Evolution of Leaf Fusion in Honeysuckle (Periclymenum,Lonicera)

Abstract

Premise of research. Fused or connate leaves are a well-known phenomenon observed across a limited number of angiosperm clades, and no study has attempted to examine this form of fusion from an evolutionary, morphometric, or functional perspective. We examined leaf fusion in honeysuckles, specifically the Periclymenum clade (∼22 species) of Lonicera (Caprifoliaceae, Dipsacales), which exhibits variation in leaf shape, degree of fusion, and the position of fused leaves. As fused leaves co-occurred with reproductive structures, evolutionary correlations between leaf fusion and inflorescence architecture were also examined. Methodology. Variation of leaf fusion was assessed using elliptical Fourier analysis, and multiple individuals of 19 of the 22 species of Periclymenum were sampled. As fused leaves occurred only on reproductive shoots, a suite of inflorescence characters were also studied. A phylogeny for Periclymenum was reconstructed using published sequence data, and this tree was used for ancestral character state reconstructions and correlation analyses. Pivotal results. Leaves directly subtending inflorescences of Periclymenum were free or fused but were rarely partially fused. Fused leaves were ancestral to Periclymenum and were lost in parallel. Leaf fusion was not correlated with inflorescence architecture features, yet inflorescence architecture has become more complex among Periclymenum species. Conclusions. While free leaves are the ancestral condition in the Dipsacales, fused leaves have been gained at least three times and lost at least twice. Given the proximity of fused leaves to reproductive structures, particularly in Lonicera, fused leaves may play a role in protection or in the discovery of flowers and fruits by pollinators and seed dispersers, respectively.