Analysis of pollen-style interactions in Petunia hybrida; the determination of variance in male reproductive success

Summary A factorial cross design was used to evaluate the influence of differences among pollen donors and recipients on variation in stylar attrition of pollen tubes in self-fertile plants of Petunia hybrida. Pollinations to flower buds were used to reduce the inhibitory ability of the style and these crosses were compared with flower pollinations to assess the degree of stylar influence on pollen fertilization ability. There was less pollen tube attrition after bud pollinations than after flower pollinations, indicating that styles of buds were less able to inhibit pollen tubes. The variance component for plants acting as pollen donors tended to be greater after flower pollinations than after bud pollinations. The lower variance in male success after bud pollinations indicates that differences among pollen donors after flower pollinations were due to stylar inhibition of pollen rather than differences in pollen vigor. Since the level of variation in pollen growth after pollination to flowers was greater among clones than among ramets within clones, the differences in pollen fertilization ability are probably genetically based.     

Self-incompatibility and interspecific incompatibility: relationships in intra- and interspecific crosses of Zinnia elegans Jacq. and Z. angustifolia HBK (Compositae)

Summary Intraspecific and reciprocal interspecific crosses involving Zinnia angustifolia clones and Z. elegans lines showed that in both species, sporophytic self-incompatibility (SI) systems were present. Intensity of SI varied among clones and lines, and high self seed set was associated with a concomitant decrease in callose fluorescence in papillae and pollen tubes. Incomplete stigmatic inhibition of pollen germination and tube growth was observed in reciprocal interspecific crosses and associated with callose synthesis, suggesting S-gene activity. Seed set and progeny obtained following Z. angustifolia×Z. elegans matings was comparable to intraspecific compatible matings of Z. angustifolia although the rate of pollen tube growth through the style was slower. In Z. elegans × Z. angustifolia matings, additional prezygotic barriers were present and acted between pollen tube penetration of the stigma and syngamy. SI X SI interspecific incompatibility was essentially unilateral, with no embryos or progeny obtained when Z. elegans was the pistillate parent. It was hypothesized that nonfunctioning of Z. elegans × Z. angustifolia crosses was due to S-gene expression at the stigmatic surface and to other isolating mechanisms in the stylar or ovarian transmitting tissue.Scientific Article No. A-4448, Contribution No. 7439 of the Maryland Agricultural Experiment Station, Department of HorticultureA portion of this paper was presented in the report: Boyle TH, Stimart DP (1986) Incompatibility relationships in intra- and interspecific crosses of Z. elegans Jacq. and Z. angustifolia HBK (Compositae). In: Mulcahy D (ed) Biotechnology and ecology of pollen. Springer, New York     

Interspecific incompatibility due to stylar barriers in tuber-bearing and closely related non-tuber-bearing Solanums

Summary The phenomenon of interspecific incompatibility between various wild tuber-bearing and closely related non-tuber-bearing Solanum species was studied. One area of investigation included an examination of possible protein interactions in the incompatibility reaction using SDS electrophoresis. Pollen tube inhibition and morphology were examined in conjunction with biochemical analysis. Two sets of crosses were examined: interspecific tuber-bearing species crosses and interspecific tuber-bearing × non-tuber-bearing species crosses. These crosses had consistent pollen tube inhibition in the upper one-third of the style. The upper third of the styles of incompatibly pollinated, compatibly pollinated, and unpollinated styles was studied under fluorescence microscopy to observe pollen tube growth and morphology. Interspecific tuber-bearing × non-tuber-bearing species crosses demonstrated consistent pollen tube inhibition just below the stigma with frequent pollen tube swelling and bursting and extensive callose deposition along the pollen tube wall. Interspecific tuber-bearing species crosses had pollen tube inhibition further down the style with pollen tube tip tapering and extensive callose deposition. Stylar proteins of the lower two-thirds of the styles were analyzed with SDS electrophoresis. No unique protein differences were found to be specifically associated with the interspecific incompatibility reaction in this portion of the style.Cooperative investigation of the U.S. Department of Agriculture, Agricultural Research Service, and the Wisconsin Experiment Station. Supported in part by the USDA, Cooperative States Research Service Competitive grant no. 83-CRCR-1-1253     

