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Pollen germination throughout the seasons is examined.

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 ([R.sup.2] = 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.

We are studying gene expression in generative cells using isolated generative cells of lily pollen as our experimental material. Our study indicates that the generative cell displays differential gene expression. Certain transcripts show strict cell specific expression, whereas others show preferential expression in generative cells. One of the preferentially expressed cDNA clones from the generative cell shows a significant nucleotide sequence similarity with a human nucleotide excision repair protein. It is of interest to note that the generative cell, the progenitor of two sperm cells, showed a higher level of the transcripts than the vegetative cell. These results suggest that an active DNA repair process responsible for maintaining the genetic integrity of germ line DNA exists in the generative cells of pollen. One cDNA clone, LGC1 showed generative cell specific expression. The level of LGC1 transcripts was very high in generative cells. In sire hybridization results clearly demonstrate the generative cell specific localization of LGC1 transcripts. LGC1 is the first germ line specific gene to be identified in flowering plants. The availability of this gene has provided us with a molecular marker to study the regulation of cell-specific expression in generative cells of pollen.

Maize (Zea mays L.) pollen can be stored up to several hours in inert medium such as paraffin oil. However, biological activity and genetic manipulation are almost precluded. In aqueous media, maize pollen loses viability within a few minutes when in conditions different from those promoting germination. We have evidence that viability of maize pollen can be preserved in a hypertonic aqueous solution at 0 degree C. Detrimental if individually applied, low temperature and high sucrose concentrations, together, allow storage of maize pollen up to 10 to 12 hr in an aqueous medium. Such stored pollen can then germinate on agar medium. The lack of in vivo fertilization by liquid pollination, at first interpreted by an osmotic hypothesis, led us to the development of a new, rapid procedure for in vitro germination. Short-term storage of maize pollen in aqueous medium is now possible and should support new techniques of genetic manipulation.

One of the major concerns about genetically modified (GM) crops is that they might spread their genes to nearby weeds or organic crops. Some governments have responded by recommending that [...]

Just a little bit of the wet stuff can wreck pollen and leave cotton flowers unpollinated. Sterile flowers soon fall off the plant without forming the bolls that are the [...]

1 Subalpine meadow communities are influenced by edaphic and microclimatic gradients, and should be affected by climate change that affects these local controls. We used pollen preserved in meadow soils to investigate the long-term interaction of these factors in a 1-ha subalpine meadow in the Olympic Mountains, Washington, USA. 2 To describe the spatial scale at which soil pollen is related to neighbouring vegetation, pollen in 46 soil surface samples was compared with plant cover within concentric circular plots (0.5-1 m radii). Regression analysis of percentage pollen with plant cover at different distances from the surface sample had the highest correlation with radii < 1 m, confirming that entomophilous meadow species have very small relevant pollen source areas. 3 We analysed pollen records in soil columns at three sites representing a range of growing season lengths and community types within the meadow. Soils had aggraded by the deposition of eolian silt. Radiocarbon dates of pollen concentrates were similar to or older than radiocarbon dates on charcoal pieces at the same depth, suggesting negligible downwards movement of pollen in the soil profile. 4 The oldest soil pollen profile was from the wettest microsite, currently dominated by a snowbed Carex nigricans community. This site was occupied by a dry Juniperus community prior to c. 6000 BP (before present), when it shifted to more mesic communities dominated by Poaceae and Polemonium. The first appearance of a snowbed Carex nigricans community at this site, c. 2500-1500 BP, suggests a change to cooler and/or wetter regional climate. 5 High levels of Polygonum bistortoides at all sites indicated a shift to long snow-free periods and mesic summer conditions during an interval corresponding to the Medieval Warm Period (c. 1200-700 BP). After 500 BP (during the Little Ice Age) Carex nigricans re-established in the wet microsite, while relatively little change occurred at the other two sites. Overall, the greater magnitude of change at this microsite than at the longer growing-season sites indicates that, in these subalpine meadows, short growing season sites are most sensitive to regional climate change. Keywords: Little Ice Age, Medieval Warm Period, pollen source area, vegetation-climate equilibrium

