Tom J. de Jong

Geitonogamy : The neglected side of selfing

Flowers of many angiosperm species are well adapted to avoid self-pollination, for instance by temporal and spatial separation of pollen and stigma within the same flower. However, such adaptations do not prevent the transfer of pollen between flowers on the same plant (geitonogamy). Recent empirical studies, aided by advances in field techniques, statistical methods and modelling, show that geitonogamy often is substantial and increases with plant size. Selfing by geitonogamy incurs a fitness cost of reduced pollen export, and recent reports show that seed set can suffer as well, even in self-incompatible species. Geitonogamy has important implications for sex-allocation theory, the evolution of dioecy and other issues in evolutionary biology.

Plant size, geitonogamy and seed set in Ipomopsis aggregata

SummaryWe used powdered fluorescent dyes to estimate receipt of self vs. outcross pollen in the self-incompatible species Ipomopsis aggregata (Polemoniaceae). Flowers on small and large plants received equal amounts of outcross pollen, whereas flowers on large plants received more self pollen, so the proportion of self pollen delivered through geitonogamy increased with plant size. In natural populations emasculation of all flowers on a plant raised average seed set per flower from 5.19 to 6.99 and also raised fruit set, though not significantly. From these results one expects a negative correlation between plant size and seeds per flower. The opposite trend was observed in a sample of plants in the field, suggesting that deleterious effects of geitonogamy on female fecundity in large plants can be overruled by other factors such as size-related fruit or seed abortion. Results are discussed in relation to the evolution of gynodioecy.

Threshold size for flowering in different habitats: Effects of size-dependent growth and survival

For organisms with indeterminate growth, life history theory predicts that in environments where organisms experience high survival rates or gain fecundity with age or size, natural selection favors delayed maturity. In semelparous perennial plants the onset of reproduction is regulated by a threshold size for flowering. We tested this prediction by comparing sand dune populations of the facultative biennial herb Cynoglossum officinale. We collected data on flowering probability, survival, and growth rate in relation to plant size in two habitat types, open areas and poplar thickets, in Meijendel, The Netherlands, and in Holkham, England. Survival of established rosettes was highest in Holkham and lowest in open areas in Meijendel. Relative growth rates in Holkham were about three times as high as those in Meijendel. These findings agreed with the differences in threshold sizes found among the sites: the Holkham field population harbored higher threshold sizes than the Meijendel sites. We used the field data to compare optimal threshold sizes for flowering predicted by three existing models. All three models gave the same rank order for threshold sizes as found in the field: Meijendel open < Meijendel poplar thicket < Holkham. One model, which maximized population growth rate λ, predicted optimal threshold sizes that agreed very well with threshold sizes found in the field. The predictions of the two other models, both maximizing R0, were consistently lower than the threshold sizes observed in Meijendel, while their predictions for Holkham were too high.

Limiting factors for seed production in Cynoglossum officinale

SummaryOver three years of study, small plants of Cynoglossum officinale consistently produced more flowers per unit of dry weight than large plants. In contrast to earlier results, weight of all seeds tended to increase more than proportional to size. As a result a positive correlation existed between seed set per flower and plant size. The correlation between the mean number of pollinator visits per flower and size was positive but not significant. In a field experiment we found that resources rather than pollen were limiting seed set. Thus, it is unlikely that enhanced pollination of the largest plants causes the size-dependency of seed set per flower. Alternative hypotheses are discussed briefly.

Is it profitable for biennials to live longer than two years

Abstract A stochastic model for simulating a biennial plant population is developed. Results of simulation demonstrate that in variable environments delay of flowering can be profitable in two ways: by increasing population growth and by decreasing the rate of extinction. The variability of the environment is an important parameter that influences the optimal life-history strategy.

Selection for biennial life histories in plants

The applicability of traditional models of life-history evolution is restricted because models assume constant environments without density-dependence. In these models the geometric growth rate (λ) of a lineage is the natural choice for the fitness criterion. Because in growing populations λ is very sensitive to changes in generation time, biennials appear at a disadvantage when compared to species with annual reproduction. We propose an alternative fitness criterion for the situation in which recruitment is limited by both the availability of seeds and the (constant) rate at which ‘safe sites’ for establishment are created by small-scale disturbances. Under the assumptions of the model the evolutionarily stable strategy is to maximize expected seed production, irrespective of the length of the life cycle. For monocarpic plants, this implies that delay of flowering is favourable if the increased seed production compensates for the death of rosettes.

