Unlike most highly outcrossing flowering plants, milkweeds (Asclepiadaceae) have exceptionally low pollen-ovule ratios. We counted the number of pollen grains contained within a pollinium sac and the number of ovules contained within an ovary for 38 species of Asclepiadaceae. Across four tribes of the Asclepiadoideae, the number of pollen grains per pollinium varied from 14 to 445, and the number of ovules per ovary varied from 4 to 229. Nevertheless, the pollen-ovule ratio was constrained within a narrow range, generally 1 to 2. Similar constraints on pollen-ovule ratios occur within mimosoid legumes (Acacia, Calliandra, Inga) whose pollen also is dispersed in clusters (polyads). Our data are consistent with the view that agglutination of pollen evolves in concert with ovule number to minimize the likelihood of receiving mixed pollen loads and are not consistent with the view that pollen-ovule ratios reflect breeding system or pollination efficiency. Trends in pollen grain and ovule number do not appear to reflect taxonomic affinities, but rather ecogeographical pressures.

Communicating Editor: Paul Wilson