EVOLUTION OF ANGIOSPERM POLLEN. I. INTRODUCTION

摘要

This paper is the first in a series that documents the diversity, distribution, and evolution of palynological characters across angiosperms in a contemporary phylogenetic context, using modern optimization methods. The objectives of the series are: (1) to describe the diversity of pollen morphologies across the angiosperms; (2) to estimate ancestral palynological character states, diagnostic characters, and synapomorphies for monophyletic groups; (3) to highlight and interpret inferred patterns and processes of evolution in palynological characters; and (4) to provide a framework for the placement of enigmatic taxa (including fossil taxa) based on pollen morphology. This first paper examines the methods available for such a study and presents an overview of palynological characters across angiosperms as a whole. Using a well-supported, recent, molecular phylogeny, we consider the effects of coding strategy, method of optimization, and starting tree topology upon inference of trait evolution. Coding strategy and optimization method had significant effects upon inferred ancestral character states, the latter probably due to the different evolutionary models applied. Phylogenetic topology had little effect upon inferred ancestral character states, because the uncertainty in topology at this level involved only nodes where few character state changes occurred. Several palynological characters showed consistent, structured patterns in the context of phylogeny: angiosperms are distinguished from other seed plants by character states including supratectal elements echinate and less than 1 mu m in size, and infratectum structure columellate; eudicots, as recognized in previous studies, may be defined by globose, isopolar, radially symmetrical grains with three equatorial apertures. We present a framework for the remainder of the series, in which the angiosperms are divided into nine monophyletic and paraphyletic groups each having a similar level of pollen variability, and a set of recommendations for the analysis of these groups. The series will provide a reference for future palynological and systematic studies and an approach that may be replicated for other character sets.