Fruit syndromes in Viburnum: correlated evolution of color, nutritional content, and morphology in bird-dispersed fleshy fruits

Premise A key question in plant dispersal via animal vectors is where and
why fruit colors vary between species and how color relates to other fruit
traits. To better understand the factors shaping the evolution of fruit
color diversity, we tested for the existence of syndromes of traits
(color, morphology, and nutrition) in the fruits of Viburnum. We
placed these results in a larger phylogenetic context and reconstructed
ancestral states to assess how Viburnum fruit traits
have evolved across the clade. Results We find that
blue Viburnum fruits are not very juicy, have high lipid
content, and large, round endocarps surrounded by a small quantity of
pulp. Red fruits display the opposite suite of traits: they are very juicy
with low lipid content and smaller, flatter endocarps. The
ancestral Viburnum fruit may have gone through a
sequence of color changes before maturation (green to yellow to red to
black), though our reconstructions are equivocal. In one major clade
of Viburnum (Nectarotinus), fruits mature synchronously
with reduced intermediate color stages. Most transitions between fruit
colors occurred in this synchronously fruiting clade. Conclusions It is
widely accepted that fruit trait diversity has primarily been driven by
the differing perceptual abilities of bird versus mammal
frugivores. Yet within a clade of largely bird-dispersed fruits, we find
clear correlations between color, morphology, and nutrition. These
correlations are likely driven by a shift from sequential to synchronous
development, followed by diversification in color, nutrition, and
morphology. A deeper understanding of fruit evolution within clades will
elucidate the degree to which such syndromes structure extant fruit
diversity.