Leaf morphology and anatomy of Huarpea andina (Asteraceae): insights into the evolution of xeromorphy within Barnadesioideae

Abstract

The earliest diverging lineage of Asteraceae is Barnadesioideae, which represents an evolutionary bridge with the closely related family Calyceraceae. Barnadesioideae consist of 10 morphologically diverse genera that range from seasonal annual herbs to trees more than 30 m tall. They are united by possessing axillary spines and unusual ‘barnadesioid trichomes’, plus by the absence of two inverted repeats of the chloroplast genome that are found in all other investigated members of Asteraceae. One of the most peculiar genera of Barnadesioideae is Huarpea and its single species, H. andina. This species grows scattered as a small subshrub up to 10 cm tall and is confined to the very dry, rocky ‘llanos de altura’ (open plains) in San Juan Province, Argentina, at 3000–4500 m. Leaf morphology and anatomy provide clues as to how H. andina has adapted for survival in this challenging environment of extremely dry summers, snow-covered winters, and dramatic wind and temperature fluctuations. Gas exchange from the leaves is regulated by stomata protected along the adaxial midrib and in abaxial crypts resulting from tightly revolute leaf margins. A thick hypodermis provides support to withstand strong winds. A molecular phylogeny of Barnadesioideae was the basis for reconstructing the evolutionary history of three xeromorphic characters within the subfamily, using ancestral-state reconstruction, stochastic character mapping, and tests of correlated evolution. Phylogenetic analyses indicated that these xeromorphic characters evolved independently only a few times in Barnadesioideae, and they showed no significant correlated evolution with phytogeographic distribution or habitat moisture. The distinctive leaf anatomy of H. andina, therefore, represents one of the few independent acquisitions of xeromorphic characters within the subfamily.