Relationships of pollen size,pistil length and pollen tube growth rates in Rhododendron and their influence on hybridization

Summary Pollen size and pistil length data have been collected for 93 species of Rhododendron (Ericaceae) belonging to a number of different subgeneric taxa. For a sample of eight species in section Vireya, pollen tube growth in the style after selfor interspecific pollination has been quantified. Pollen volume and the time taken for pollen tubes to reach the ovary were both related to pistil length. Pollen-tube growth rates were generally greater for species with longer pistils and larger pollen. Increasing temperature increased the rate of pollen-tube growth. There was no detectable effect of pollen tube density on tube growth rate in the style. After interspecific pollinations tube growth rates in foreign styles could be faster or slower than in self styles. A semisterile individual with two viable pollen grains per tetrad and a plant grafted as scion to a longer-styled stock both showed more rapid pollen-tube growth than expected on the basis of pistil size. Data collected for 26 species in section Vireya showed that where extreme disparity of pollen/pistil size causes failure of interspecific crosses, one or more bridging species with intermediate pollen/pistil size can generally be selected.     

Self-incompatibility in a distylous species of Rubiaceae: is there a single incompatibility response of the morphs?

Heterostyly is a genetically controlled floral polymorphism usually associated with an incompatibility system. This set of features is known to occur in several angiosperm families, but some aspects of its biology has not been well studied. The present study investigates cellular aspects of the pollen–pistil interaction after compatible and incompatible pollinations of Psychotria nuda, to increase our knowledge of heteromorphic self-incompatibility (HetSI). The use of bright field, fluorescence and transmission electron microscopy methods allowed us to demonstrate that pollen tubes behave differently after incompatible and compatible pollinations. Pollen tubes were particularly distinct after incompatible pollinations of L- and S-morph flowers. Relative to compatible pollen tubes, incompatible L-morph tubes had a drastic reduction in cellular contents, but no cell rupture. Incompatible S-morph tubes exhibited dense cytoplasm in apical regions, as well as in other regions, accompanied by a rupture of the apex. These results support the hypothesis that L- and S-morph flowers have different incompatibility mechanisms during HetSI.     

Unilateral incongruity in crosses involvingLycopersicon pennellii andL. esculentum is distinct from self-incompatibility in expression,timing and location

Both interspecific and intraspecific mechanisms restrict the exchange of genes between plants. Much research has focused on self incompatibility (SI), an intraspecific barrier, but research on interspecific barriers lags behind. We are using crosses betweenLycopersicon esculentum andL. pennellii as a model with which to study interspecific crossing barriers. The crossL. esculentum×L. pennellii is successful, but the reciprocal cross fails. Since the cross can be successfully made in one direction but not the other, gross genomic imbalance or chromosomal abnormality are precluded as causes. We showed that the lack of seed set observed in the crossL. pennellii×L. esculentum is due to the inability of pollen tubes to grow more than 2–3 mm into the style, whereas S1 crosses show continued slow pollen tube growth but, also, fail to set seed. These results indicate that the unilateral response is a barrier distinct from SI, differing from SI in the timing and location of expression in the style. We therefore suggest that this unilateral response in theL. pennellii×L. esculentum cross is more accurately referred to as unilateral incongruity (UI) rather than interspecific incompatibility. Periclinal chimeras were used to determine the tissues involved in UI. The results of crosses with the available chimeras indicate that the female parent must beL. pennellii at either LI (layer 1) or both LI and LII (layer 2) and the male parent must beL. esculentum at either LII or both LI and LII to observe UI similar to that seen in theL. pennellii×L. esculentum cross. Pollinations with a mixture of pollen fromL. pennellii and from transgenicL. esculentum plants harboring a pollen-specific GUS reporter gene marker were used to ascertain whether the growth of the pollen tubes of either species was modified as a possible means of overcoming UI. We found no evidence of communication between the two types of pollen tubes to either enhance or restrict all pollen tube growth.     

