Evolution of pollen morphology in Calyceraceae and insights into its early evolutionary differentiation

Abstract

Pollen morphology has been a fundamental tool in the study of evolution and phylogeny in angiosperms. As sister to Asteraceae, Calyceraceae provides critical insights into understanding the early diversification of the sunflower family. To date, the most comprehensive palynological studies of Calyceraceae have been based on just a few species and lack a phylogenetic framework. Here we present an evolutionary study of pollen morphology in Calyceraceae based on an almost complete sampling and in light of recent phylogenetic hypotheses for this family. Twenty-seven palynological characters were established after morphological studies with scanning electron and bright field microscopy for 44 species of Calyceraceae. Character analyses included phylogenetic signal and reconstruction of ancestral states. Morphology of the mesocolpium, amb, and apertures support most genera, and provide support for the two main clades of Calyceraceae. We propose five pollen types, namely I–V. Most genera are nearly uniform in pollen morphology except for Gamocarpha, which includes four pollen types. Pollen types I and III, related to the most recent common ancestors (MRCAs) of Acicarpha and Gamocarpha respectively, possess the most apomorphic character states. Pollen types II and V, related to Calycera and Moschopsis MRCAs respectively, retain the most plesiomorphic character states. The major evolutionary changes between pollen of Calyceraceae and its sister group (whether it is Asteraceae or Barnadesioideae) involve mesoaperture development and exine structure. Our results support an independent origin of the mesocolpal concavities in Calyceraceae and its sister group (Asteraceae or Barnadesioideae), and for colpal ledges in Goodeniaceae and Calyceraceae.