Extremely Long-Lived Stigmas Allow Extended Cross-Pollination Opportunities in a High Andean Plant

High-elevation ecosystems are traditionally viewed as environments in which predominantly autogamous breeding systems should be selected because of the limited pollinator availability. Chaetanthera renifolia (Asteraceae) is an endemic monocarpic triennial herb restricted to a narrow altitudinal range within the high Andes of central Chile (3300–3500 m a.s.l.), just below the vegetation limit. This species displays one of the larger capitulum within the genus. Under the reproductive assurance hypothesis, and considering its short longevity (monocarpic triennial), an autogamous breeding system and low levels of pollen limitation would be predicted for C. renifolia. In contrast, considering its large floral size, a xenogamous breeding system, and significant levels of pollen limitation could be expected. In addition, the increased pollination probability hypothesis predicts prolonged stigma longevity for high alpine plants. We tested these alternative predictions by performing experimental crossings in the field to establish the breeding system and to measure the magnitude of pollen limitation in two populations of C. renifolia. In addition, we measured the stigma longevity in unpollinated and open pollinated capitula, and pollinator visitation rates in the field. We found low levels of self-compatibility and significant levels of pollen limitation in C. renifolia. Pollinator visitation rates were moderate (0.047–0.079 visits per capitulum per 30 min). Although pollinator visitation rate significantly differed between populations, they were not translated into differences in achene output. Finally, C. renifolia stigma longevity of unpollinated plants was extremely long and significantly higher than that of open pollinated plants (26.3±2.8 days vs. 10.1±2.2, respectively), which gives support to the increased pollination probability hypothesis for high-elevation flowering plants. Our results add to a growing number of studies that show that xenogamous breeding systems and mechanisms to increase pollination opportunities can be selected in high-elevation ecosystems

Effects of pollination treatment on the reproduction of two populations of <i>Chaetanthera renifolia</i>.

<p>ANCOVA summary of effects of pollination treatment (autonomous self-pollination vs. hand cross-pollination) and site (La Parva and Piedra Numerada) on the percentage of achene set and achene weight of <i>Chaetanthera renifolia</i> plants. Plant size (diameter) was entered as a covariate. Significant effects are indicated in bold.</p

Effects of pollination treatment and site on stigmas longevity of <i>Chaetanthera renifolia</i>.

<p>Two-way ANOVA of the effects of treatment (pollinator-excluded vs. open-pollinated) and site (La Parva and Piedra Numerada) on stigma longevity (# days) of <i>Chaetanthera renifolia</i> capitula. Significant effects are indicated in bold.</p

Effect of pollination (unpollinated vs. open-pollinated) on stigma longevity of <i>Chaetanthera renifolia</i>.

<p>Mean stigma longevity of open-pollinated (white bars, <i>n</i> = 18) and pollinator–excluded plants (grey bars, <i>n</i> = 18) at La Parva (A) and Piedra Numerada (B) populations of <i>Chaetanthera renifolia</i>. Bars are means ± SE. *** Indicates significant differences (<i>P</i><0.01).</p

List and frequencies of insect pollinator taxa observed on <i>Chaetanthera renifolia</i> capitula.

<p>Number of individuals of each taxon observed (<i>N</i>), and percentage (%) of the total number of visits that each pollinator taxon made to <i>Chaetanthera renifolia</i> capitula from La Parva and Piedra Numerada.</p

Capitulum and disc size (mean ± SD) of <i>Chaetanthera renifolia</i> and other coexisting congeners at the same Andean basin in central Chile.

<p>The slope aspect, elevation (m asl) and number of sampled capitula are indicated for each population.</p><p>*Indicates significant differences (<i>P</i><0.001, <i>t</i>-test) for comparisons of <i>C. renifolia</i> floral traits vs. each of the species.</p

Effect of supplemental hand-pollination on the reproduction of two populations of <i>Chaetanthera renifolia.</i>

<p>ANCOVA summary of the effects of pollination treatment (open-pollinated vs. supplemental hand pollination) and site (La Parva and Piedra Numerada) on the percentage of achene set and achene weight of <i>Chaetanthera renifolia</i> plants. Plant size (diameter) was entered as a covariate. Significant effects are indicated in bold.</p

Effects of pollination treatments (control vs. supplemental hand-pollination) on reproduction of <i>Chaetanthera renifolia</i>.

<p>Mean percentage of achene set (A and B) and achene weight (mg) (C and D) at La Parva and Piedra Numerada. Open bars: control (<i>n</i> = 30); Grey bars: hand supplementary pollinated (<i>n</i> = 30). Bars are means ± SE. *** Indicates significant differences (<i>P</i><0.001).</p

Effects of pollination treatments (spontaneous self-pollination vs. hand cross-pollination) on reproduction of <i>Chaetanthera renifolia</i>.

<p>Mean percentage of achene set and achene weight at La Parva (A and C) and Piedra Numerada (C and D). Open bars: autogamous self-pollinated (<i>n</i> = 18); Grey bars: hand cross-pollination (<i>n</i> = 18). Bars are means ± SE. *** Indicates significant differences (<i>P</i><0.001).</p