Pollen tube growth in the self‐compatible sweet cherry genotype, ‘Cristobalina’, is slowed down after self‐pollination

Sweet cherry is a self‐incompatible fruit tree species in the Rosaceae. As other species in the family, sweet cherry exhibits S‐RNase‐based gametophytic self‐incompatibility. This mechanism is genetically determined by the S‐locus that encodes the pollen and pistil determinants, SFB and S‐RNase, respectively. Several self‐compatible sweet cherry genotypes have been described and most of them have mutations at the S‐locus leading to self‐compatibility. However, ‘Cristobalina’ sweet cherry is self‐compatible due to a mutation in a pollen function modifier that is not linked to the S‐locus. To investigate the physiology of self‐compatibility in this cultivar, S‐locus segregation in crosses involving ‘Cristobalina’ pollen, and pollen tube growth in self‐ and cross‐pollinations, were studied. In the crosses with genotypes sharing only one S‐haplotype, the non‐self S‐haplotype was inherited more frequently than the self S‐haplotype. Pollen tube growth studies revealed that the time to travel the whole length of the style was longer for self‐pollen tubes than for cross‐pollen tubes. Together, these results suggest that ‘Cristobalina’ pollen tube growth is slower after self‐pollination than after cross‐pollination. This reproductive strategy would allow self‐fertilisation in the absence of compatible pollen but would promote cross‐fertilisation if cross‐compatible pollen is available, a possible case of cryptic self‐incompatibility. This bet‐hedging strategy might be advantageous for an ecotype that is native to the mountains of the Spanish Mediterranean coast, in the geographical limits of the distribution of this species. ‘Cristobalina’ blooming takes place very early in the season when mating possibilities are scarce and, consequently, self‐compatibility may be the only possibility for this genotype to produce offspring.     

Interactions of S alleles in sporophytically controlled self incompatibility of Brassica

Summary The expressed activity in pollen and stigma was determined for both S alleles of sixteen S-alíele heterozygous genotypes and for one of the two S alleles of two additional heterozygotes. Activities were measured using pollen tube penetration and seed set data from reciprocal crosses between each S-allele heterozygote and its two corresponding S-allele homozygotes.In pollen the S-allele activities ranged from zero to 100% inhibition of pollen tube penetration and seed set, and in the stigma they ranged from 8 to 100% inhibition. Of the sixty-eight S-allele activities measured, thirty-three (48%) were 90 to 100% inhibition, nine (13%) were 80 to 89% inhibition and one to five were within each ten-unit range below 80% inhibition.In an S-allele heterozygote, each subset of two S alleles had an activity for each allele in both pollen and stigma which was highly repeatable among duplicate pollinations within and among successive years. Each subset of two S alleles had a specific S-allele interaction in the pollen, and the same or another specific interaction in the stigma. In pairings with six other S alleles, allele S ₂ had four calculated levels of activity in pollen that ranged from 88 to 94%, and five levels in the stigmas between 15 and 94%. When paired in a heterozygote, alleles S ₃ and S ₅ had activities ranging between 42 and 59%, representing mutual weakening of S-allele activity. Also, heterozygote S ₁₅ S ₃ had pollen activities, respectively, of 25 and 6%, i.e. mutual weakening in the pollen.These results indicate that in heterozygous combination with a series of other S alleles, each S-allele may have activity in pollen and also in stigma that potentially is between zero and 100% inhibition. They further indicate that the defined sexual-organ X S-allele-interaction Types I, II, III and IV are extremes; all intermediate variations including complete weakening of both alleles are possible. Recessiveness is weakening of the activity of but one of the two S alleles. The pollen tube penetrations into the style and seed set were highly correlated.Department of Plant Breeding and Biometry Paper No. 683     

Fluorescence and scanning electron microscopic study on self-incompatibility in distylous Averrhoa carambola L.

