The mechanism of delayed selfing in Collinsia verna (Scrophulariaceae)

Abstract

Collinsia verna, blue-eyed Mary, has floral attributes of an outcrossing species, yet most flowers readily self-pollinate under greenhouse conditions. Here we describe the mechanism of self-pollination in C. verna via changes in relative positions of the stigma and anthers and late timing of receptivity, resulting in delayed selfing. Each flower contains four anthers that dehisce sequentially over ∼1 wk. Pollen that is not collected by pollinators accumulates in the keel petal and retains high viability (>80% pollen germination) up to the time of corolla abscission. The stigmatic surface does not become receptive until after the third anther dehisces. This overlap in the sexual phases is concurrent with a change in herkogamy during floral development. In most flowers (70%), the stigma has moved to the front of the keel and is positioned near the anthers when the third anther dehisces. Under field conditions, fruiting success of plants within pollinator exclosures was ∼75% of the fruiting success in open-pollinated plants (33% fruiting success via autogamy vs. 44% fruiting success, respectively). Collinsia verna plants in pollinator exclosures exhibit variation in autogamy rates within natural populations (range 0-80%). In addition, only half of naturally pollinated, receptive flowers examined had pollen tubes growing in their styles. In contrast, shortly after corolla abscission, nearly all flowers examined (96%) had pollen tubes in their styles. Thus we find that in C. verna, autogamy occurs late in floral development, which has the potential to provide substantial reproductive assurance, and that individuals vary in their ability to set fruit through this mechanism. We suggest that delayed selfing mechanisms may be overlooked in other species and that variable pollinator availability may play a significant role in the maintenance of mixed mating in species with delayed selfing, such as C. verna.

Variation in the timing of autonomous selfing among populations that differ in flower size, time to reproductive maturity, and climate

“…Flowers were checked every 8 h, beginning when the fl ower was in bud and ending when the corolla was shed. We noted the fl oral stage at each time period as in Kalisz et al (1999) with the stage (0 -4) representing the number of anthers that had dehisced. We noted when the corolla opened, when it was shed, and the position of the stigma relative to the anthers.…”

Population at the edge: increased divergence but not inbreeding towards northern range limit in Acer campestre

“…Additionally, rates of dispersal determine mating patterns and spatial genetic structure of a population. Limited dispersal accompanied by low density of pollen donors can lead to pollen limitation syndrome, which often drives inbreeding through increased self-fertilisation (in self-compatible species) or mating between relatives (Kalisz et al 1999;Rajora et al 2002;Johnson et al 2009). In consequence, a decreased heterozygosity (due to inbreeding) facilitates expression of deleterious genes, having negative impact on the fitness and, consequently, increasing extinction risk (Newman and Pilson 1997;Herlihy and Eckert 2002;Reed and Frankham 2003;Vilas et al 2006;Gargano et al 2009).…”

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

“…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).…”