Pollen sterols are associated with phylogeny and environment but not with pollinator guilds.

Pengjuan Zu,Geraldine A Wright,Elynor Moore,W David Nes,Samuel Pironon,Iain W Farrell,Charlotte Phillips,Dudley I Farman,Philip C Stevenson,Orlando Schwery,Ian Ondo,Hauke Koch

doi:10.1111/nph.17227

Abstract

Summary Phytosterols are primary plant metabolites that have fundamental structural and regulatory functions. They are also essential nutrients for phytophagous insects, including pollinators, that cannot synthesize sterols. Despite the well‐described composition and diversity in vegetative plant tissues, few studies have examined phytosterol diversity in pollen.We quantified 25 pollen phytosterols in 122 plant species (105 genera, 51 families) to determine their composition and diversity across plant taxa. We searched literature and databases for plant phylogeny, environmental conditions, and pollinator guilds of the species to examine the relationships with pollen sterols.24‐methylenecholesterol, sitosterol and isofucosterol were the most common and abundant pollen sterols. We found phylogenetic clustering of twelve individual sterols, total sterol content and sterol diversity, and of sterol groupings that reflect their underlying biosynthesis pathway (C‐24 alkylation, ring B desaturation). Plants originating in tropical‐like climates (higher mean annual temperature, lower temperature seasonality, higher precipitation in wettest quarter) were more likely to record higher pollen sterol content. However, pollen sterol composition and content showed no clear relationship with pollinator guilds.Our study is the first to show that pollen sterol diversity is phylogenetically clustered and that pollen sterol content may adapt to environmental conditions.

Highlights

Phytosterols are a class of lipids with key metabolic and ecological functions for plants (Nes & McKean, 1977; Vanderplanck et al, 2020a)
We looked up the inferred ages of seven clades from the large phylogeny of spermatophytes documented by Zanne et al (2014): Angiospermae (243 million years ago (Ma)), Monocotyledoneae (171 Ma), Eudicotyledoneae (137 Ma), Superrosidae (118 Ma), Rosidae (117 Ma), Superasteridae (117 Ma), and Asteridae (108 Ma)
We profiled 25 phytosterols in the pollen of 122 plant species from 51 families, including representatives of Gymnosperms, Nymphaeales, Monocots, Ranunculids, Caryophyllales, Asterids, and Rosids (Fig. 2; Table S1). These phytosterols can be arranged into biosynthetic pathways with three main distinct branches (i.e. C-24 – no substitution, C-24 methyl and C-24 ethyl groups, Fig 3; see Fig. 1 for illustrations of the group structure)

Summary

Introduction

Phytosterols are a class of lipids with key metabolic and ecological functions for plants (Nes & McKean, 1977; Vanderplanck et al, 2020a). The structural variation and composition of sterols in plant tissues is important for phytophagous insects since they cannot synthesize sterols de novo, and depend upon specific plants to obtain the required sterols from their diet to sustain their development (Behmer & Elias, 1999, 2000; Lang et al, 2012). This may be especially important for pollen feeding insects that require specific sterols.

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