Variation In Pollen Production Research Articles

Pollen biology of the serpentine-endemic Orobanche nowackiana (Orobanchaceae) from Albania

The holoparasite flowering plant Orobanche nowackiana Markgr. is a rare endemic plant that parasitises the Ni hyperaccumulator species Alyssum murale Waldst. and Kit. in Komjan Mt. (Albania). The purpose of this study was to establish baseline data concerning aspects of its pollen biology. To achieve this goal three objectives were addressed: (1) describe pollen morphology; (2) study pollen production and fertility/sterility; (3) study the localisation of metals in anthers and pollen. Pollen morphology was investigated with light microscope and scanning electron microscope (SEM) observations. The pollen grains are 3-colpate, most often oblate-spheroidal, with long colpi reaching the poles. The ornamentation is microreticulate. The anther and pollen grains were micromorphologically analysed by SEM coupled with an energy-dispersive X-ray probe (SEM-EDX). Low concentration of Ni was recorded for anthers (0.38%) and pollen (0.1–5.6%). Variation in pollen production was found for the flowers of the same individual. The mean pollen production per flower and stamen was 59365 and 14938 pollen grains respectively. The sterile pollen was above the limit considered as a normal abortion and was between 10.1 and 38.0%. From a palynological point of view our results are important for taxonomy and support keeping the species in the genus Phelipanche.

Beyond the heterostylous syndrome.

Beyond the heterostylous syndrome.

Temporal variation in phenotypic gender and expected functional gender within and among individuals in an annual plant.

Adaptive explanations for variation in sex allocation centre on variation in resource status and variation in the mating environment. The latter can occur when dichogamy causes siring opportunity to vary across the flowering season. In this study, it is hypothesized that the widespread tendency towards declining fruit-set from first to last flowers on plants can similarly lead to a varying mating environment by causing a temporal shift in the quality (not quantity) of siring opportunities. A numerical model was developed to examine the effects of declining fruit-set on the expected male versus female reproductive success (functional gender) of first and last flowers on plants, and of early- and late-flowering plants. Within- and among-plant temporal variation in pollen production, ovule production and fruit-set in 70 Brassica rapa plants was then characterized to determine if trends in male and female investment mirror expected trends in functional gender. Under a wide range of model conditions, functional femaleness decreased sharply in the last flowers on plants, and increased from early- to late-flowering plants in the population. In B. rapa, pollen production decreased more rapidly than ovule production from first to last flowers, leading to a within-plant increase in phenotypic femaleness. Among plants, ovule production decreased from early- to late-flowering plants, causing a temporal decrease in phenotypic femaleness. The numerical model confirmed that declining fruit-set can drive temporal variation in functional gender, especially among plants. The discrepancy between observed trends in phenotypic gender in B. rapa and expected functional gender predicted by the numerical model does not rule out the possibility that male reproductive success decreases with later flowering onset. If so, plants may experience selection for early flowering through male fitness.

Human impact on vegetation at the Alpine tree-line ecotone during the last millennium: lessons from high temporal and palynological resolution

Three mires and a small lake in the Swiss and Austrian Alps were studied palynologically at high resolution, covering the last 1,000, 400, 50 and 1,200 years, respectively. Methodological lessons include: (1) Sub-decadal resolution in upper, little-decomposed peat layers reveals recurrent marked fluctuations in both percentages and influx of regional tree-pollen types, reflecting variations in pollen production rather than in plant-population sizes. (2) Intermittent, single-spectrum pollen maxima in samples of sub-decadal resolution indicate pollen transport in clumps. This type of pollen transport may remain unrecognized in sections with lower sampling resolution, which may then lead to inappropriate interpretation in terms of plant-population sizes. (3) The detection of short-lived phases of human impact in decomposed peat requires sampling intervals as close as 0.2 cm. (4) PAR (pollen influx) may reflect vegetation dynamics more faithfully than percentages. Reliable PAR, however, is difficult to achieve in Alpine mires due to past human impact on peat growth, even when complex depth–age modelling techniques are used. Critical comparison of PAR with percentages is therefore essential. (5) Careful consideration of spatial scales in pollen signals (local–regional and subdivisions) is essential for a realistic palaeo-ecological interpretation. Results in terms of past human impact on vegetation are summarized as follows: (1) Trends in pollen types reflecting regional human action are in general agreement with earlier findings for the western Swiss Alps, allowing for regional differences. (2) All mires in the Alps investigated here and in an earlier study experienced human impact during the last millennium. The studied small lake, lying in sub-alpine pasture, records forest dynamics at a lower elevation since a.d. 800.

