Pollen Selection Research Articles

Pollen selection - past, present and future

Pollen selection - past, present and future

Pollen selection — past, present and future

A series of studies, recently reviewed, has established that approximately 60% of the structural genes which are expressed in the sporophytic portion of the angiosperm life cycle are also expressed and exposed to selection in the pollen. Given the haploidy and large population sizes of pollen grains, a substantial portion of the sporophytic genome could thus be periodically exposed to a bacterial type of mass screening. This extraordinary possibility is often subject to some skepticism which may, of course, be justified. However, recent attempts to apply models appear to be inappropriate in this context, in part because these attempts overlook an important source of genetic variation, and also because they assume fixed values for selection and fitness. More recently, studies of pollen/pollen interactions have suggested that what Linskens termed the “programic phase” may represent an arena for important, and largely unexplored phenomena, some of which are discussed here.

Effects of pollen selection on progeny vigor in a Cucurbita pepo x C. texana hybrid

We examined the effects of pollen selection for rapid pollen-tube growth on progeny vigor. First, we crossed a wild gourd (Cucurbita texana) to a cultivated zucchini (Cucurbita pepo cv 'Black Beauty') to produce an F1 and then an F2 generation. Half of the F1 seeds were produced by depositing small loads of C. texana pollen onto the stigmas of C. pepo. These small pollen loads were insufficient to produce a full complement of seeds and, consequently, both the fast- and the slow-growing pollen tubes were permitted to achieve fertilization. An F2 generation was then produced by depositing small loads of F1 pollen onto stigmas of F1 plants. The F2 seeds resulting from two generations of small pollen loads are termed the non-selected line because there was little or no selection for pollen-tube growth rate on these plants. The other half of the F1 and F2 seeds were produced by depositing large pollen loads (>10 000 pollen grains) onto stigmas and then allowing only the first 1% or so of the pollen tubes that entered the ovary to fertilize the ovules. We did this by excising the styles at the ovary at 12-15 h after pollination. The resulting F2 seeds are termed 'the selected line' because they were produced by two generations of selection for only the fastest growing pollen tubes. Small pollen loads from the F2plants, both the selected and the non-selected lines, were then deposited onto stigmas of different C. pepo flowers, and the vigor of the resulting seeds was compared under greenhouse and field conditions. The results showed that the seeds fertilized by pollen from the selected line had greater vegetative vigor as seedlings and greater flower and fruit production as mature plants than the seeds fertilized by pollen from the non-selected line. This study demonstrates that selection for fast pollen-tube growth (selection on the microgametophyte) leads to a correlated increase in sporophyte (progeny) vigor.

Anemophilous plants select pollen from their own species from the air.

In wind-tunnel experiments, Niklas (1985) has demonstrated the ability of anemophilous plants to select pollen from their own species from the airstream. However, there have been no field experiments to establish whether this operates in nature. We surveyed the pollents on the stigmas of four different, coextensive, dioecious anemophilous species surrounded by a Pinus radiata plantation. The alien pine pollen was overrepresented relative to background levels on only one of the four species. For all three indigenous species with both male and female plants in the area, the highest proportion - in all cases more than 40% - of the pollen found on their stigmas came from their own species. One indigenous species lacked male plants in the area; consequently, the results from this species are difficult to interpret. However, for all four species, there was at least 15% pine pollen, and some pollen of other indigenous species. These results suggest that there is some pollen selection, but that the mechanisms are may be not as effective as Niklas has suggested.

Pollen selection: a transgenic reconstruction approach.

A transgenic reconstruction experiment has been performed to determine the feasibility of male gametophytic selection to enhance transmission of genes to the next sporophytic generation. For tobacco pollen from a transgenic plant containing a single hygromycin-resistance (hygromycin phosphotransferase, hpt-) gene under control of the dc3 promoter, which is active in both sporophytic and gametophytic tissues, 3 days of in vitro maturation in hygromycin-containing medium was sufficient to result in a 50% reduction of germinating pollen, as expected for meiotic segregation of a single locus insert. Pollination of wild-type plants with the selected pollen yielded 100% transgenic offspring, as determined by the activity of the linked kanamycin-resistance gene--present within the same transferred T-DNA borders--under control of the nos promoter. This is direct proof that selection acting on male gametophytes can be a means to alter the frequency of genes in the progeny.

