The demonstration of postzygotic self-incompatibility in Asclepias L. prompted us to reconsider explanations for several unusual features of the Apocynaceae. With such late-acting self-incompatibility, mixed loads of self- and cross-pollen can be extremely wasteful by causing abortion of whole fruits and consequent wastage of both cross-pollen and cross-fertilized ovules. We contend that the evolution of pollinia and loss of carpel fusion in certain Apocynaceae represent adaptations to prevent or compensate for these negative effects of mixed pollen loads. Trends in the agglutination of pollen can be seen in the tetrads of Apocynum L., which are transported on sticky band-like translators; the masses of tetrads of Periplocoideae, which are deposited onto the sticky scoop portion of a spoon-like translator; the pollinia of Secamonoideae, which have a clip-like translator but no outer covering; and the pollinia of Asclepiadoideae, which are completely enclosed by a waxy outer covering. In many genera of the Rauvolfioideae, the ovary is syncarpous, but in most Apocynoideae and all Periplocoideae and Secamonoideae, the gynoecium consists of two carpels that are free in the ovule-bearing region but fused in the upper region to produce a compitum, which enables pollen placed on one stigmatic area to supply pollen tubes to both ovaries. In at least some Asclepiadoideae, however, a compitum is lacking, although postgenital fusion between the carpel apices still takes place. Parallel trends in pollen delivery and receipt also occur in taxa from other angiosperm families in which late-acting self-incompatibility systems have been implicated.
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Vol. 94 • No. 2