Floral cost vs. floral display: Insights from the megadiverse Myrtales suggest that energetically expensive floral parts are less phylogenetically constrained

Premise of the study: Floral display describes the effect of flower size combined with the number of flowers per inflorescence. There is strong evidence that a floral-display trade-off operates under energetic constraint, with few-flowered inflorescences likely to have larger flowers than many-flowered inflorescences. Flower size can be estimated by different variables; thus, we propose that the variable for flower size that is most highly (negatively) correlated with the number of flowers per inflorescence will also be the best estimate of floral cost. Ranking the correlation with the phylogenetic signal of the variable can provide additional insight into the evolution of floral display.

Methods: The Myrtales were chosen as a model order based on age, worldwide distribution, and diversity of reproductive strategies. Ninety-nine species representing all families and one quarter of generic diversity across its geographic and ecological range were sampled to reconstruct a phylogeny based on rbcL and ndhF sequences. Correlation coefficients were calculated for flower size variables vs. the number of flowers per inflorescence. Phylogenetic signal was measured for all variables and for floral display.

Key results: Flowers per inflorescence showed significant negative correlation with the following flower size variables (weakest to strongest): filament length < anther size < flower depth < flower diameter. As the correlation of each character with number of flowers per inflorescence rose (suggesting increased cost), the values for phylogenetic signal diminished (suggesting less constraint).

Conclusions: We conclude that energetically costly floral characters appear to be less phylogenetically constrained, while low-cost floral characters maintain higher levels of phylogenetic inertia.