Effect of Pollen Selection for Alachlor Tolerance in Maize

Pollen selection can lead to correlated responses in the subsequent sporophytes because many genes are expressed in both haploid and diploid phases. This study was conducted to evaluate the effects of pollen selection for tolerance to the herbicide alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide] in maize (Zea mays L.). Tassels of the single cross hybrid K55 (tolerant) x Ky226 (susceptible) were grown in absence (control) and in presence (selected) of alachlor (1 g L -1 ). Mature pollen from this process was used to make backcrosses to Ky226 (susceptibility being recessive). The selected population outperformed the control population (88 vs. 81%; P ≤ 0.01) for percentage of normal seedlings (i.e., not injured by alachlor) when compared in a field trial treated with the herbicide (5 kg a.i. ha -1 ). Pollen of plants grown without alachlor was analyzed in vitro; in presence of the herbicide in the medium (50 mg L -1 ) the selected population exceeded the control population for pollen tube length (334 vs. 308 μm; P ≤ 0.01). Two cycles of pollen selection were also conducted in the single cross hybrid K55 x H99 (susceptible) following the backcross procedure to the susceptible parent. Seeds of control and selected populations were exposed to alachlor (1 g L -1 ), and seedlings were grown in the greenhouse. For percentage of normal seedlings, we obtained a gain (16%) in the first cycle and an additional gain (10%) in the second cycle. The results indicate that genes controlling alachlor tolerance in maize are expressed in both haploid and diploid phases and that pollen selection can be effectively applied to improve the trait.

Effect of Pollen Selection for Alachlor Tolerance in Maize

Pollen selection can lead to correlated responses in the subsequent sporophytes because many genes are expressed in both haploid and diploid phases. This study was conducted to evaluate the effects of pollen selection for tolerance to the herbicide alachlor [2‐chloro‐N‐(2,6‐ diethylphenyl)‐N‐(methoxymethyl) acetamide] in maize (Zea mays L.). Tassels of the single cross hybrid K55 (tolerant) ✕ Ky226 (susceptible) were grown in absence (control) and in presence (selected) of alachlor (I g L−1). Mature pollen from this process was used to make backcrosses to Ky226 (susceptibility being recessive). The selected population outperformed the control population (88 vs. 81%; P ≤ 0.01) for percentage of normal seedlings (i.e., not injured by alachlor) when compared in a field trial treated with the herbicide (5 kg a.i. ha−1). Pollen of plants grown without alachlor was analyzed in vitro; in presence of the herbicide in the medium (50 mg L−1) the selected population exceeded the control population for pollen tube length (334 vs. 308 μm; P ≤ 0.01). Two cycles of pollen selection were also conducted in the single cross hybrid KS5 ✕ H99 (susceptible) following the backcross procedure to the susceptible parent. Seeds of control and selected populations were exposed to alachlor (I g L−1), and seedlings were grown in the greenhouse. For percentage of normal seedlings, we obtained a gain (16%) in the first cycle and an additional gain (10%) in the second cycle. The results indicate that genes controlling alachlor tolerance in maize are expressed in both haploid and diploid phases and that pollen selection can be effectively applied to improve the trait.

Effect of Pollen of Five Different Male Palm Trees on the Chemical Characteristics of Jiroft Mazafati Date

The type of pollen of date palm trees could affect the chemical characteristics of the fruit, and determine the quality and quantitative aspects. Careful selection of proper pollen for pollination of date palm is very important. This research was conducted for the selection of the best pollinizers and to study the effects of five varieties of male trees from the Shahdad area on the chemical characteristics of `Jiroft Mazafati' date. A completely random block was used for analysis of the 2 years of data (1991–1993). The study sight was in the Jiroft area of Kerman Province, Iran. Samples of fruit were collected after the pollination process and the required information recorded. Total dry matter, fruit moisture, total sugar content, fruit ash, and fruit pH were measured. Statistical calculations analysis and compound variations and evaluations of the treatments by Duncan's test indicate that, statistically, the different treatments have no effect on the fruit pH, and the difference are not significant. There are statistically significant effects on the other chemical characteristics of the fruit.

The Effect of Pollens of Five Different Male-palm Trees on the Physical Characteristics of Jiroft Mazafati Date C.V.

