Systematic palynology in Ebenaceae with focus on Ebenoideae: Morphological diversity and character evolution

Abstract

This study examines the diversity and character transformations in pollen and orbicule morphology of Ebenaceae, with a focus on subfamily Ebenoideae (ca. 600 sp.). 62 specimens comprising all three genera of Ebenoideae ( Diospyos, Euclea, Royena), were studied using LM and SEM. Bayesian phylogenetic analysis was performed on molecular sequence data to establish an evolutionary hypothesis that was then used as an evolutionary framework to identify synapomorphies and trace evolutionary trends of palynological data with Bayesian posterior mapping and principle component analyses (PCA). Ebenoideae pollen is generally shed as monads (permanent tetrads in two species), medium-sized, prolate-spheroidal to subprolate and tricolporate. A substantial amount of variation is found in pollen size, equatorial outline (lobate, subtriangular, circular and hexagonal) and sexine ornamentation type ((micro)rugulate, striate, granulate and gemmate). Moreover, orbicules were present on the inner locule wall in all specimens examined. Their abundance, degree of fusion with tapetal membrane and aggregation vary considerably. We can conclude that Ebenaceae pollen is more heterogeneous than previously assumed. We traced palynological synapomorphies for groups at different taxonomic levels: subfamily level (pollen size, pollen wall stratification and aperture morphology), generic level (size, equatorial outline and sexine ornamentation types) and subgeneric clades (size, ectocolpus morphology, equatorial outline and sexine ornamentation subtypes) respectively. The granular infratectum and the unique sculpturing pattern on the orbicule walls are the most discriminating pollen features for subfamily Ebenoideae.