Self-Sterility and Cryptic Self-Fertility in Campsis radicans (Bignoniaceae)

Abstract

Campsis radicans is almost completely self-sterile following pollinations of pure self-pollen, but when self- and cross-pollen are present in similar amounts, ca. one in six viable seeds is sired by self-pollen. The amount of selfing depends in part on the identity of the cross-pollen donor in the pollen mixture. Selfed seeds are smaller on average than outcrossed seeds. Seed sizes in fruits from mixed (self + cross) pollinations are more variable than those from cross-pollinations, with a larger number of small, often inviable seeds. Most of these small seeds are presumed to be products of selfing. Selfed seeds do not occur in particular positions along the length of the fruit. After 6-7 wk seedling growth, outcrossed seedlings were taller and heavier than selfed seedlings. Selection for the avoidance of selfing is likely to have been important in the evolution of protandry in Campsis and perhaps in the large "overproduction" of flowers, which permits selective fruit abortion. We discuss the possible roles of late-acting self-incompatibility and inbreeding depression in preventing fruit production following pure self-pollinations. We propose the term "pistillate sorting" to refer to events taking place in or on a pistil that cause the parentage of viable seeds to be other than a random sample from the pollen deposited on the stigma.