Evolution of breeding systems and fruits in New World Galium and relatives (Rubiaceae)

Dioecy occurs in only about 6% of angiosperms, yet it has evolved many times from hermaphroditism. Polygamy is an even more uncommon condition within angiosperms, in which both unisexual and bisexual flowers occur within a species. Polygamy, dioecy, and hermaphroditism all occur within a New World clade of Galium (Rubiaceae), in which dioecy is hypothesized to have evolved from hermaphroditism via polygamy. At least five sections of Galium as traditionally defined by fruit morphology occur within this group. We tested the monophyly of sections defined by fruit morphology and sought to determine origins and pathways of breeding systems within this group. •

We obtained chloroplast (rpoB-trnC, trnC-psbM, trnL-ndhJ) and nuclear ribosomal (external transcribed spacer) DNA sequences for 89 taxa from the Cruciata-Galium-Valantia (CGV) clade to estimate the phylogeny. Ancestral states for breeding systems, fruit types, and fruit hairs were reconstructed using parsimony and likelihood analyses. •

We identified nine well-supported lineages of New World Galium taxa. However, none of the sections traditionally defined by fruit morphology are monophyletic. Dioecy is inferred to have arisen at least three times from hermaphroditism; polygamy is inferred to have arisen at least twice from dioecy and at least six times from hermaphroditism. •

Polygamy appears to be a terminal condition in the CGV clade and not a pathway to dioecy. Fruit characters traditionally used in the taxonomy of this group have arisen multiple times within this clade of Galium and are not reliable indicators of shared evolutionary history.