The negative effects of litter of parent plants of Cirsium vulgare on their offspring: autotoxicity or immobilization?

It is shown that litter of flowering plants of Cirsium vulgare inhibits the growth of seedlings of the same species at concentrations as low as 0.4% (g litter/g soil). The inhibition of growth cannot be fully compensated by adding nutrients, which indicates that autotoxicity may occur. It must be concluded, however, from an analysis of the results that immobilization of nutrients by microorganisms is the main cause of growth reduction if litter is added to the soil. In a field experiment under nutrient rich conditions plant litter did not affect germination, survival or growth of seedlings. The hypothesis is put forward that under conditions of nutrient deficiency and at high population densities, immobilization of nitrogen by Cirsium vulgare litter may influence plant growth. Results are discussed in relation to those of Stachon and Zimdahl (1980) and Wilson (1981) who claimed allelopathy for the related Cirsium arvense.

Bidirectional selection on threshold size for flowering in Cynoglossum officinale (hound's-tongue)

Previous studies have demonstrated that many facultative biennials show variation in threshold size for flowering. In order to quantify the genetic variation for this character, we performed a two-way artificial selection experiment on threshold size in Cynoglossum officinale. The parental generation, established from seed from a natural population, showed large variation in threshold sizes (2.6−13.4 g). After one generation of selection for low threshold sizes all plants in the F1 flowered at sizes below 3.2 g. In the high selection line none of the F1 plants under 3.6 g flowered, and there were very large nonflowering plants (up to 17.2 g). By interpreting the logistic regression of flowering probability on plant size as a cumulative frequency distribution of threshold sizes, we could derive the frequency distributions of threshold sizes in each generation. These were used to estimate the narrow-sense heritabilities by the standard procedure for artificial selection on a quantitative character. This gave ‘heritabilities’ of 0.35 and 0.32 for the high and low selection lines, respectively. A previously constructed optimization model, applied to the population in our study area, predicted strong selection against low threshold sizes, but a fairly equal fitness for threshold sizes around and above the optimum. This flat fitness profile may explain why the natural population of Cynoglossum officinale harbours extensive genetic variation for this character.

Genetic variation in chrysanthemum for resistance to Frankliniella occidentalis

In a choice‐experiment, 42 chrysanthemum cultivars were screened for resistance to Frankliniella occidentalis (Pergande). Oviposition preference, two types of feeding damage and thrips numbers per flower were recorded as measures of resistance. A large genetic variation in thrips resistance was found among the cultivars screened. The amount of feeding damage was strongly determined by oviposition preference. Besides, a positive correlation was found between the oviposition preference in non‐flowering chrysanthemums (number of eggs) and flowering chrysanthemums (number of thrips per flower). Thrips feeding on young, developing tissues, causes growth damage because affected cells are unable to expand and leaves become distorted. Thrips feeding on older, expanded leaves causes cells to become filled with air, resulting in ‘silver’ damage. The amounts of growth‐ and ‘silver’ damage were negatively correlated suggesting that thrips chose either young or older leaves to feed on. The order of resistance among cultivars did not change during the experiment. In order to get more insight in resistance mechanisms the influence of some plant‐ and flower characters on resistance was examined. The plant characters height, number of leaves, flower production and flower weight were all negatively correlated with resistance. It is suggested that tall chrysanthemum cultivars with many and large flowers may invest less in defence than smaller cultivars, and therefore are more damaged by thrips.

Population growth and survival of western flower thrips Frankliniella occidentalis Pergande (Thysanoptera, Thripidae) on different chrysanthemum cultivars: Two methods for measuring resistance

Two methods were used to measure the resistance of 5 chrysanthemum cultivars to thrips Frankliniella occidentalis Pergande (Thysanoptera, Thripidae). As a measure of resistance of a cultivar, growth and survival of the insect population were taken.