Why do flowers wilt?

Resources salvaged when flowers wilt on a perennial plant could promote reproduction by, in preference order, the same flowers (Hypothesis 1), adjacent flowers on the same plant (Hypothesis 2), or during the next flowering season by the same plant (Hypothesis 3). We tested the above hypotheses for Blandfordia grandiflora, a perennial species, where some plants included flowers that were allowed to wilt, while equivalent flowers on other plants were prevented from wilting. The abilities of these plants to produce seed were determined by liberally pollinating all flowers. To test Hypotheses 1 and 2, seed set per flower and per plant were compared between plants with and without wilting flowers. To specifically test Hypothesis 3, reproduction was prevented in all flowers. For each experiment, flowering was monitored in the same plants during the next flowering season, thus also enabling Hypothesis 3 to be tested. The results were consistent with Hypothesis 3, but not with Hypotheses 1 and 2. Hence, we verified, for the first time, that plants may benefit from salvaging resources from wilting flowers and re-using these resources for subsequent reproduction. However, contrary to expectations, plants re-used these resources to promote reproduction during subsequent flowering, and not during current flowering by either the same flowers or other flowers on the same plant. The plants must have transferred resources from wilting flowers to underground corms and roots, which provided resources necessary for subsequent flowering. This is likely part of a general plant strategy to salvage resources invested in reproduction during one flowering season and reuse these resources during subsequent flowering.

How does pollination mutualism affect the evolution of prior self-fertilization? A model

The mode of pollination is often neglected regarding the evolution of selfing. Yet the distribution of mating systems seems to depend on the mode of pollination, and pollinators are likely to interfere with selfing evolution, since they can cause strong selective pressures on floral traits. Most selfing species reduce their investment in reproduction, and display smaller flowers, with less nectar and scents (referred to as selfing syndrome). We model the evolution of prior selfing when it affects both the demography of plants and pollinators and the investment of plants in pollination. Including the selfing syndrome in the model allows to predict several outcomes: plants can evolve either toward complete outcrossing, complete selfing, or to a stable mixed-mating system, even when inbreeding depression is high. We predict that the evolution to high prior selfing could lead to evolutionary suicides, highlighting the importance of merging demography and evolution in models. The consequence of the selfing syndrome on plant-pollinator interactions could be a widespread mechanism driving the evolution of selfing in animal-pollinated taxa.

Effects of mating system on adaptive potential for leaf morphology in Crepis tectorum (Asteraceae)

BACKGROUND AND AIMS

A shift from outcrossing to selfing is thought to reduce the long-term survival of populations by decreasing the genetic variation necessary for adaptation to novel ecological conditions. However, theory also predicts an increase in adaptive potential as more of the existing variation becomes expressed as homozygous genotypes. So far, relatively few studies have examined how a transition to selfing simultaneously affects means, variances and covariances for characters that might be under stabilizing selection for a spatially varying optimum, e.g. characters describing leaf morphology.

METHODS

Experimental crosses within an initially self-sterile population of Crepis tectorum were performed to produce an outbred and inbred progeny population to assess how a shift to selfing affects the adaptive potential for measures of leaf morphology, with special emphasis on the degree of leaf dissection, a major target of diversifying selection within the study species.

KEY RESULTS

Three consecutive generations of selfing had a minor impact on survival, the total number of heads produced and the mean leaf phenotype, but caused a proportional increase in the genetic (co)variance matrix for foliar characters. For the degree of leaf dissection, the lowest 50th percentile of the inbred progeny population showed a disproportionate increase in the genetic variance, consistent with the recessive nature of the weakly lobed phenotype observed in interpopulation crosses. Comparison of inbreeding response with large-scale patterns of variation indicates a potential for selection in a (recently) inbred population to drive a large evolutionary reduction in degree of leaf dissection by increasing the frequency of particular sibling lines.

CONCLUSIONS

The results point to a positive role for inbreeding in phenotypic evolution, at least during or immediately after a rapid shift in mating system.