Various combinations of intermorph, selfing and intramorph pollinations were carried out in Averrhoa carambola and the pollinated pistils were observed under fluorescence and scanning electron microscopes in time-course experiments. In both compatible and incompatible pollinations, similar behavior of pollen germination and penetration was observed in the first 4 h after pollination. In compatible intermorph pollination, pollen tubes were found at the base of the transmitting tract of the style at 8 h and 24 h after pollination in both the pin and thrum morphs. With thrum flowers, selfing resulted in pollen tubes being uniformly arrested at the junction between the stigmatic and stylar tissues. Penetration of pollen tubes into the upper portion of style was observed in thrum intramorph pollination and, when the second member was treated as the gynoecial tissue under the same pollination, penetration of tubes was further enhanced. Pin flowers, on selfing, resulted in pollen tube penetration farther down the style than was the case with thrum selfing. Intramorph pollination of pin morph behaved in a similar manner to selfing and was not affected by genotypes. Beside the stamen-style dimorphism, the receptive surface of the cob stigma was larger in pin than thrum flowers. While pin pollen was round, thrum pollen was oblong in shape with pin to thrum ratio on the polar axis being 1.2 and on the equatorial axis 0.8. The stigma of pin morph belonged to the dry type, while that of the thrum resembled the wet type.     

Development of SSR markers for studies of diversity in the genus <Emphasis Type="Italic">Fagopyrum</Emphasis>

The numbers of SSR markers and their utilization have not been determined and investigated as extensively in Fagopyrum species as compared to other crop species. The current report presents 136 new SSR markers in Fagopyrum esculentum ssp. esculentum and their application to related species in the genus Fagopyrum. Of the 136 SSRs, 10 polymorphic SSR markers were utilized in a genetic diversity analysis of a common buckwheat population consisting of 41 accessions of diverse origin. The study showed observed (H _O) and expected (H _E) heterozygosities ranging from 0.071 to 0.924 (mean = 0.53) and from 0.073 to 0.902 (mean = 0.412), respectively. Forty-one of the 136 SSRs amplified sequences in other Fagopyrum species, including the cymosum and urophyllum groups. The phylogenetic relationships revealed using the SSRs was consistent with results obtained using other marker systems, with one exception. The sequence and diversity information obtained using these new SSRs and their cross-transferability to related Fagopyrum species will increase our understanding of genetic structures and species relationships within the Fagopyrum genus.     

Progress and prospects for interspecific hybridization in buckwheat and the genus Fagopyrum

Cultivated buckwheat, such as common (Fagopyrum esculentum Moench.) and tartary (Fagopyrum tataricum (L.) Gaertn.) buckwheat, is one of the most versatile crops for forage and food and has several benefits for human health. Interspecific hybridization between Fagopyrum species is of great importance to improvement of buckwheat. Hybridization would allow the transfer of agronomical beneficial characteristics from wild Fagopyrum species, including self-pollination and increased fertility, frost tolerance, and higher content of beneficial compounds. However, conventional breeding methods are only partially applicable because of the self-incompatibility and incompatibility barriers between different species. Present review summarizes the morphology of self-incompatibility, the genetic and cellular basis of incompatibility between different Fagopyrum species. In many interspecific crosses hybrid embryos are aborted after successful pollination due to post-zygotic incompatibility. The use of in vitro embryo rescue after interspecific hybridization has been successful in circumventing breeding barriers between Fagopyrum species. Methods applied successfully for the construction of interspecific hybrids are discussed in detail.     