Effects of male fecundity, interindividual distance and anisotropic pollen dispersal on mating success in a Scots pine (Pinus sylvestris) seed orchard

Quantifying the effect of pollen dispersal and flowering traits on mating success is essential for understanding evolutionary responses to changing environments and establishing strategies for forest tree breeding. This study examined, quantitatively, the effects of male fecundity, interindividual distance and anisotropic pollen dispersal on the mating success of Scots pine (Pinus sylvestris), utilizing a well-mapped Scots pine seed orchard. Paternity analysis of 1021 seeds sampled from 87 trees representing 28 clones showed that 53% of the seeds had at least one potential pollen parent within the orchard. Pronounced variation in paternal contribution was observed among clones. Variations in pollen production explained up to 78% of the variation in mating success, which was 11.2 times greater for clones producing the largest amount of pollen than for clones producing the least pollen. Mating success also varied with intertree distance and direction, which explained up to 28% of the variance. Fertilization between neighboring trees 2.3 m apart was 2.4 times more frequent than between trees 4.6 m apart, and up to 12.4 times higher for trees downwind of the presumed prevailing wind direction than for upwind trees. The effective number of pollen donors recorded in the seed orchard (12.2) was smaller than the theoretical expectation (19.7). Based on the empirical observations, a mating model that best describes the gene dispersal pattern in clonal seed orchards was constructed.

Variation in pollen production and pollen viability in natural populations of gynodioecious Beta vulgaris ssp. maritima: evidence for a cost of restoration of male function?

Gynodioecious species are defined by the co-occurrence of two clearly separated categories of plants: females and hermaphrodites. The hermaphroditic category may, however, not be homogeneous, as male fitness may vary among hermaphrodites as a result of many biological factors. In this study, we analysed estimates of pollen quantity and viability in the gynodioecious Beta vulgaris ssp. maritima, comparing hermaphrodites bearing a male-fertile cytotype and hermaphrodites bearing cytoplasmic male sterility (CMS) genes, which are counteracted by nuclear restoration factors. We show that: (i) pollen quantity continuously varies among restored hermaphrodites, suggesting a complex genetic determination of nuclear restoration; (ii) pollen viability was lower in restored (CMS) hermaphrodites than in non-CMS hermaphrodites, probably because of incomplete restoration in some of these plants; and (iii) pollen quantity and viability also varied among hermaphrodites with male-fertile cytotypes, possibly a result of a silent cost of restoration. Finally, we discuss the consequences of these results for pollen flow and the dynamics of gynodioecy.

Vegetation history, human impact and climate change during the last 700 years recorded in annually laminated sediments of Lac Pavin, France

This paper presents a 1.80-m-long varved sediment record documenting the vegetation history by pollen and diatom analysis recovered from Lake Pavin, a volcanic crater lake in the Auvergne (French Massif Central). High-resolution sampling and a varve-based age model allowed the reconstruction of ecosystem responses to both regional climate change and human impact. According to the present varve chronology, the Lake Pavin sequence covers the last 700 years, thus representing the late Subatlantic period (i.e. from the Late Medieval onwards). The pollen record largely reflects intensive human influence (clearing, cultivation and grazing) on the lake surrounding area. The arboreal pollen spectrum of the entire sequence is co-dominated by Fagus and Quercus, except during the 20th century. However, whilst the Quercus record is relative stable, several major fluctuations are discernable in the Fagus curve. An increase in Pinus and Picea shortly before of the turn the 20th Century, indicates afforestation by conifers. The varve counting indicates that the most prominent signal in the Fagus curve occurs during the period between about 1540 and 1670 AD. During this interval, the initial drop in the Fagus curve is estimated to have taken 5–10 years, with the subsequent recovery occurring over a similar period. Since there is no clear evidence of tree population replacement, such rapid signals imply that the fluctuations in the Fagus curve reflect variations in pollen production, being influenced by the cooler temperatures experienced during a cold spell of the Little Ice Age.