The type of pollens of the date palm trees could affect on the outward and physical characteristics of date fruits, so far to determine its qualitative and quantitative aspects (15). Therefore careful selection of proper pollen for pollination of the date palms is considered of high importance. This research work was conducted to select the best pollinizers and to study the effects of pollens of five varieties of male trees, from Shahdad area, on the outward and physical characteristics of `Jiroft Mazafati'. For analyzing the results of this experiment, a completely randomized block statistical project was implemented for a period of 2 years from 1991 to 1993. The site of this experiment was in Jiroft area in Kerman Province. In this experiment, after pollination samples of fruit were collected during unripe (Kharak), ripe, and dry fruit stages and required information were recorded. Fruit length and diameter, seed length and diameter, weight of fruit, weight of seed, pulp weight, ration of fruit pulp to seed, ratio of fruit length to diameter, volume of fruit, weight of one full cluster of fruits, and finally the time of ripening were measured. Statistical calculations, analysis of compound variations and evaluation of treatments by Duncan's test indicate that different treatments of pollens will have no effect from statistical point of view on the ratio of fruit pulp to seed and the ratio of fruit length to diameter, and there are no significant differences. But there are significant effects on the other outward and physical characteristics of the fruit, and the differences from statistical point of view are considerable. The type of pollens are effective on the ripening time of the fruit and this difference ranges from 15 to 25 days for different treatments, which is important from early ripening point of view of the product.

Sporophytic and Gametophytic Components of Thermotolerance Affected by Pollen Selection

This study was undertaken to assess the effect of in vivo male gametophytic selection on components of thermotolerance in maize. The F1 between a tolerant (Pa33) and a susceptible (B37) genotype was used as pollen source. Male gametophytic selection was carried out by applying a two-h treatment of 50°C to immature tassels in the field. The resulting F2 and backcross generations were evaluated for sporophytic and gametophytic components of thermotolerance, including cellular membrane stability, heat shock protein synthesis, pollen germination, and tube growth rate. Both expression of heat shock proteins 102, 84, and 17 kDa and pollen germination ability under a high-temperature regime were significantly higher in the selected than in the nonselected progenies. No response to selection was detected for cellular membrane stability and pollen tube growth rate. These results suggest that pollen selection can serve to improve at least some gametophytic and sporophytic components of thermotolerance.

EFFECTS OF MATERNAL AND PATERNAL ENVIRONMENT AND GENOTYPE ON OFFSPRING PHENOTYPE IN SOLIDAGO ALTISSIMA L.

To predict the possible evolutionary response of a plant species to a new environment, it is necessary to separate genetic from environmental sources of phenotypic variation. In a case study of the invader Solidago altissima, the influences of several kinds of parental effects and of direct inheritance and environment on offspring phenotype were separated. Fifteen genotypes were crossed in three 5 × 5 diallels excluding selfs. Clonal replicates of the parental genotypes were grown in two environments such that each diallel could be made with maternal/paternal plants from sand/sand, sand/soil, soil/sand, and soil/soil. In a first experiment (1989) offspring were raised in the experimental garden and in a second experiment (1990) in the glasshouse. Parent plants growing in sand invested less biomass in inflorescences but produced larger seeds than parent plants growing in soil. In the garden experiment, phenotypic variation among offspring was greatly influenced by environmental heterogeneity. Direct genetic variation (within diallels) was found only for leaf characters and total leaf mass. Germination probability and early seedling mass were significantly affected by phenotypic differences among maternal plants because of genotype (genetic maternal effects) and soil environment (general environmental maternal effects). Seeds from maternal plants in sand germinated better and produced bigger seedlings than seeds from maternal plants in soil. They also grew taller with time, probably because competition accentuated the initial differences. Height growth and stem mass at harvest (an integrated account of individual growth history) of offspring varied significantly among crosses within parental combinations (specific environmental maternal effects). In the glasshouse experiment, the influence of environmental heterogeneity and competition could be kept low. Except for early characters, the influence of direct genetic variation was large but again leaf characters (= basic module morphology) seemed to be under stricter genetic control than did size characters. Genetic maternal effects, general environmental maternal effects, and specific environmental maternal effects dominated in early characters. The maternal effects were exerted both via seed mass and directly on characters of young offspring. Persistent effects of the general paternal environment (general environmental paternal effects) were found for leaf length and stem and leaf mass at harvest. They were opposite in direction to the general environmental maternal effects, that is the same genotypes produced "better mothers" in sand but "better fathers" in soil. The general environmental paternal effects must have been due to differences in pollen quality, resulting from pollen selection within the male parent or leading to pre- or postzygotic selection within the female parent. The ranking of crosses according to mean offspring phenotypes was different in the two experiments, suggesting strong interaction of the observed effects with the environment. The correlation structure among characters changed less between experiments than did the pattern of variation of single characters, but under the competitive conditions in the garden plant height seemed to be more directly related to fitness than in the glasshouse. Reduced competition could also explain why maternal effects were less persistent in the glasshouse than in the garden experiment. Evolution via selection of maternal effects would be possible in the study population because these effects are in part due to genetic differences among parents.

