Summary
The ability to produce seeds when pollinators or potential mates are scarce is thought to be one of the main advantages of self-fertilization in flowering plants. However, whether autonomous selfing increases seed set in natural populations has seldom been tested, and even fewer studies have evaluated the advantage of selfing across a gradient of pollen availability. This study examines the fertility consequences of autonomous selfing in Aquilegia canadensis (Ranunculaceae), a shortlived, spring-flowering perennial typically found in small, patchy populations on rock outcrops. We used a pollinator exclusion experiment to confirm reports that A. canadensis has a well-developed capacity for autonomous selfing resulting from incomplete protogyny and close proximity of stigmas and anthers during dehiscence. Flowers excluded from pollinators set 87% as many seeds per carpel (X̄ ± 1 SE = 7.1 ± 1.4 seeds) as hand-pollinated flowers (8.1 ± 1.3 seeds), and seed production in unpollinated flowers correlated negatively with the distance between stigmas and anthers (r = -0.46). Autonomous selfing could be potentially valuable in providing reproductive assurance because only 2.7 ± 0.5 pollen grains were deposited on each stigma before anther dehiscence, compared to 134.1 ± 17.9 pollen grains by the end of anther dehiscence. However, prevention of autonomous selfing by anther removal before dehiscence did not decrease seed set, even for plants at low plant densities where outcross pollen may have been in short supply. Emasculated flowers set as many seeds per carpel (9.3 ± 0.9) as intact flowers (8.4 ± 1.1), indicating that sufficient cross pollen is deposited for full seed set. These results do not support the hypothesis that autonomous selfing by A. canadensis has been selected because it provides reproductive assurance.
Methodology
This study was conducted during May and June 1996 in two areas located near the Queen’s University Biological Station in Leeds and Grenville County, eastern Ontario, Canada. One site (‘‘QUBS’’) is a 100-ha peninsula on Lake Opinicon, 2 km west of Chaffey’s Lock (44°35'N latitide, 76°19'W longitude). Aquilegia canadensis is patchily distributed on the many small granite outcrops scattered throughout this area, and plants occur in a wide range of densities. The second site is located along the abandoned Lindsay Lake road (‘‘LLR’’) 8 km southwest of QUBS (44°33'N, 76°22'W). Here, we used plants growing on a large 1-ha outcrop. In both populations, bumble bees and small solitary bees were the most common flower visitors, although visitors were only observed in the denser patches.
Capacity for autonomous self-fertilization—We used pollinator exclusion combined with hand-pollinations to quantify the capacity of A. canadensis for autonomous self-fertilization. Twenty-nine plants with at least three unopened buds were haphazardly selected (12 from QUBS and 17 from LLR) and excluded from pollinators with cylinders of coarse wire mesh covered in fine bridal veil. Each of the three buds on each plant were randomly assigned one of three pollination treatments: (1) all stigmas were hand self-pollinated at the onset of anther dehiscence; (2) stigmas were left unpollinated; or (3) stigmas were left unpollinated and all anthers were removed from the flower before they dehisced. Just after the last anther dehisced, we measured several traits for each flower receiving treatments 1 and 2, including the maximum length of a randomly chosen spur, the length of a randomly chosen sepal, style length measured as the distance from the stigma on the longest style to the point at which the nectar spurs fuse (about the basal end of the style), and the distance between the anther on the longest stamen and the stigma on the shortest style (herkogamy). Fruits were collected just before the mature follicles dehisced, and the full seeds in each were counted. The capacity for autonomous self-fertilization was assessed with a paired t test comparing seed production in flowers receiving treatments 1 and 2.
Timing of pollen deposition on stigmas—The potential value of autonomous self-fertilization in increasing seed set was assessed by comparing the number of pollen grains on stigmas just before anther dehiscence with pollen receipt at the end of anther dehiscence. One recently opened flower was tagged on each of 46 haphazardly selected plants at QUBS. We removed two stigmas from each flower just before anther dehiscence and another two just after the last anther had dehisced. Floral measurements were performed as above. Pollen grains were counted at 100X on stigmas crushed under glass coverslips in fuschin jelly (Dafni, 1993, p. 72).