Pollination governs a plant's mating options by establishing the diversity and intensity of male-gamete exchange between plants. For animal-pollinated plants, the pattern of pollen dispersal arises from the pollinator's interactions with floral organs during each flower visit and with the pollen it carries during transport. These interactions typically cause some portions of a flower's pollen to experience different transport conditions than other portions. We model three classes of heterogeneous transport conditions to assess their effects on pollen dispersal. Two classes occur when the pollen carried by individual pollinators becomes spatially structured. 'Horizontal' structure arises when areas of a pollinator's body differ in their frequency or intensity of contact with anthers and/or stigmas. Alternatively, 'vertical' structure develops when pollinators do not disturb their pollen load, causing pollen to accumulate on the pollinator's body in layers, so that much pollen is temporarily inaccessible to stigmas. The third class of heterogeneity occurs when the pollinators that visit a flower differ consistently with respect to pollen transfer from pollinator to stigmas. These classes of heterogeneous transport produce similar mean patterns of pollen dispersal, which differ from that expected if transported pollen acts as a homogeneous population. In particular, near and far recipient flowers receive proportionately more of the donor pollen reaching stigmas than predicted for a single population. Reanalysis of data in the literature illustrates that predictions based on heterogeneous transport conditions are more representative of observed dispersal patterns than predictions of the traditional characterization of pollen dispersal We also demonstrate that stochastic variation between flowers in pollen donation and receipt does not affect the arithmetic mean pattern of pollen dispersal, although it does reduce the geometric mean. Although the different types of heterogeneous transport conditions produce similar mean patterns of pollen dispersal, the influences of specific floral and pollinator characteristics on dispersal can depend on the type(s) of heterogeneity involved. In particular, pistil characteristics should affect dispersal independently of stamen characteristics when horizontal heterogeneity governs transport dynamics, but not when pollen accumulates in layers. This relation is likely reflected in many aspects of floral divergence among related taxa pollinated by different vectors and convergent design among unrelated taxa served by similar vectors. Key words: animal pollination; flower design; pollen carryover; pollen deposition; pollen layering; pollen removal; pollination; pollinator grooming; stochastic variation., INTRODUCTION The amount and variety of pollen that a plant exports and imports establish the scope for many components of plant mating (reviewed by Harder and Barrett 1996). From the [...]

Pollen containment may be necessary to prevent the dissemination of novel genes from transgenic crops into sexually compatible land races or wild relatives in locations where these are grown or occur naturally in the same vicinity. Routine maize (Zea mays L.) breeding activities employ controlled pollinations and are sometimes done in areas where !and races or wild relatives are known to occur. The ability of researchers to control pollen movement and to thereby control the potential flow of novel genes from transgenic maize to land races or wild relatives was investigated. Using white- and yellow-seeded inbreds, pollen control was measured in two mating designs. The ability to control pollen was assessed by observing seed color in pollinations on adjacent plantings intended to trap uncontrolled pollen. In one experiment, the yellow-seeded maize contained a transgene. In this experiment contaminant seeds observed in the white maize were analyzed for the presence of recombinant DNA and the gene expression product. The results from these experiments indicated that routine plant breeding activities can be conducted with completely effective pollen containment if the transgenic line is detasseled and serves as the female for pollination with a nontransgenic mule inbred. However, precautions in addition to those used in these experiments are necessary to provide complete control of pollen dissemination if a transgenic male is used to make crosses and [approximately equals] 0.1% outcrossing to adjacent rows is deemed unacceptable., Plant breeding research has undergone great changes in the last decade. Plant breeders have implemented numerous advances in crop improvement technology to increase the speed of genetic improvement and to [...]

A 260-cm sediment core from C.C. Pond, Talladega County, Alabama, yields a pollen sequence spanning the last 33,000 years. Pollen deposition reflects changes in local forest composition through the full-glacial interval. The data are consistent with significant displacement of boreal forest during maximum glaciation, as well as the existence of a more temperate interstadial period preceding the full-glacial.

Helianthus annuus and H. petiolaris are two common annual sunflower species that frequently form hybrid swarms in nature. First generation hybrids suffer highly reduced pollen fertility, at least in part due to chromosomal structural rearrangements differentiating the two species. Previous studies in the Rieseberg lab have demonstrated the importance of rearrangements in limiting molecular marker introgression in synthetic hybrid lineages between Helianthus annuus and H. petiolaris. Here we examine patterns of molecular marker introgression in 139 individuals representing three naturally occurring narrow hybrid zones between these two species in eastern Nebraska. Seeds were collected from plants closest to the H. annuus side of the hybrid zone that appeared to be of mixed parentage, based on morphological intermediacy. Seeds were germinated and plants grown to maturity in the greenhouse. Percent pollen viability and a suite of 13 morphological traits that differentiate the parental species were scored in all plants. All individuals were also surveyed for the presence of species-specific mapped molecular markers characteristic of H. petiolaris. Composite graphical genotypes for each of the three hybrid zones were constructed and compared with those from synthetic hybrid lineages. We found that patterns of molecular marker introgression are far more complex in natural populations than in synthetic lineages. Nonetheless, a number of similarities are evident, both in terms of the frequency and genomic location of introgressed markers. In synthetic lineages, introgression was markedly reduced in non-collinear versus collinear linkages. This pattern is far less striking, though still apparent in natural hybrids. Furthermore, particular linkage blocks that resisted introgression in synthetic lineages also do so in natural hybrids. We also found that pollen viability declines in relation to the number of molecular markers introgressed. Although this relationship is predicted with high levels of introgression, we see evidence of decreased pollen viability with as few as three introgressed molecular markers. Hybrid indices were calculated from morphological data and compared to introgressed marker frequencies and pollen viability. Overall, hybrid indices are poorly correlated with both pollen viability and frequency of molecular marker introgression, suggesting that morphological intermediacy is a poor indicator of hybridity in this system.

