Evolutionary Analysis of a Key Floral Trait in Aquilegia Canadensis (Ranunculaceae): Genetic Variation in Herkogamy and Its Effect on the Mating System

Abstract

The mating system of flowering plant populations evolves through selection on genetically based phenotypic variation in floral traits. The physical separation of anthers and stigmas within flowers (herkogamy) is expected to be an important target of selection to limit self-fertilization. We investigated the pattern of phenotypic and genetic variation in herkogamy and its effect of selffertilization in a broad sample of natural populations of Aquilegia canadensis, a species that is highly selfing despite strong inbreeding depression. Within natural populations, plants exhibit substantial phenotypic variation in herkogamy caused primarily by variation in pistil length rather than stamen length. Compared to other floral traits, herkogamy is much more variable and a greater proportion of variation is distributed among rather than within individuals. We tested for a genetic component of this marked phenotypic variation by growing naturally pollinated seed families from five populations in a common greenhouse environment. For three populations, we detected a significant variation in herkogamy among families, and a positive regression between parental herkogamy measured in the field and progeny herkogamy in the greenhouse, suggesting that there is often genetic variation in herkogamy within natural populations. We estimated levels of self-fertilization for groups of flowers that differed in herkogamy and show that, as expected, herkogamy was associated with reduced selfing in 13 of 19 populations. In six of these populations, we performed floral emasculations to show that this decrease in selfing is due to decreased autogamy (within-flower selfing), the mode of selfing that herkogamy should most directly influence. Taken together, these results suggest that increased herkogamy should be selected to reduce the production of low-quality selfed seed. The combination of high selfing and substantial genetic variation for herkogamy in A. canadensis is enigmatic, and reconciling this observation will require a more integrated analysis of how herkogamy influences not only self-fertilization, but also patterns of outcross pollen import and export.KEY WORDS: Aquilegia canadensis, autogamy, heritability, herkogamy, inbreeding depression, outcrossing, self-fertilization.Evolution of the tremendous diversity of mating systems exhibited by flowering plants involves natural selection acting on phenotypic variation in plant traits such as floral morphology, devel-

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

“…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).…”

“…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.…”

“…However, several studies found herkogamy to have a strong genetic basis (e.g., Herlihy and Eckert 2007;Luo and Widmer 2013), and a comprehensive review concluded that the trait is typically highly heritable and evolvable (Opedal et al 2017). Moreover, the profound functional implications of sexual organ position Keller et al 2012Keller et al , 2014Keller et al , 2016) justify speculating about possible adaptive explanations for the existence of high-herkogamous plants.…”

“…To address question iii, we estimated the relative fitness of selfed offspring, w, using an approach analogous to Opedal et al (2016), based on the populationlevel inbreeding coefficient, F, and the selfing rate based on progeny arrays, s m p 1 2 t m (Ritland 1990;Ågren and Schemske 1993;Herlihy and Eckert 2002). Inbreeding coefficients reflect the breeding history of populations over several generations, where sustained high levels of inbreeding increase values of F (Fyfe and Bailey 1951;reviewed by Jarne and David 2008).…”

“…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). Such delayed selfing represents a mechanism whereby selfing can provide reproductive assurance without interfering with outcrossing opportunities in early anthesis (Lloyd 1992;Lloyd and Schoen 1992;Kalisz et al 1999Kalisz et al , 2004Eckert et al 2006) Thus, delayed selfing enables the benefits of selfing via increasing total reproductive output, without incurring the discounting costs associated with using gametes for selfing that could have been otherwise outcrossed (Herlihy and Eckert 2002). Delayed selfing was proposed to always be favored by selection (Lloyd 1992), offering an explanation for mixed mating as a best-ofboth-worlds solution to the problem of unreliable pollinator service (Kalisz et al 2004;Morgan and Wilson 2005).…”