No abstract

The prevalence of nonrandom fertilization due to postpollination events has rarely been studied in natural populations, despite important implications for outcrossing rates, mate choice, and plant fitness. Nonrandom paternity within fruits can be caused by both unequal fertilization and unequal embryo abortion. Using self‐compatible Hibiscus moscheutos, we studied the potential for nonrandom fertilization by comparing growth rates of pollen‐tubes from different donors. The branched style of Hibiscus allowed within‐flower comparisons between pollen donors. Relative pollen‐tube growth rates were determined by applying pollen from pairs of donors to different stigmas on adjacent stylar branches. We then measured the number of callose plugs per tube in cross‐sectional transects across the style after 3 hr. We demonstrate that rates of callose plug formation can be used as a sensitive indicator of relative pollen‐tube growth rate. Differences between pollen donors were common and repeatable. Self‐pollen‐tubes grew slower than outcross pollen‐tubes in some crosses and faster in others. Allozyme variation in glucose phosphate isomerase was used to show that individuals with fast‐growing pollen‐tubes sired a disproportionate number of seeds following mixed pollinations (up to 72%). Since seed abortion was negligible, we conclude that variation in pollen‐tube growth rates leads to nonrandom paternity within fruits.

The potential influence of pollen‐tube competition on offspring “quality” has received considerable attention in recent years. Yet the prevalence of pollen competition in natural populations is largely unknown because few investigators have actually measured rates of pollen deposition on stigmas. In this study, we assess the potential for pollen‐tube competition in natural populations of the self‐compatible, pollinator‐dependent herbaceous perennial, Hibiscus moscheutos. Individual flowers averaged two to four visits per 15 min by potential pollinators (Ptilothrix and Bombus), and about 34% of these visits involved contact with a stigma. The median number of pollen grains deposited on virgin stigmas per contact visit was 70 grains (values ranged from 0 to 889), and flowers averaged about four contact visits per hour. Approximately 360 pollen grains must reach stigmas for full seed set to occur in a typical flower (an average ovary has 139 ovules, and 2.6 pollen grains are required per seed). Within 2 and 3 hr exposure to pollinators, 65% and 97% of the flowers received excess pollen (>360 grains) and median stigmatic pollen loads exceeded the number of ovules by a factor of 4.0 and 5.4, respectively. Based on 3 yr observations, it is concluded that pollen competition may frequently occur in this species.