GENETIC CONSTRAINTS ON FLORAL EVOLUTION IN A SEXUALLY DIMORPHIC PLANT REVEALED BY ARTIFICIAL SELECTION

Sexual dimorphism is one of the most widespread and recognizable patterns of phenotypic variation in the biotic world. Sexual dimorphism in floral display is striking in the dioecious plant Silene latifolia, with males making many, small flowers compared to females. We investigated this dimorphism via artificial selection on two populations to determine whether genetic variation exists within populations for flower size and the extent of the between-sex correlation, whether a flower size and number trade-off exists within each sex, and whether pollen and ovule production vary with flower size. We selected for decreased flower size (calyx width) in females and increased flower size in males and measured the response to selection in size and correlated responses in flower dry mass, flower number, and pollen or ovule number per flower. Four bouts of selection in each of two selection programs were performed, for a total of three selection lines to decrease size, three to increase it, and two control lines. Flower size always significantly responded to selection and we always found a significant correlated response in the sex not under selection. Selection decreased but did not eliminate the sexual dimorphism in flower dry mass and number. A negative relationship between flower size and number within each sex was revealed. Whereas ovule number showed a significant correlated response to selection on flower size, pollen number did not. Our results indicate that although substantial additive genetic variation for flower size exists, the high between-sex genetic correlation would likely constrain flower size from becoming more sexually dimorphic. Furthermore, floral display within each sex is constrained by a flower size and number trade-off. Given this trade-off and lack of variation in pollen production with flower size, we suggest that sexual dimorphism evolved via sexual selection to increase flower number in males but not females.

GENETIC CONSTRAINTS ON FLORAL EVOLUTION IN A SEXUALLY DIMORPHIC PLANT REVEALED BY ARTIFICIAL SELECTION

Sexual dimorphism is one of the most widespread and recognizable patterns of phenotypic variation in the biotic world. Sexual dimorphism in floral display is striking in the dioecious plant Silene latifolia, with males making many, small flowers compared to females. We investigated this dimorphism via artificial selection on two populations to determine whether genetic variation exists within populations for flower size and the extent of the between-sex correlation, whether a flower size and number trade-off exists within each sex, and whether pollen and ovule production vary with flower size. We selected for decreased flower size (calyx width) in females and increased flower size in males and measured the response to selection in size and correlated responses in flower dry mass, flower number, and pollen or ovule number per flower. Four bouts of selection in each of two selection programs were performed, for a total of three selection lines to decrease size, three to increase it, and two control lines. Flower size always significantly responded to selection and we always found a significant correlated response in the sex not under selection. Selection decreased but did not eliminate the sexual dimorphism in flower dry mass and number. A negative relationship between flower size and number within each sex was revealed. Whereas ovule number showed a significant correlated response to selection on flower size, pollen number did not. Our results indicate that although substantial additive genetic variation for flower size exists, the high between-sex genetic correlation would likely constrain flower size from becoming more sexually dimorphic. Furthermore, floral display within each sex is constrained by a flower size and number trade-off. Given this trade-off and lack of variation in pollen production with flower size, we suggest that sexual dimorphism evolved via sexual selection to increase flower number in males but not females.

Tassel Morphology as an Indicator of Potential Pollen Production in Maize

Adequate pollen production is an essential prerequisite for achieving high yields in commercial corn (Zea mays L.) production and for insuring high levels of genetic purity in the production of hybrid seed. Documenting the timing and intensity of pollen shed are fundamental to these goals, but methods to describe patterns of pollen release from maize tassels are limited and laborious. Our objective was to explore characteristics of tassel morphology that could be used as simple and indirect measures of pollen production per plant under field conditions. The progress of tassel development was documented using a nine-stage scale based on easily-quantified morphological characteristics. Genetic variation among hybrids and inbreds as well as environmental variation across planting densities and years was correlated with levels of pollen production. This analysis revealed that a change in tassel dry weight during pollen shed was not an accurate measure of pollen production per tassel. Likewise, no single morphological characterization captured all the genetic and environmental variation in pollen production per tassel. But a combination of morphological traits incorporated into a Tassel Area Index (TAI) accounted for up to 89% of the variation in pollen production among hybrids in response to population density, and 64% of the variation in pollen production among inbred heterotic groups. Because data collection is simple, quick, and non-destructive, the Tassel Area Index approach is well-suited for distinguishing genetic variation in pollen production and relative responses to treatments under field conditions. The accuracy of the technique could be increased, if necessary, by incorporating additional information about flower density or pollen production per anther. But this would entail a much greater investment of time and resources.

