Phylogenetic and functional signals in gymnosperm ovular secretions

Gymnosperm are either wind-pollinated (anemophilous) or wind-insect-pollinated (ambophilous). Regardless of pollination mode, ovular secretions play a key role in pollen capture, germination, and growth; they are likely also involved in pollinator reward. Little is known about the broadscale diversity of ovular secretions across gymnosperms, and how these may relate to various reproductive functions. This study analyses the sugar and amino acid profiles of ovular secretions across a range of ambophilous (cycads and Gnetales) and anemophilous gymnosperms (conifers) to place them in an evolutionary context of their possible functions during reproduction. Ovular secretions from 13 species representing all five main lineages of extant gymnosperms were sampled. HPLC techniques were used to measure sugar and amino acid content. Multivariate statistics were applied to assess whether there are significant differences in the chemical profiles of anemophilous and ambophilous species. Data were compared with published chemical profiles of angiosperm nectar. Chemical profiles were placed in the context of phylogenetic relationships. Total sugar concentrations are significantly higher in ovular secretions of ambophilous species than wind-pollinated taxa such as Pinaceae and Cupressophyta. Ambophilous species have lower amounts of total amino acids, and a higher proportion of non-protein amino acids compared to anemophilous lineages, and are also comparable to angiosperm nectar. Results suggest that early gymnosperms likely had ovular secretion profiles that are a mosaic of those associated with modern anemophilous and ambophilous species. Ginkgo, thought to be anemophilous, has a profile typical of ambophilous taxa, suggesting that insect pollination either exists in Gingko, but is undocumented, or that its ancestral populations were insect pollinated. Chemical profiles of ovular secretions of ambophilous gymnosperms show a clear signal of pollinator-driven selection, including higher levels of carbohydrates than anemophilous taxa, lower levels of amino acids, and the presence of specific amino acids, such as β-alanine, that are known to influence insect feeding behaviour and physiology.