Haplo-diploid gene expression and pollen selection for tolerance to acetochlor in maize

The objectives of this research were to determine if genes controlling the reaction to the herbicide acetochlor in maize (Zea mays L.) are active during both the haploid and the diploid phases of the life cycle and if pollen selection can be utilized for improving sporophytic resistance. Pollen of eight inbred lines, previously characterized through sporophytic analysis for the level of tolerance to acetochlor, showed a differential reaction to the herbicide forin vitro tube length; moreover, such pollen reactions proved to be significantly correlated (r =0.786(*),df=6) with those of the sporophytes producing the pollen. Pollen analysis of two inbred lines (i.e. Mo17, tolerant, and B79, susceptible) and their single cross showed that thein vitro pollen-tube length reaction of the hybrid was intermediate between those of two parents. An experiment on pollen selection was then performed by growing tassels of Mo17xB79 in the presence of the herbicide. Pollen obtained from treated tassels showed a greater tolerance to acetochlor, assessed asin vitro tube length reaction, than pollen obtained from control tassels. Moreover, the backcross [B79 (Mo17xB79)] sporophytic population obtained using pollen from the treated tassels was more tolerant (as indicated by the fresh weight of plants grown in the presence of the herbicide) than was the control backcross population. The two populations did not differ when grown without the herbicide. These findings indicate that genes controlling the reaction to acetochlor in maize have haplodiploid expression; consequently, pollen selection can be applied for improving plant tolerance.

Sporophytic response to pollen selection for Alachlor tolerance in maize

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.

Pollination density - effects on pollen germination and pollen tube growth in Betula pubescens Ehrh. in northern Sweden.

The aim of this study was to test the hypothesis that increased pollination enhances the probability of'pollen competition'in Betula pubescent. In an in vitro test, the germination of pollen from three B. pubescens trees, increased with increasing pollen density (r2 = 0·82, 0·81 and 0·82, respectively). Pollen tubes grew at different rates in styles when testing different paternal/maternal combinations. Evidence was obtained to suggest that these differences might be attributed to both maternal and paternal effects. Further, increasing the number of tubes per style tended to increase the length of the longest tube per style, but had no significant effect on the mean length of the tubes. The r values, for the correlations of lengths with numbers of pollen tubes per style, were lower in two natural B pubescens populations than in the data from a controlled pollination experiment, is concluded that selection among male gametes may occur after supplementary pollination in B. pubescens, but in natural situations, pollen deposition is usually too low for pollen Selection to be a common occurrence.

Clonal Repeatability of In Vitro Pollen Tube Growth Rates in Oenothera organensis (Onagraceae)

The concept that pollen selection is an important force driving angiosperm evolution has received a lot of attention in recent years. In order for pollen competition to have evolutionary significance, heritable variation for pollen tube growth rate must exist. In this study, I tested the clonal repeatability, a measure of broad-sense heritability, of in vitro pollen tube growth rate in Oenothera organensis. Although I found significant phenotypic differences in pollen tube growth rate, the calculated clonal repeatability value of 9.4% indicates that heritability of pollen tube growth rate in this species is quite low. In contrast, both flower identity and plant identity had highly significant effects on pollen tube growth rate, suggesting that the environment may be more important than genotype in determining pollen performance in this species.

Clonal repeatability of in vitro pollen tube growth rates in Oenothera organensis (Onagraceae)

The concept that pollen selection is an important force driving angiosperm evolution has received a lot of attention in recent years. In order for pollen competition to have evolutionary significance, heritable variation for pollen tube growth rate must exist. In this study, I tested the clonal repeatability, a measure of broad‐sense heritability, of in vitro pollen tube growth rate in Oenothera organensis. Although I found significant phenotypic differences in pollen tube growth rate, the calculated clonal repeatability value of 9.4% indicates that heritability of pollen tube growth rate in this species is quite low. In contrast, both flower identity and plant identity had highly significant effects on pollen tube growth rate, suggesting that the environment may be more important than genotype in determining pollen performance in this species.

