Interspecific pollen transfer as a mechanism of competition: effect of Castilleja linariaefolia pollen on seed set of Ipomopsis aggregata

Abstract

We investigated whether the deposition of Castilleja linariaefolia (Pursh) V. Grant pollen on flowers of Ipomopsis aggregata (Benth.) in DC. reduced I. aggregata's seed set. Ipomopsis flowers were hand-pollinated with either pure conspecific pollen (the control) or with C. linariaefolia pollen applied prior to, simultaneously with, or following conspecific pollen. Flowers that were simultaneously given C. linariaefolia and I. aggregata pollen received 39% less conspecific pollen than flowers in the control, but did not set fewer seeds per fruit. Flowers that were given C. linariaefolia pollen first received 30% less conspecific pollen and set 38% fewer seeds per fruit than the control. These results suggest that there is a priority effect; deposition of C. linariaefolia pollen on I. aggregata flowers results in competition, but only when C. linariaefolia pollen is deposited first. Unlike pollen of another competitor of I. aggregata (Delphinium nelsonii Greene. Per.), C. linariaefolia pollen caused competition without inducing an increased rate of stigma closure. Only 12% of I. aggregata's stigmas closed when C. linariaefolia pollen was deposited, and seed set of I. aggregata was reduced relative to the control whether their stigmas closed or not. The low rate at which C. linariaefolia pollen induces closure of I. aggregata's stigmas could be an adaptation to reduce the negative effects of competition.Key words: Castilleja linariaefolia, competition, hand pollination, hummingbirds, Ipomopsis aggregata, pollination.

Increased relative abundance of an invasive competitor for pollination, Lythrum salicaria, reduces seed number in Mimulus ringens

“…The reproductive success of native species may also be lowered if pollinators deposit pollen grains of the invasive onto the stigma of the native species, blocking or clogging the stigmatic surface or interfering with the successful germination of conspeciWc pollen (Waser 1978;Waser and Fugate 1986;Galen and Gregory 1989;Caruso and Alfaro 2000;Brown and Mitchell 2001). However, it is also possible that the presence of heterospeciWc grains will have little inXuence on seed number (Campbell and Motten 1985;Murcia and Feinsinger 1996;Jakobsson et al 2008).…”

Relationships between the floral neighborhood and individual pollen limitation in two self-incompatible herbs

“…We nevertheless think that this relationship deserves attention. A negative relationship between heterospeciWc Xower density and reproduction could be the result of at least two processes: a decrease in the rate of visitation of Armeria Xowers if heterospeciWc Xowers are more attractive (e.g., Chittka and Schürkens 2001;Brown et al 2002;Totland et al 2006) and/or an increase in the transfer of heterospeciWc pollen if pollinators visit diVerent species during foraging bouts (e.g., Galen and Gregory 1989;Caruso and Alfaro 2000;Jakobsson et al 2007). The latter process could negatively aVect reproduction through stigma clogging or chemical interference (e.g., Galen and Gregory 1989;Murphy 2000).…”

“…A special case of facilitation is the "magnet-species eVect," where nonrewarding plant species pollinated by deceit receive more visits when they grow close to attractive plant species than when they grow alone (Laverty 1992;Johnson et al 2003;Juillet et al 2007). A high density of heterospeciWc Xowers can aVect pollination negatively through improper pollen transfer, which occurs when pollinators switch between species (Galen and Gregory 1989;Caruso and Alfaro 2000;Jakobsson et al 2007). HeterospeciWc pollen can cause decreased seed production through chemical interference (Sukhada and Jayachandra 1980;Murphy 2000) or stigma surface blocking (Rathcke 1983;Galen and Gregory 1989).…”

The Influence of Garden Size and Floral Cover on Pollen Deposition in Urban Community Gardens

“…Heterospecific pollen deposition can reduce fruit or seed production of the flowering plant being pollinated, especially if the heterospecific pollen arrives before conspecific pollen (i.e. pollen from a different flower or a different plant of the same species) arrives (Galen and Gregory 1989;Caruso and Alfaro 2000). Though heterospecific pollen may not limit the amount of conspecific pollen deposited, it competes for limited stigmatic space capable of hydrating conspecific grains.…”

“…Studies on heterospecific pollen transfer usually focus on the effect of the pollen load or pollen type on seed production and fruit size (Galen and Gregory 1989;Caruso and Alfaro 2000). These studies often include carefully controlled applications of different ratios of conspecific pollen, in which the amount of heterospecific pollen is known.…”