Abstract

BackgroundThe detailed knowledge of plant anatomical characters and their variation among closely related taxa is key to understanding their evolution and function. We examined anatomical variation in 46 herbaceous taxa from the subfamily Campanuloideae (Campanulaceae) to link this information with their phylogeny, ecology and comparative material of 56 woody tropical taxa from the subfamily Lobelioideae. The species studied covered major environmental gradients from Mediterranean to Arctic zones, allowing us to test hypotheses on the evolution of anatomical structure in relation to plant competitive ability and ecological preferences.Methodology/Principal FindingsTo understand the evolution of anatomical diversity, we reconstructed the phylogeny of studied species from nucleotide sequences and examined the distribution of anatomical characters on the resulting phylogenetic tree. Redundancy analysis, with phylogenetic corrections, was used to separate the evolutionary inertia from the adaptation to the environment. A large anatomical diversity exists within the Campanuloideae. Traits connected with the quality of fibres were the most congruent with phylogeny, and the Rapunculus 2 (“phyteumoid”) clade was especially distinguished by a number of characters (absence of fibres, pervasive parenchyma, type of rays) from two other clades (Campanula s. str. and Rapunculus 1) characterized by the dominance of fibres and the absence of parenchyma. Septate fibres are an exclusive trait in the Lobelioideae, separating it clearly from the Campanuloideae where annual rings, pervasive parenchyma and crystals in the phellem are characteristic features.Conclusions/SignificanceDespite clear phylogenetic inertia in the anatomical features studied, the ecological attributes and plant height had a significant effect on anatomical divergence. From all three evolutionary clades, the taller species converged towards similar anatomical structure, characterized by a smaller number of early wood vessels of large diameter, thinner cell-walls and alternate intervessel pits, while the opposite trend was found in small Arctic and alpine taxa. This supports the existing generalization that narrower vessels allow plants to grow in colder places where they can avoid freezing-induced embolism, while taller plants have wider vessels to minimize hydraulic resistance with their greater path lengths.

Highlights

  • The detailed knowledge of plant anatomical characters and their variation among closely related taxa is key to understanding their evolution and function [1]

  • Variation in anatomical structure is a result of several forces such as: the adaptation of species to the prevailing conditions in their habitats [2], phenotypic plasticity as an ability of individuals with an identical genotype to develop differently - based on specific conditions during their ontogeny [3,4], and evolutionary constraints in which taxa that share part of their evolutionary history possess similar ‘blue-prints’ [5,6,7]

  • The large anatomical variability of the subfamily Campanuloideae found within the studied material was partitioned into phylogenetic relatedness and ecological adaptation

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Summary

Introduction

The detailed knowledge of plant anatomical characters and their variation among closely related taxa is key to understanding their evolution and function [1]. One of the critical features of comparative studies on plant trait variations in relation to ecological adaptations is the extent of phylogenetic relatedness among taxa [8], which makes them partly dependent in any statistical inference. Understanding the evolution of plant structures requires separation of evolutionary inertia from a true adaptation to the environment. This is commonly done by comparing analyses made with and without phylogenetic corrections [10]. This approach is based on discounting all of the variation that could possibly be explained by phylogenetic relatedness of the studied species [9], before studying the effect of other potential predictors such as the species’ ecological preferences. The species studied covered major environmental gradients from Mediterranean to Arctic zones, allowing us to test hypotheses on the evolution of anatomical structure in relation to plant competitive ability and ecological preferences

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