Oenothera villaricae Dietrich and Oe. picensis Dietrich, complete translocation heterozygotes, are fully interfertile, giving rise to six discrete classes of true-breeding hybrid from a reciprocal cross. Associated with each parent and hybrid is a characteristic abortive nonstaining pollen fraction easily distinguished from fully developed pollen under the light microscope. Significant reciprocal differences in the abortive fraction between different hybrid accessions suggest partial cytoplasmic control of this phenomenon. Pollen abortion is most severe in the [F.sub.1] hybrid generation, and ameliorates with successive generations of hybrid self-fertilization. Analysis of variance suggests a combined nucleocytoplasmic basis for the pollen abortion, with significant effects also noted for selfed hybrid generation. Correlated with the observation of increased pollen abortion in Oenothera hybrids are meiotic observations of broken chromosome rings (chains, univalents), asymmetric anaphase distributions and trinucleate tetrads. To test the hypothesis that the pollen abortion is due to abnormal meiosis, meiotic disjunction frequency was quantified for each parent, [F.sub.1] and [F.sub.9] selfed hybrid accessions. Analysis of variance shows levels of significance comparable to those noted for pollen stainability for effects of nucleus, cytoplasm and selfed hybrid generation on disjunction frequency. A high degree of correlation ([r.sup.2] = 0.984) is noted between disjunction frequency and pollen stainability. We conclude that the abortive pollen grains are indeed the products of nondisjunctional meiotic events, which themselves are consequences of genome-plastome incompatibility.

Populations of Clarkia tembloriensis have been documented to vary with respect to pollen tube growth rates. Previous investigations have demonstrated that pollen grows more rapidly in an outcrossing population (Idria-2) than in a selling population (Cantua Creek-2). In the present study a series of pollinations were made between plants from the two populations and their hybrids to determine whether differences in pollen tube growth rates and numbers of tubes in the style were due to pollen effects, pistil effects, or both. Reciprocal crosses of all combinations were made. For one experiment flowers were pollinated and their stigma and style samples were removed at 2 hour intervals for 12 hours and at 24 hours post pollination. Fluorescence microscopy was used to determine the number of pollen tubes at the stigma/style junction, 1 cm below the stigma, and at the base of the style. In this experiment Idria-2 had significantly more pollen tubes in all positions of the style than Cantua Creek-2 styles and this trend was maintained through time (3-way ANOVA). Pollen tube numbers in hybrids were intermediate to, and significantly different from Idria-2 and Cantua Creek-2. The source of pollen used did not have an effect on pollen tube numbers in the style except at the stigma/style junction (3-way ANOVA). In a second experiment pollen tube growth rates were determined by measuring the time taken for pollen tubes to germinate, grow the length of the style, and emerge from the cut ends of detached styles. In crosses of pollen and style from the same population, hybrid pollen grew slightly but not significantly faster (0.2902 cm/h) than Idria-2 pollen (0.2804 cm/h) which grew significantly faster than Cantua Creek-2 pollen (0.2378 cm/h). In crosses of pollen and styles from different populations, all pollen ,types tended to grow faster through hybrid styles (0.2845 cm/h), slightly slower through Idria-2 sales (0.2739 cm/h) and significantly slower through Cantua Creek-2 styles (0.2181 cm/h). However. there were slight and significant effects due to pollen type. Our study suggests that the pistil of Clarkia tembloriensis has the strongest influence on pollen tube numbers and growth through the style. However, pollen source also has a small but significant effect on pollen tube growth rates and numbers especially in the upper part of the style.

The extent of pollen limitation in natural populations is still problematic for many herbaceous plants. Burd (1994) demonstrated from survey data that pollen limitation was found in 62% of plant species; however, none of the studies in his survey investigated pollen limitation in a large sample of populations from different habitat types. Trillium grandiflorum is a bee-pollinated, herbaceous perennial common in a variety of habitats in southern Ontario. Controlled hand-pollinations demonstrated that the species is self-incompatible. Comparisons of seed set from open and supplemental hand-pollinations in 18 populations from large woodlands, small woodlands and disturbed sites (roadsides and hedgerows) were undertaken to evaluate the intensity of pollen limitation among populations and habitat types. There was significant variation in the intensity of pollen limitation among populations. However, the extent of pollen limitation was not significantly associated with habitat type, presumably because generalist pollinators that visit T. grandiflorum occur in all three habitat types. Evidence was obtained that T. grandiflorum plants occurring at low density were more susceptible to pollen limitation than those occurring at high density.

SUPERMAN (FLO-10) is a floral mutant in Arabidopsis thaliana that normally produces female sterile flowers. This phenotypic aberration results from a combination of increased stamen number and reduced or abnormal carpels that are non-functional. The flowers of SUPERMAN contain two lateral and four median stamens, as seen in wild-type plants; however, they also contain several additional stamens. The amount of pollen produced from each of the three types of anthers of SUPERMAN and the viability of this pollen was examined and compared to wild-type (cv. Columbia) to determine the consequences of this mutation on male fertility. Both the lateral and the median anthers of SUPERMAN and wild-type plants produced similar amounts of pollen per anther and demonstrated no significant difference in viability. Per anther, the additionals of SUPERMAN produced significantly less pollen than those of the laterals and medians. Furthermore, the pollen produced from these additional anthers was significantly less viable. Currently we are testing the fertility of pollen produced from the various anther types and the germination of the resulting seed.