Procedures for identifying S-allele genotypes of Brassica

Summary Procedures are described for efficient selection of: (1) homozygous and heterozygous S-allele genotypes; (2) homozygous inbreds with the strong self- and sib-incompatibility required for effective seed production of single-cross F₁ hybrids; (3) heterozygous genotypes with the high self- and sib-incompatibility required for effective seed production of 3- and 4-way hybrids.From reciprocal crosses between two first generation inbred (I₁) plants there are three potential results: both crosses are incompatible; one is incompatible and the other compatible; and both are compatible. Incompatibility of both crosses is useful information only when combined with data from other reciprocal crosses. Each compatible cross, depending on whether its reciprocal is incompatible or compatible, dictates alternative reasoning and additional reciprocal crosses for efficiently and simultaneously identifying: (A) the S-allele genotype of all individual I₁ plants, and (B) the expressions of dominance or codominance in pollen and stigma (sexual organs) of an S-allele heterozygous genotype. Reciprocal crosses provide the only efficient means of identifying S-allele genotypes and also the sexual-organ x S-allele-interaction types.Fluorescent microscope assay of pollen tube penetration into the style facilitates quantitation within 24–48 hours of incompatibility and compatibility of the reciprocal crosses. A procedure for quantitating the reciprocal difference is described that maximizes informational content of the data about interactions between S alleles in pollen and stigma of the S-allele-heterozygous genotype.Use of the non-inbred I_o generation parent as a known heterozygous S-allele genotype in crosses with its first generation selfed (I₁) progeny usually reduces at least 7 fold the effort required for achieving objectives 1, 2, and 3, compared to the method of making reciprocal crosses only among I₁ plants.Identifying the heterozygous and both homozygous S-allele genotypes during the I₁ generation facilitates, during subsequent inbred generations, strong selection for or against modifier genes that influence the intensity of self- and sib-incompatibility. Selection for strong self and sib incompatibility can be effective for both homozygous inbreds and also for the S-allele heterozygote, thus facilitating production of single-cross F₁ hybrids and also of 3-and 4-way hybrids.Department of Plant Breeding and Biometry paper No. 690     

The diversity of interspecific pollen-pistil incongruity in Nicotiana

Nicotiana tabacum was used as a pistillate parent and crossed with three self-compatible species, N. rustica, N. repanda and N. trigonophylla, which were previously reported to have pollen tubes unilaterally inhibited by N. tabacum pistil. Temporal and morphological observations revealed distinct differences of pollen tube behavior among these incongruous crosses. Pollen tubes of N. repanda were arrested in stigma and those of N. rustica in the middle of the style. On the other hand, pollen tubes of N. trigonophylla continued growing at a slow rate. Tubes of N. repanda and N. rustica showed morphological abnormalities such as swelling, thick wall, and irregular callose deposition. In addition, tubes of N. rustica often elongated in reverse direction and wound about in the middle of the style. Although the tubes of N. trigonophylla were apparently normal in morphology, they were distributed throughout the transmitting tissue, differing from the self-pollination of N. tabacum in which they were confined to the peripheral region of it. The diversity of pollen tube behavior indicates that physiological causes of incongruity are different among the three crosses. Bud pollination enabled pollen tubes to reach the ovary in all crosses, indicating that the N. tabacum pistil acquired its ability to inhibit foreign pollen tube elongation with its development. When interspecific hybrids between N. tabacum and the other three species were pollinated by parental species, tubes reached the ovary in all crosses, but the elongation rate of tubes slowed down and morphology was abnormal.     

Effects of Protein Phosphatase Inhibitors and Calcium Antagonists on Self-Incompatible Reaction in Buckwheat

Isolated pistils of dimorphic buckwheat (Fagopyrum esculentum Moench.) flowers were treated with phosphatase inhibitors (ocadaic acid and cantharidin) and with calcium antagonists (verapamil, La³⁺, and A23187). They were subsequently cross- or self-pollinated, and the growth of pollen tubes was observed under the fluorescence microscope. All treatments suppressed inhibition of pollen tubes growth suggesting that protein phosphatases and calcium signaling may be involved in self-incompatibility signal transduction in buckwheat. This revised version was published online in July 2006 with corrections to the Cover Date.     

