- Queen‘s University
- University of Guelph
Summary
In self-compatible plants, small populations may experience reduced outcrossing owing to decreased pollinator visitation and mate availability. We examined the relation between outcrossing and population size in eastern Ontario populations of Aquilegia canadensis. Experimental pollinations showed that the species is highly self-compatible, and can achieve full seed-set in the absence of pollinators via automatic self-pollination. We estimated levels of outcrossing (t) and parental inbreeding coefficients (F) from allozyme variation in naturally pollinated seed families for 10 populations ranging in size from 32 to 750 reproductive individuals. The proportion of seeds produced through outcrossing was generally low (mean = 0.29 ± 0.02 SE) and varied widely among populations (range = 0.00–0.83). Accordingly, estimates of F were large (mean = 0.26 ± 0.05) and significantly greater than zero in seven populations. As expected, four small populations (N < 40) outcrossed less (0.17 ± 0.03) than six large populations (N > 90; 0.38 ± 0.03). However, parental plants were not significantly more inbred in small than large populations (P= 0.18). There was no difference in the germination of seeds from hand self- and cross-pollinations. However, population genetic estimates of inbreeding depression for survival expressed from seed to reproductive maturity were very high (mean δ = 1 − relative fitness of selfed seed = 0.88 ± 0.14). The combination of self-compatibility and automatic self-pollination makes the mating system of A. canadensis sensitive to variation in ecological factors that affect the likelihood of cross-pollination.
Methodology
Study populations
We studied 10 populations ranging in size from 32 to 750 plants located in eastern Ontario, Canada (Table 1). Eight were from the Admiralty Islands in the St Lawrence River near Gananoque. Another (CLR) was located on the boundary of Charleston Lake Provincial Park, about 3 km north-east of the town of Outlet. The remaining population (WMZ) was located at the Queen’s University Biological Station, about 2 km west of Chaffey’s Lock. Populations were usually spatially discrete and separated from nearby populations by at least 100 m, usually much more. All populations occurred on exposed rock outcrops with little tree or shrub cover.
The size of each population (N) was estimated as the number of reproductive individuals at peak flowering (Table 1). Seven were surveyed in 1995, 1996 and 1997. Population WMZ was only surveyed in 1996. To test for an association between N and mating system parameters, populations were classified as either large (N > 90, n = 6) or small (N < 40, n = 4) on the basis of their size in 1995 (1996 for WMZ). The large-size class represents the upper quartile of the distribution of population sizes in eastern Ontario. The small-size class represents the smallest size of population for which one can obtain enough seed families to estimate the mating system accurately. Although there was some fluctuation in the size of each population between years, relative size was strongly correlated between years (Kendall’s coefficient of concordance: W = 0.921, P= 0.011). The populations that were classified as small in 1995 and subsequently resurveyed stayed small in the following years, except ANC which more than doubled in size from 1995 to 1997 (Table 1).
Self-compatibility and autofertility
Self-compatibility and the capacity for automatic self-fertilization were assessed in the largest population (CLR) in 1995. We randomly chose 39 plants, each with at least three unopened flowers, and excluded them from pollinators with a wire mesh cylinder covered with fine bridal veil. For each plant, one bud was randomly assigned to each of three treatments: self-pollinated, cross-pollinated or unpollinated. Because some buds did not mature into flowers, some plants did not receive all three treatments. Flowers were self-pollinated within 12 h of when the first anther started shedding pollen (i.e. at the onset of stigma receptivity) by brushing all stigmas with fresh pollen from three anthers taken from the same flower or a different flower on the same inflorescence. Flowers were cross-pollinated in the same fashion less than 12 h before the first anther started shedding pollen using one anther from each of three different pollen donors growing about 2 m away. To avoid contaminating the stigmas of cross-pollinated flowers with self-pollen, all anthers were removed before they started to shed pollen. The removal of anthers from flowers at this stage does not reduce seed production (Eckert & Schaefer, 1998). To control for any potential negative effects of anther removal on seed weight and/or viability, we also removed all anthers from flowers immediately after self-pollination. Flowers were left unpollinated to assess their capacity for automatic self-fertilization. Results from a subsequent experiment showed that A. canadensis is not apomictic, and that pollinators do not get into the exclusion cages (Eckert & Schaefer, 1998). To determine whether the proximity of anthers to stigmas influences the capacity for automatic self-fertilization, the distance between the stigma of the shortest style and the tip of the anther on the longest stamen on unpollinated flowers was measured to 0.1 mm using calipers just after the last anther had started shedding pollen. The filaments of all anthers are still fully turgid at this point.