Effects of floral morphology and display on outcrossing in Blue Columbine, Aquilegia caerulea (Ranunculaceae)

Abstract

The hermaphrodite sexual system possessed by most flowering plants creates the potential for close inbreeding through self-pollination. While many species strongly limit self-fertilization with self-incompatibility, about one-third of hermaphrodite taxa are selfcompatible (Richards 1986) and the balance between outcrossing and self-fertilization will depend, in part, on the relative deposition of self-and cross-pollen on stigmas (Holsinger 1992). This, in turn, can be influenced by both intrinsic and extrinsic factors, especially features of floral morphology and display.Floral features, such as the spatial and temporal separation of male and female functions within flowers (herkogamy and dichogamy, respectively), are generally expected to promote outcrossing, either through reducing within-flower selfing (autogamy, e.g. Darwin 1876;Müller 1883;Proctor & Yeo 1973;Faegri & van der Pijl 1979;Richards 1986) or betweenflower selfing (geitonogamy; Holsinger 1996). Aspects of floral display are also expected to influence the level of outcrossing. Both flower size and display size may enhance opportunities for outcrossing by increasing pollinator visitation Summary 1. The effect of variation in floral morphology and display on the level of outcrossing was investigated in a Rocky Mountain population of the self-compatible, protandrous, perennial herb, Aquilegia caerulea. ANOVA revealed substantial variation among 36 plants in floral traits including herkogamy (CV = 35%), protandry (23%), pollen production (12%), flower size (8%), the daily number of flowers in male phase (58%) and autofertility, seed production in the absence of pollinators (192%). 2. Whether this floral variation affected the outcrossing rates of individual plants was assessed using both a categorical analysis, in which outcrossing rates were estimated separately for groups of plants that differed with respect to floral features, and a correlation analysis involving outcrossing rates estimated for individual plants from large progeny arrays (n ≥ 40). Both analyses indicated significant positive correlations between the outcrossing rate and herkogamy (r = + 0·71) and protandry (+ 0·54), negative correlations with the number of flowers in male phase during each day of female phase (-0·42) and autofertility (-0·36), but no correlation with flower size or pollen production. 3. Partial correlation indicated that both herkogamy and dichogamy, although weakly intercorrelated, were positively correlated with the outcrossing rate and that these effects were only partly mediated by their negative correlation with autofertility. 4. The extent to which floral features and the outcrossing rate covaried across the flowering sequence within plants was also examined in this study. As expected for a plant with strong protandry and sequential blooming, the number of flowers in male phase decreased strongly with flowering sequence position. However, categorical analysis did not support the prediction that the first flowers to open on an inflorescence outcrossed at a low...

Effect of variation in herkogamy on outcrossing within a population of Gilia achilleifolia

“…The results support the role of anther-stigma separation in decreasing individual selfing rate, as reported in other diverse taxa such as Lycopersicon pimpinellifolium (Rick et al, 1978), Datura stramonium (Motten and Stone, 2000), and Aquilegia caerulea (Brunet and Eckert, 1998).…”

“…Recently, researchers have begun to investigate the effects of floral variation on individual outcrossing rates within a population (Barrett et al, 1994;Brunet and Eckert, 1998;Vrieling et al, 1999;Motten and Stone, 2000). Current procedures for estimating population-level outcrossing rates are quite robust if multiple loci are included in the study (Ritland and Jain, 1981).…”

“…One way to avoid the problem is to use an artificial plot in which the genotypic frequencies are controlled (eg Motten and Stone, 2000). Alternatively, one can compare the outcrossing-rate estimates of two groups differing in some focal trait (Brunet and Eckert, 1998). We adopted the latter approach for our study.…”

Mixed Mating in Homostylous Species: Genetic and Experimental Evidence from an Alpine Plant with Variable Herkogamy,Primula halleri

“…The lack of a significant difference in the mean outcrossing rates of the two herkogamy classes is surprising for two reasons. First, most other studies that considered herkogamy variation among individuals found significantly diminished outcrossing in low-herkogamous individuals (e.g., Karron et al 1997;Brunet and Eckert 1998;Takebayashi et al 2006;Herlihy and Eckert 2007). Second, the absolute amount of herkogamy difference between classes was shown to greatly affect seed set under pollinator exclusion , demonstrating that the autonomous component of selfing is affected by herkogamy.…”

“…In self-compatible species, herkogamy has been mainly proposed as a mechanism to limit selfing, but it also influences sexual interference between male and female floral functions (Barrett 2002). Herkogamy negatively affects seed set when pollinators are absent (e.g., Luijten et al 1999;Moeller 2006;de Vos et al 2012), and several genetic analyses confirmed the expected positive correlation between herkogamy and outcrossing rates, both among (e.g., Rick et al 1977;Holtsford and Ellstrand 1992;Luo and Widmer 2013) and within populations (e.g., Karron et al 1997;Brunet and Eckert 1998;Takebayashi et al 2006;Herlihy and Eckert 2007;Weber et al 2012; but see Medrano et al 2005Medrano et al , 2012. Importantly, herkogamy is known to decrease during anthesis in some species, potentially increasing opportunity for autonomous selfing in later stages of anthesis (Luijten et al 1999;Armbruster et al 2002;Duan et al 2010;de Vos et al 2012).…”