinate sculpturing. Striate exine patterns and prolate shapes characterized the pollen of the Agrimoniinae, except for the microechinate-verrucate pattern and subprolate to spheroidal shapes observed in Hagenia. Pollen characters are most useful at the generic level and, when mapped on to a molecular phylogenetic tree of the tribe, are concordant with a monophyletic Agrimoniinae and a clade comprising Margyricarpus + Acaena + Polylepis + Cliffortia + Sanguisorba in the Sanguisorbinae. Outgroup comparison indicated that operculate colpi, three apertures, and polymorphism for striate or microverrucate exines represented primitive states for tribe Sanguisorbeae.