Does inbreeding promote evolutionary reduction of flower size? Experimental evidence from Crepis tectorum (Asteraceae)

PREMISE OF THE STUDY

Small, autogamous flowers have evolved repeatedly in the plant kingdom. While much attention has focused on the mechanisms that promote the shift to autogamy, there is still a paucity of information on the factors that underlie the reduction of flower size so prevalent in selfing lineages. In this study of Crepis tectorum, I examine the role of inbreeding, acting alone or together with selection, in promoting evolutionary reduction of flower size.

METHODS

Experimental crosses were performed to produce progeny populations that differed in inbreeding and (or) selection history. Progenies were grown in two different environments and scored for flower size and other characters.

KEY RESULTS

Inbreeding depressed flower and fruit size, but also caused changes in flowering time and the number of heads produced. Despite some inconsistencies in the results for the last progeny generation, the decline in flower size was persistent over generations, consistent across environments, and similar in magnitude to the effects of selection for small flower size and the floral reduction inferred to have taken place during the shift toward autogamy within the study species. The floral size reduction was largely independent of changes in overall vigor, and there was considerable adaptive potential in flower size (measured by sib analyses and parent-offspring comparisons) after inbreeding.

CONCLUSIONS

The results of this study indicate that inbreeding can promote evolutionary reduction of flower size and highlight the close, persistent association between flower and fruit size in the study species.

Costs of reproduction in life history of a perennial plant Carex secalina

We tested a hypothesis based on life history theory that examines reproductive costs incurred by individuals in consecutive years of their life. A multi-year dataset of resource allocation to vegetative and reproductive structures was analysed in Carex secalina — a perennial, monoecious sedge, reproducing only sexually. In a four-year garden experiment, we assessed above-ground biomass at the end of each season and reproductive allocation expressed as the total length of male and female spikes. The study was aimed at determining how size and age of a plant relates to its reproduction, and how the rate of reproduction affects the year-toyear biomass change in Carex secalina. We observed that after each reproductive episode, individuals had significantly smaller sizes and produced a lower number of generative tillers. The total production of reproductive structures decreased significantly with age in all populations. Moreover, the decrease in plant biomass was greater when more reproductive structures were produced in a previous year, which indicates that the plants incur costs of reproduction in terms of above-ground biomass production.

Visibility vs. biomass in flowers: exploring corolla allocation in Mediterranean entomophilous plants

BACKGROUND AND AIMS

While pollinators may in general select for large, morphologically uniform floral phenotypes, drought stress has been proposed as a destabilizing force that may favour small flowers and/or promote floral variation within species.

METHODS

The general validity of this concept was checked by surveying a taxonomically diverse array of 38 insect-pollinated Mediterranean species. The interplay between fresh biomass investment, linear size and percentage corolla allocation was studied. Allometric relationships between traits were investigated by reduced major-axis regression, and qualitative correlates of floral variation explored using general linear-model MANOVA.

KEY RESULTS

Across species, flowers were perfectly isometrical with regard to corolla allocation (i.e. larger flowers were just scaled-up versions of smaller ones and vice versa). In contrast, linear size and biomass varied allometrically (i.e. there were shape variations, in addition to variations in size). Most floral variables correlated positively and significantly across species, except corolla allocation, which was largely determined by family membership and floral symmetry. On average, species with bilateral flowers allocated more to the corolla than those with radial flowers. Plant life-form was immaterial to all of the studied traits. Flower linear size variation was in general low among conspecifics (coefficients of variation around 10 %), whereas biomass was in general less uniform (e.g. 200-400 mg in Cistus salvifolius). Significant among-population differences were detected for all major quantitative floral traits.

CONCLUSIONS

Flower miniaturization can allow an improved use of reproductive resources under prevailingly stressful conditions. The hypothesis that flower size reflects a compromise between pollinator attraction, water requirements and allometric constraints among floral parts is discussed.