Selection for tolerance to copper during pollen formation in Mimulus guttatus Fischer ex DC

In Mimulus guttatus, copper tolerance is determined largely by a single gene and is expressed in both the sporophyte and microgametophyte. This study explores the extent to which selection during pollen formation affects copper tolerance in the sporophytic generation. Two sets of plants heterozygous for copper tolerance, produced by reciprocal crosses between different copper-tolerant or sensitive families, and the plant on which the original observations were based, were cloned and grown in control or copper-supplemented solutions. Pollen viability and the number of tolerant progeny produced in backcrosses to sensitive plants were compared. In addition, the effect of copper treatment on pollen viability in vitro was compared for plants tolerant, sensitive and heterozygous for copper tolerance. The extent to which in vitro pollen viability decreased in response to copper treatment corresponded to the copper tolerance of the pollen source. When grown with added copper, four of the five plants showed significant reductions in pollen viability, ranging from 18% to 48% of control values. The reductions in pollen viability were correlated with an increase in tolerant progeny (r= 0.679, p=0.004). Increases in tolerant progeny could be large, ranging from 119% to 170% of that of controls, but were usually smaller than was predicted from the reductions in viable pollen. In addition, plants derived from reciprocal crosses differed significantly in the extent to which pollen viability was decreased and sporophytic tolerance increased. Thus, while selection during pollen formation could increase sporophytic tolerance, sporophytic factors, perhaps including cytoplasmic or epigenetic ones, moderated the effectiveness of pollen selection for copper tolerance.

Use of Radiation to Transfer Alien Chromosome Segments to Wheat

Ionizing radiation can accomplish the transfer of genetic information from species so distantly related to wheat (Triticum aestivum L. em Thell.) that their chromosomes pair very little, if at all, with those of wheat, even in the absence of the homoeologous‐pairing suppressor Ph1. In a successful transfer, the alien segment must almost always replace a homoeologous wheat segment, but radiation induces translocations largely at random; therefore automatic selection in favor of desirable translocations must be provided if the size of the project is to be kept within reasonable limits. Pollen selection will occur if seeds or plants monosomic for both an alien chromosome and one of its wheat homoeologues are irradiated. Making the plants also deficient for Ph1 may increase the number of suitable transfers. High‐frequency occurrence of the desired alien character in M2 head‐rows from plants grown from irradiated seed can identify favorable transfers with little cytological work. Irradiation of plants shortly before meiosis, using them to pollinate ditelosomics or double ditelosomics for the wheat arm or chromosome concerned, and cytologically examining offspring which have the alien character can not only identify the desirable transfers, but also reveal the lengths of the alien segments involved.

Pollen selection for Alternaria resistance in oilseed brassicas: responses of pollen grains and leaves to a toxin of A. brassicae

The effects of destruxin B, a host-specific toxin of Alternaria brassicae that causes black spot disease in oilseed brassicas, were studied on in vitro pollen germination and pollen-tube growth of Brassica campestris var "brown sarson", B. juncea, B. napus cvs "Westar" and "Cresor", B. nigra and Sinapis alba. Pollen grains of B. nigra, B. juncea and B. campestris were the most sensitive and those of S. alba the least sensitive to the toxin. Effects of the toxin were also studied on the leaves of these species, and the degree of sensitivity of leaves of different species was comparable to that of their pollen grains. The results on the responses of pollen grains as well as leaves to the toxin are in agreement with the degree of susceptibility/resistance of these species to A. brassicae reported in the literature, indicating that the genes imparting susceptibility/restistance are expressed in the pollen, a prerequisite for pollen selection. Results are also presented which show that the toxin fed to the cut end of isolated inflorescence axis is readily taken up by the developing pollen and results in the inhibition of germination of susceptible pollen. This technique offers a simple and effective method for application of selection pressure to eliminate pollen grains susceptible to the toxin from effecting fertilization.

Isolation of two independent allyl alcohol resistant Adh-1 null mutants in tomato following selection of pollen and seeds

The Adh-1 null mutant (B15-1-8) isolated previously was used to establish conditions that allow the selection of ADH-deficient pollen grains and seeds of tomato. New Adh-1 null mutants were then selected among the progenies derived from crosses between the genetically unstable tomato lines Yv ms, Sulfurea and 2s-Iso and the Adh-1 null B15-1-8 mutant. Treatment of 158 500 F 1 seeds with allyl alcohol led to the identification of a new Adh-1 null mutant (OYO 424). Similarly, fertilization of B15-1-8 with allyl alcohol-treated pollen collected from the mutable lines resulted in the recovery of another Adh-1 null mutant (BOY 421). Like the mutant B15-1-8, OYO 424 and BOY 421 were found to lack Adh-1 activity in both pollen and seeds. Genetic analysis showed that the mutations in both mutants were allelic to the Adh-1 locus on chromosome 4. Southern analysis, using as a probe Adh-1 genomic clones spanning the entire coding region and 2 kb of the upstream region, suggested that the mutations of the Adh-1 alleles in B15-1-8, OYO 424 and BOY 421 were due to small alterations and not to the insertion of a transposable element. Remarkably, the mutant allele of B15-1-8 was highly unstable, reverting to the wild type allele at a frequency of 5 × 10 −4.