Pollen-ovule ratios in milkweeds (Asclepiadaceae): an exception that probes the rule

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