Mating system and reproductive biology in Campanulastrum americanum

Kilkenny, F. F., and L. F. Galloway. 2008. Reproductive success in varying light environments: direct and indirect effects of light on plants and pollinators. Oecologia 155: 247-255. (pdf reprint)
Abstract.– Plant populations often exist in spatially heterogeneous environments. Light level can directly affect plant reproductive success through resource availability or by altering pollinator behavior. It can also indirectly influence reproductive success by determining floral display size which may in turn influence pollinator attraction. We evaluated direct and indirect effects of light availability and measured phenotypic selection on phenological traits that may enhance pollen receipt in the insect-pollinated herb Campanulastrum americanum . In a natural population, plants in the sun had larger displays and received seven times more visits than plants in the shade. Using experimental arrays to separate the direct effects of irradiance on insects from their response to display size, we found more visits to plants in the sun than the shade, but no association between number of visits each flower received and display size. Plants in the sun were not pollen limited but pollen-augmented shade flowers produced 50% more seeds than open-pollinated flowers. Phenological traits, which may influence pollen receipt, were not under direct selection in the sun. However, earlier initiation and a longer duration of flowering were favored in the shade, which may enhance visitation in this pollen-limited habitat.

Galloway, L. F., and J. R. Etterson. 2007. Inbreeding depression in an autotetraploid herb: a three cohort field study. New Phytologist 173: 383-392. (pdf reprint)
Summary.
•  Autotetraploids are predicted to have reduced inbreeding depression relative to diploids. However, recent theory and information on genomic changes following autopolyploidy suggest that inbreeding depression may be closer to diploids.
•  In three consecutive years, self and outcross pollinations were conducted on autotetraploid Campanulastrum americanum , seeds were planted into native sites, and biennial offspring were followed through seed production.
•  Inbred individuals had lower germination rates, reduced survival, were smaller, and flowered later, producing fewer fruits with fewer seeds. Inbred offspring had 6% of the cumulative fitness of outcross offspring. Although performance varied substantially among cohorts, inbreeding depression for cumulative fitness was relatively constant with d ranging only from 0.92-0.95.
•  C. americanum, like many outcrossing species, expressed very high levels of inbreeding depression. This supports the hypothesis that inbreeding depression of some autotetraploids may be similar to diploids. Furthermore, few studies have measured temporal variation in inbreeding depression. Constant inbreeding depression given a six-fold range in cohort performance suggests that inbreeding depression may be relatively robust to environmental variation experienced by natural populations.

Kruszewski, L., and L. F. Galloway. 2006. Explaining outcrossing rate in Campanula americana (Campanulaceae): geitonogamy and cryptic self-incompatibility. International Journal of Plant Sciences 167: 455-461. (pdf reprint)
Abstract.– A flexible mating system may enable self-compatible plants to prevent self-fertilization under certain circumstances. Campanulastrum americanum is a self-compatible, protandrous herb. Although within-plant pollen transfer is likely and self pollen can produce a full set of seeds, selfing in natural populations is rare. We investigated explanations for the high outcrossing rate. One possibility was that self pollen contacts the stigma rarely. We surveyed floral displays in nature to gauge the potential for within-plant pollen transfer. Floral displays in nature are large enough to enable frequent geitonogamous pollinator visits and subsequent self-pollination. Alternatively, the high outcrossing rate could result from cryptic self-incompatibility, a mechanism that favors outcross pollen over self pollen. To determine if outcross pollen has a seed-siring advantage over self pollen, we pollinated maternal plants with equal mixtures of self and outcross pollen. Genotyping the offspring revealed that outcross pollen sired significantly more seeds than self pollen. We explored whether differential growth rates of self and outcross pollen tubes produce cryptic self-incompatibility. Growth rates did not differ, indicating that cryptic self-incompatibility occurs by some other mechanism. Cryptic self-incompatibility enables C. americanum to reduce inbreeding when outcross pollen is present, yet ensure reproduction when only self pollen is available.

Lau, J., and L. F. Galloway. 2004. Effects of low-efficiency pollinators on plant fitness and floral trait evolution in Campanula americana (Campanulaceae). Oecologia 141: 577-583. (pdf reprint)
Abstract. – Floral visitors vary in their pollination efficiency and their preferences for floral traits. If low-efficiency pollinators decrease the amount of pollen available to higher efficiency visitors, then low-efficiency visitors may actually have negative fitness consequences for the plants that they visit. We used experimental arrays in two populations to determine the floral preferences and the fitness effects of low-efficiency (or "ugly") pollinators on Campanula americana . These ugly pollinators (halictid bees) preferentially visited flowers with pollen over flowers that had had their pollen removed. C. americana pollen color varies quantitatively from light tan to dark purple, and we found that natural variation in pollen color influenced the magnitude of halictid preferences for flowers with pollen. In general, preferences for flowers with pollen were stronger when the ugly pollinators foraged in arrays of flowers with tan-colored pollen than in arrays with purple-colored pollen. When plants received few visits by efficient Bombus pollinators, visits by ugly pollinators significantly decreased siring success relative to plants where visits by ugly pollinators were prevented. In contrast, ugly pollinators did not influence siring success when higher efficiency pollinators were more abundant. Thus, the relationship between low-efficiency pollinators and the plants that they visit varies from commensalistic to antagonistic depending on the presence of other pollinators in the community. Our findings suggest that the negative fitness effects and floral preferences of low-efficiency or "ugly" pollinators may contribute to the maintenance of a pollen color polymorphism in C. americana.

