Phylogenetic relationships of the perianthless Piperales: Reconstructing the evolution of floral development

The perianthless members of the Piperales are unique among the basal lineages of angiosperms because they are mainly herbaceous plants with over 2000 species possessing highly reduced flowers. There have been several attempts to address the evolution of the flower morphology in the group, but no previous study has included aDNA based estimate of phylogeny. Here we present a robust reconstruction of the phylogenetic relationships of the genera in the perianthless Piperales using DNA sequence data from three genes (rbcL, atpB, and 18S) and two genomes (nuclear and chloroplast). We estimated the likelihood values of ancestral character states of mature floral structures. Developmental sequences also were analyzed using step matrices under specific models of character state change to examine the origin of meristic differences of the androecium and gynoecium. Developmental lability is the general theme in the evolution of floral merosity in the perianthless Piperales, with different developmental processes giving rise to morphologically identical mature stages. Our findings confirm this notion based on several distinct patterns: ( 1) nonidentical ontogenies give rise to homoplasious, six-staminate androecia of Saururaceae and some Piperaceae most likely through terminal addition; (2) identical ontogenies produce the homoplasious two-staminate flowers in Peperomia and Piper umbellatum through deletion; and ( 3) nearly identical ontogenies produce homplasious tricarpellate gynoecia of Piper and Anemopsis + Houttuynia clade, and the apocarpous gynoecium of Saururus may be secondarily derived, both end products occurring through deletion. We note that changes in organ number and the degree of carpel fusion have been important during the evolutionary history of the perianthless Piperales as well as the basal angiosperms in general and should be studied more extensively. Overall we emphasize the lability of developmental pathways, especially in the flower, and endorse the methodological utility of developmental sequences for directing future investigations of floral evolution.