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

Abstract

The diverse features of floral morphology are often thought to be well-designed mechanisms to manipulate plant mating systems. We evaluated the effectiveness of one such mechanism, anther-stigma separation (herkogamy), in controlling variation in the level of outcrossing among plants in a population of Gilia achilleifolia. Variation in outcrossing rates within populations has the potential to influence the coevolution between inbreeding depression and mating system. Using four polymorphic allozymes, we compared the outcrossing-rate estimates of two groups of individuals under natural conditions: one group with low herkogamy and another with high herkogamy. The high herkogamy group had a higher outcrossing rate (0.572) than the low herkogamy group (0.335). This suggests that the withinpopulation variation in outcrossing rate could potentially cause the previously observed association between herkogamy and inbreeding depression (Takebayashi and Delph, 2000). A previous study of floral traits among G. achilleifolia populations failed to detect a relationship between herkogamy and outcrossing rate, demonstrating that the functionality of traits may be obscured in amongpopulation studies as a consequence of uncontrolled environmental variation. Additionally, the effect of herkogamy on outcrossing rate in delayed selfers such as G. achilleifolia may be particularly prominent when pollinator availability is low. Our population-level estimate of outcrossing rate (0.444) was somewhat lower than an estimate from the same population, 15 years prior to our study (0.75), suggesting that pollinator availability may fluctuate among years. Both within-year and among-year variation in the outcrossing rate may have a strong influence on matingsystem evolution.

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.…”

Maintenance of mixed mating after the loss of self-incompatibility in a long-lived perennial herb

“…Kalisz et al, 2012) or spatial separation of the sexes within flowers (Webb and Lloyd, 1986; e.g. Karron et al, 1997;Takebayashi et al, 2006), the stigmas of L. cavanillesii are fully surrounded by, and in contact with, the anthers, and they appear to be receptive as soon as the anthers dehisce. We might thus have expected SC individuals to self-fertilize all their ovules autonomously, precluding opportunities for outcrossing.…”

Phylogenetic Relationship between Degree of Self-compatibility and Floral Traits in Lilium longiflorum Thunb. (Liliaceae)

“…Loss of selfincompatible individuals from a population is often associated with changes to the mating system, particularly selection favoring selfing, and the loss of SI may be accompanied by changes in the floral traits that promote or are associated with increased rates of selfing. For example, an increase in the rate of selfing at species and population level has been associated with lower pollen: ovule ratios (Cruden, 1977;Galloni et al, 2007;Jürgens et al, 2002), reduced investment in floral attractants (Lyons and Antonovics, 1991;Tang and Huang, 2007;Wyatt, 1984), reduced heterogamy (Takebayashi et al, 2006), and close anther-stigma separation (Holtsford and Ellstrand, 1992). In four Hypochaeris species, self-compatible species have smaller flower heads than self-incompatible species (Ortiz et al, 2006), and a high frequency of selfcompatible individuals in Leptosiphon jepsonii populations is associated with shorter corolla tubes and smaller corolla lobes (Goodwillie and Ness, 2005).…”