Galloway, L. F., J. R. Etterson, and J. L. Hamrick. 2003. Outcrossing rates and inbreeding depression in the herbaceous autotetraploid Campanula americana. Heredity 90: 308-315. (pdf reprint)
Abstract. – Polyploidy in angiosperms is frequently associated with an increase in self-compatibility. Self-fertilization can enhance polyploid establishment, and theory predicts reduced inbreeding depression in polyploids relative to diploids. Therefore we may expect mating systems that promote self-fertilization or mixed-mating in polyploid species. However few studies have measured polyploid mating systems and inbreeding depression. We report the outcrossing rate and inbreeding depression for Campanula americana , a self-compatible protandrous herb. Allozyme genotypes suggest that C. americana is an autotetraploid with tetrasomic inheritance. We found the multilocus outcrossing rate, tm = 0.938, did not differ from unity. This result was unexpected since previous work demonstrated that pollinators frequently move from male- to female-phase flowers on the same plant, i.e. geitonogamy. Self and outcross pollinations were conducted for three populations. Offspring were germinated in controlled conditions and grown to maturity in pots in nature. Inbreeding depression was not significant for most seed and germination characters. However, all later life traits except flowering date differed between inbred and outcrossed individuals resulting in a 26% reduction in cumulative fitness for inbred plants. Limited early- and moderate later-life inbreeding depression suggest it is buffered by the greater levels of heterozygosity found in an autotetraploid. Campanula americana appears to have a flexible mating system where within flower protandry and/or cryptic self-incompatibility result in a high outcrossing rate when pollinators are abundant but self-compatibility and limited inbreeding depression maintain reproductive success when mates are limited.

Evanhoe, L., and L. F. Galloway. 2002. Floral longevity in Campanula americana (Campanulaceae): a comparison of morphological and functional gender phases. American Journal of Botany 89: 587-591. (pdf reprint)
Abstract. – Plastic responses to pollination and/or pollen removal may shift a flower’s realized longevity closer to an optimal longevity that maximizes reproductive output per unit resource input. In particular, conditional responses to pollen removal and pollen deposition are expected in flowers of protandrous species in which the lengths of the male and female phases may be adjusted independently. We investigated plasticity in floral longevity in Campanula americana, a protandrous, insect-pollinated herb. In greenhouse studies, we found that the longevity of the morphological male phase was shortened by pollen removal and that the longevity of the morphological female phase was shortened by pollen deposition. In a natural population, male and female sexual functions saturated within a few hours of morphological gender phase onset. In contrast to theory, morphological gender phases did not terminate immediately upon saturation of sexual function. These findings are discussed in the context of current floral longevity theory.

Galloway, L. F., T. Cirigliano and K. Gremski. 2002. The contribution of display size and dichogamy to potential geitonogamy in Campanula americana. International Journal of Plant Science 163: 133-139. (pdf reprint)
Abstract. – An increase in floral display size may enhance fitness by increasing a plant’s attractiveness to pollinators. Alternatively a larger display may reduce fitness by increasing pollen movement within an individual. In dichogamous species the frequency of this geitonogamy also depends on floral gender ratio, and pollinator response to floral gender and to inflorescence architecture. We explored the relationships of floral display size and dichogamy to potential geitonogamy in Campanula americana, a protandrous herb. We observed pollinators in a natural population where individuals differed in display size, gender ratio, and distance to conspecifics, and in experimental arrays where individuals only differed in display size. In both settings, bees responded to increased floral displays by increasing the number and length of foraging bouts. The number of potentially geitonogamous visits, those to female-phase flowers after male-phase flowers, also increased with display size. In addition, although proportion of flowers visited did not increase with the floral display, the proportion of geitonogamous visits did. There was no evidence that dichogamy reduced the potential for geitonogamy. The ratio of male- to female-phase flowers did not affect bee behavior and bees did not display a gender preference. In contrast to other studies, the presentation of female-phase flowers below male-phase flowers on reproductive branches did not reduce geitonogamy. Variation in floral display size may cause the mating system to vary among populations. We found that display size varied among 12 populations and that geitonogamy may be infrequent in many populations due to small display sizes. Finally, within populations the selfing rates of larger biennial C. americana are likely to exceed those of smaller annual individuals.

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