Abstract
Delayed autonomous selfing offers a mechanism for seed production when pollination levels are low or unpredictable. At Mpala Research Centre (MRC) in Kenya, we examined the relationships between floral attraction, insect visitation, and delayed autonomous selfing through backwards stylar curvature in the co-flowering Hibiscus aponeurus and H. flavifolius. Despite producing similar pollen and nectar rewards, visitation rates and the composition of floral visitor guilds varied significantly between these species. Across four years of observations, floral visitation in H. flavifolius was dominated by bees, and in H. aponeurus by a mixture of bees, butterflies and beetles. Visitation rates to H. flavifolius flowers (range 0.17 - 2.1 visits flr-1hr-1) were two times greater than to H. aponeurus flowers (range 0 - 2.7 visits flr-1hr-1), which resulted in significantly higher pollen deposition and removal rates in H. flavifolius than in H. aponeurus. Field crosses demonstrated little pollen limitation in either species. In open-pollinated flowers, H. aponeurus displayed significantly greater stylar curvature and apparent self-pollination than did H. flavifolius. Floral attributes in H. aponeurus, such as a smaller corolla size and a downwards orientation of the stylar column, also suggest that delayed selfing is a more important mechanism of reproductive assurance in this species than in H. flavifolius. Determining whether these differences in insect visitation and stylar curvature are characteristic for these species or are unique to MRC will require comparison with populations located in other parts of the ranges, genetic tests of selfing rates, and chemical analyses of nectar, pollen, and floral volatiles.
Highlights
Delayed autonomous selfing occurs in several genera of the Malvaceae (Ruan et al 2010, 2011), but detailed exploration of its ecological importance has been limited to Hibiscus laevis (Klips & Snow 1997), Hibiscus trionum (Ramsey et al 2003; Seed et al 2006), and Kosteletzkya virginica (Ruan et al 2005, 2008a, 2008b, 2009)
Mean corolla diameter was smaller in H. aponeurus (5.6 ± 0.12 cm) than in H. flavifolius (6.2 ± 0.06 cm; t33 = 4.9, P < 0.0001), but the length of the stylar column was longer in H. aponeurus (2.9 ± 0.06 cm) than in H. flavifolius (2.1 ± 0.04 cm; t40 = 10.4, P < 0.0001)
Stylar columns were much more downwardly oriented with respect to the plane of the corolla in H. aponeurus (142.2 ± 3.1 degrees) than in H. flavifolius (11.7 ± 2.3 degrees; t55 = 8.1, P < 0.0001), giving H. aponeurus flowers a mildly zygomorphic shape when compared to H. flavifolius flowers (Fig. 1A)
Summary
Human activities that destroy natural habitats and fragment populations are reducing the size of pollinator populations and potentially decreasing the reproductive success of many plant species (Aguilar et al 2006; Eckert et al 2010; Thomann et al 2013; Vanbergen 2013; Somme et ting systems, habitat fragmentation can increase reliance on self-pollination (e.g. Brys & Jacquemyn 2012) and over time potentially can result in inbreeding depression, reduced genetic variability, and increased risk of local extinction due to the loss of adaptive potential (Stebbins 1957; Goodwillie et al 2005; Eckert et al 2010). Delayed autonomous selfing occurs in several genera of the Malvaceae (Ruan et al 2010, 2011), but detailed exploration of its ecological importance has been limited to Hibiscus laevis (Klips & Snow 1997), Hibiscus trionum (Ramsey et al 2003; Seed et al 2006), and Kosteletzkya virginica (Ruan et al 2005, 2008a, 2008b, 2009). Most Malvaceae capable of delayed selfing have flowers with styles that are surrounded by and extend beyond monadelphus stamens In these species, the styles curve out and backwards as flowers age until the stigmas contact the pollen located in the upper anthers (see images in Ruan et al 2010; Kumar et al 2014). Studies in Hibiscus (e.g. Klips & Snow 1997) suggest that this backward bending can be stopped by prior pollination, the number of pollen grains required to halt curvature and the physiological mechanism behind that interaction are not well understood (Buttrose et al 1977; Klips & Snow 1997; Seed et al 2006; Ruan et al 2008a)
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