Selection on spur shape in Impatiens capensis

Abstract

Rapid speciation within some plant families has been attributed to the evolution of floral spurs and to the effect of spur length on plant reproductive success. The flowers of Impatiens capensis (jewelweed) possess a long, curved spur in which nectar is produced and stored. Spur length and curvature varies among plants within one population. Here I document that spur shape is variable in natural populations, variation within plants is less than variation among plants, and spur shape is correlated with components of female and male reproductive success. The apparent natural selection is weakly directional in 1 of 2 years, with greatest seed production and pollen removal occurring in flowers with the greatest spur curvature. Bee pollinator visit length is longest at flowers with highly curved spurs, and they leave less nectar in these spurs than in flowers with straighter spurs. Spur angle evolution may be limited, at least in part, by opposing selection by nectar-robbers who prefer to visit flowers with greater spur curvature. Other factors that might contribute to the maintenance of spur angle variation are temporal variation in the strength of selection and potential genetic correlations of spur shape with other traits under selection.

Abiotic conditions affect floral antagonists and mutualists of Impatiens capensis (Balsaminaceae)

“…2-24 h after opening) and the other half in the female stage (ca. 36-48 h after opening) because I. capensis has different volumes of nectar in male-phase and female-phase fl owers ( Rust, 1979 ;Young, 2008 ). Nectar volume was measured using microcapillary tubes, fi rst through the corolla opening and then from the spur by snipping the end and squeezing out the nectar.…”

“…Agromyza borealis (Agromyzidae), a specialist leaf miner fl y, attacks only I. capensis and I. pallida ( Frost, 1924 ;Eastman, 1995 ). The fl owers are robbed by several insect species, including Bombus pollinators and Vespula maculifrons (Vespidae), and visits by small nectar thieves are common ( Rust, 1979 ;Eastman, 1995 ;Young, 2008 ). We follow previous work in defi ning nectar robbers as insects that pierce the fl ower or spur to take nectar, and nectar thieves as insects that are too small to contact reproductive parts.…”

Pollinators and nectar robbers cause directional selection for large spur circle in Impatiens oxyanthera (Balsaminaceae)

“…Studies on ecological and evolutionary consequences of species interactions have been mainly focusing on single pairwise effects, such as pollinator-mediated (Campbell et al 1996;Alexandersson and Johnson 2002;Sandring and Å gren 2009;Cuartas-Domínguez and Medel 2010; or nectar robber-mediated (Lara and Ornelas 2001;Urcelay et al 2006;Castro et al 2008;Young 2008;Navarro and Medel 2009) selection on floral traits. However, single pairwise interaction seldomly exists in isolation and can be interfered by other community members (Gómez 2003;Cariveau et al 2004;Lavergne et al 2005;Brody et al 2008;Sánchez-Lafuente 2007;Bartkowska and Johnston 2012).…”

“…Nectar spurs are found in many angiosperms groups (Hodges 1997) and have been known to affect plant fitness because they regulate the behavior of floral visitors (Travers et al 2003;Young 2008;. The evolution of nectar spurs represents a key innovation that has facilitated speciation (Hodges and Arnold 1995;Hodges 1997;Whittall and Hodges 2007).…”

“…Spur shape (spur length and curvature) usually experiences selection from pollinators and nectar robbers. On one hand, long and curly spurs increase the contact between flowers and pollinators and therefore enhance reproductive success (Travers et al 2003;Whittall and Hodges 2007;Young 2008;Brunet 2009;Boberg and Å gren 2009;Ellis and Johnson 2010;Sletvold and Å gren 2011). Yet, these traits incur nectar robber visitation which counteract selection on long and curved spurs by pollinators (Pan 1999;Temeles and Pan 2002;Young 2008;Navarro and Medel 2009).…”