Factorial analysis of self-incompatibility in alfalfa

Self-incompatibility (seeds/pod and seeds/floret tripped) was evaluated in the growth chamber (27°C) using a 6 × 6 factorial arrangement. Parents represented a broad range of self-compatibility and were selected on the basis of stability of autogamous seed production in the greenhouse and growth chamber. Additive genetic variation appeared to be most important, indicating that a mass selection scheme such as recurrent phenotypic selection would be effective in increasing levels of self-incompatibility in at least some alfalfa populations. Narrow-sense heritability estimates for seeds/pod and seeds/floret tripped were 0.23 (SE = 0.09) and 0.28 (SE = 0.06), respectively. There is a direct relationship between autogamous seed production and pollen production, however, results of this experiment indicate that there may be considerable variation in pollen production, germination and vigor among self-incompatible clones. Increased seed set resulting from self-incompatible × self-incompatible crosses could be a function of greater gamete selection in the self-incompatible parents (due to deleterious genes), improved pollen germination, and hybrid vigor in pollen tube growth. Key words: Medicago sativa L., autogamy, allogamy, hybrid

Improved preservation of organic matter and pollen in eastern Mediterranean sapropels

Here we investigate the processes leading to high organic content and high pollen concentration in pelagic marine sediments. Three marine cores near the base of the Nile River cone display the classical Quaternary sequence of sapropels and hemipelagic muds. Large variations in total pollen concentration, as well as in percentage composition of the pollen assemblage are clearly linked to the two lithological types. All pollen types have much higher concentration in sapropels than in muds, by 3 or 4 orders of magnitude. Within sapropels, the percentages of Gymnosperms (Pinus and Cedrus) pollen and Pteridophyte spores, are very low and the pollen spectrum is dominated by various Angiosperms. These variations in total pollen concentration as well as in percentages cannot be accounted for by variations in pollen production, or in efficiency of transport, because they similarly involve pollen of different areas and climates, transported to the sea by different means. The cause for these variations can only be a difference in preservation of pollen. Preservation is good in sapropels for all pollen types, but generally poor in muds. We believe that sapropels were deposited under reducing bottom waters which allowed a better preservation of the organic matter including pollen.

Variations in Pollen Production and Seed Set of Male Sterile Alfalfa Somaclones.

Variations in Pollen Production and Seed Set of Male Sterile Alfalfa Somaclones.

Fertility variation in Pinus sylvestris: A TEST OF SEXUAL ALLOCATION THEORY

Sexual allocation theory assumes trade‐offs and negative genetic correlations between male and female allocation in hermaphrodite plants. We tested this assumption by studying variation in male and female fertility in two experimental populations of Pinus sylvestris. In these populations, the genotypes have been vegetatively replicated, which allowed separation of the genetic and environmental components of variation. The genetic components of variation accounted for 36% of the total variation in pollen production in the two populations, and for 54% of cone production. As assumed by sexual allocation theory, the genetic correlation of pollen and cone production was negative in both populations (‐0.59 and ‐0.15). However, positive environmental correlations between these traits (0.43 and 0.45) resulted in no phenotypic correlation in one population and a positive phenotypic correlation in the other.

Male fertility variation in an open-pollinated plantation of Norway spruce (Piceaabies)

Male fertility variation in an open-pollinated plantation of Norway spruce (Piceaabies (L.) Karst.) was studied using six polymorphic isozyme marker loci. Significant variation in male fertilities was detected in the seed crops from 81% of the female parents. The ranking of relative contribution of male parents to the offspring varied considerably among seed crops. Pronounced heterogeneity among the fertility estimates of a male parent derived from different seed crops was also observed. The overall male fertility estimates were highly skewed, with less than 23% of trees contributing more than 50% of the male gametes to the seed sample. In addition to postpollination mechanisms, factors associated with pollination such as variation in pollen production and floral phenology among trees likely have contributed to the observed male fertility variation. These results have significant implications in forest tree improvement programs.

Characteristics of pollen production in a population of New Zealand snow‐tussock grass (Chionochloa pallens Zotov)

Intraspecific variation in pollen production and pollen size were studied within a population of Chionochloa pallens, an alpine snow-tussock grass of New Zealand. I sampled plants from two sites at the altitudinal extremes (1620 and 1070 m) of the distribution of C. pallens on Mt. Hutt, South Island. There was a highly skewed distribution of pollen production among plants; the largest 10% of plants produced over a third of the pollen. In contrast to some other species, pollen production in C. pallens can be accurately estimated by simple measures such as tussock area and number of flowering shoots. Other components of yield (number of spikelets per shoot, number of florets per spikelet, and average anther length) varied significantly among individuals, but these characteristics had a small impact on total pollen yield. There was continuous variation in pollen size among individuals; between the plants with the largest and the smallest pollen grains, there was a 21 % difference in pollen diameter and a 76% difference in pollen volume. Interplant differences in pollen size have been found in a number of plants, although the biological significance of this variation is unknown. There were few differences in pollen characteristics between sites. Plants at the low-altitude site had slightly greater pollen yield because total plant size was greater.

INTRASPECIFIC VARIATION IN POLLEN YIELD IN BROMEGRASS (POACEAE: BROMUS)

Variation in pollen production was measured within five hermaphrodite species of bromegrass (Bromus). Anther length is an excellent predictor of pollen production in this genus (R2 = 0.97). Anther length varied considerably within each of the species, both among and within individual plants. Within plants, most of the variation occurred among florets within spikelets; florets in upper spikelet positions were smaller and produced less pollen. In B. inermis, pollen production was decreased by defoliation and increased in shoots that grew on thatching ant (Formica obscuripes) mounds. Whole‐shoot pollen yield was determined by spikelet number, number of florets per spikelet, and pollen production per floret. All of these yield components must be considered in attempts to estimate pollen production accurately.

Intraspecific Variation in Pollen Yield in Bromegrass (Poaceae: Bromus)

Variation in pollen production was measured within five hermaphrodite species of bromegrass (Bromus). Anther length is an excellent predictor of pollen production in this genus (R2 = 0.97). Anther length varied considerably within each of the species, both among and within individual plants. Within plants, most of the variation occurred among florets within spikelets; florets in upper spikelet positions were smaller and produced less pollen. In B. inermis, pollen production was decreased by defoliation and increased in shoots that grew on thatching ant (Formica obscuripes) mounds. Whole-shoot pollen yield was determined by spikelet number, number of florets per spikelet, and pollen production per floret. All of these yield components must be considered in attempts to estimate pollen production accurately.

Effects of variation in inflorescence size and floral rewards on the visitation rates of traplining pollinators ofAralia hispida

In field experiments withAralia hispida inflorescences, the following variables were manipulated: number of umbels per inflorescence, number of flowers per umbel, and amounts of pollen and nectar per flower. Visitation rates by bumble bees, the principal pollinators, were then observed. In the reward-variation experiments, bees appeared to learn the positions of nectar-rich shoots, and visited them significantly more often than nectar-poor shoots. They did not respond to similar variation in pollen production. The nectar preferences developed slowly after the treatments were imposed, and bees continued to favor sites that had been occupied by nectar-rich shoots even after the treatments were discontinued. Visitation rate was approximately proportional to flower number, making it unlikely that increases in inflorescence size produced a disproportionate gain in male reproductive success (a necessary condition in certain models for the evolution of dioecy). For a fixed number of flowers per inflorescence, bees preferred inflorescences with more umbels. In pairwise choice tests of male-phase and female-phase umbels of various sizes, bees preferred male-phase umbels and larger umbels; the preference for male-phase umbels is stronger in bees that had previously fed on male-phase umbels.

Intra‐Plant Variation for Pollen Production in Male‐sterile and Fertile Alfalfa1

In Medicago saliva L. intra‐plant variation in pollen production occurred in partially male‐sterile clones, but it was not associated with either flower position on the plant or sampling date. The variation occurred at random throughout the plant. Anthers within the same flower varied for both number of pollen grains and percentage of normal pollen grains.The average number of pollen grains per anther varied among clones from about 440 to 850. The free anther had only about two‐thirds as many pollen grains as each of the remaining nine anthers. The number of normal pollen grains correlated with visual estimates of the amount of dehisced pollen, r = .854**, 11 d.f. Pollen abortion in male‐sterile plants apparently reduced internal pressure sufficiently to prevent rupture of the anther wall and extrusion of pollen.