Interspecific crosses in the genus Tulipa L.: identification of pre-fertilization barriers

 Pollen tube growth in the pistil and pollen tube penetration of ovules have both been studied in crosses between cultivars from Tulipa gesneriana L. and 12 tulip species from all eight sections of the genus Tulipa to identify pre-fertilization barriers. Depending on the cross, pollen tubes grew as far as the stigma or the style or continued growing down into the ovary. Pollen tubes penetrated none or only a few percent of the ovules of some crosses, despite the presence of many pollen tubes in the ovary. In other crosses, from which no or only a few hybrids have been obtained after seed maturation on the plant, pollen tube penetration was found in up to 79% of the ovules. Apparently, various kinds of barriers preventing fertilization or normal embryogenesis occur in interspecific tulip crosses. Received: 26 July 1996 / Revision accepted: 31 January 1997     

Heterostyly promotes compatible pollination in buckwheats: Comparisons of intraflower,intraplant, and interplant pollen flow in distylous and homostylous Fagopyrum

Premise of the Study

Heterostyly, the reciprocal positioning of stigmas and anthers in different floral morphs, has long been thought to promote intermorph pollination. However, extensive intramorph pollination occurs commonly in heterostylous species, leading to recurrent questions about the functional and evolutionary significance of heterostyly.

Methods

To identify the sources of stigmatic pollen (autogamous [intraflower], geitonogamous [intraplant], vs. interplant), we emasculated either one flower or entire plants in experimental populations of the two closely related buckwheat species, distylous Fagopyrum esculentum and homostylous F. tataricum. Differences in pollen size allowed unambiguous identification of pollen on stigmas.

Results

Only 2.4% of F. tataricum pollen and 1.5% of F. esculentum pollen arrived successfully on compatible stigmas of other plants. In the former (homostylous) species, 71.3% of the pollen load on stigmas was autogamous, 10.8% was geitonogamous, and 17.9% was interplant. In the latter (distylous) species, 37.45% of the pollen on stigmas was autogamous, 13.8% was geitonogamous, 17.0% was intramorph, and 31.75% was intermorph. The amount of incompatible pollen arriving on stigmas was greatly decreased by both one‐flower and whole‐plant emasculations, and thus, the proportion of compatible pollen deposited increased with one‐flower emasculation and increased even more with whole‐plant emasculation.

Conclusions

Our quantification of pollen‐donor sources in these two species indicated that heterostyly in Fagopyrum esculentum provided a nearly 2‐fold fitness advantage (in terms of compatible pollination) over expected (random) pollen transfers between morphs. Because of reduced herkogamy, the homostylous F. tataricum was highly autogamous.     

Ovarian self‐incompatibility in Narcissus papyraceus (Amaryllidaceae) is the result of a pre‐zygotic response

Ovarian self‐incompatibility, including pre‐ and post‐zygotic reactions, is a complex mechanism for which we still lack many details relating to its function and significance. The joint presence of ovarian self‐incompatibility with style polymorphism is a rare combination that is found in the genus Narcissus. Usually, style polymorphic species have heteromorphic (diallelic and linked to style length locus) incompatibility, which prevents fertilization between individuals of the same morph, thereby helping to maintain equal proportions of floral morphs in populations. However, when present, self‐incompatibility in Narcissus is not linked to style polymorphism and cross‐fertilization within each morph is possible. Hence, self‐incompatibility in Narcissus is of particular interest when attempting to unravel the nature of the rejection reaction and aiming to assess possible cryptic differences in the fertilization process in intra‐ and inter‐morph crosses, which might ultimately explain the wide variation of morph‐ratio in the field. We examined the breeding system of Narcissus papyraceus, a style‐dimorphic species that has biased morph ratios in most of its populations. We studied pollen‐tube growth in the pistil and ovule fate after experimentally controlled hand pollinations. The growth of pollen tubes in self‐ and intra‐ and inter‐morph crosses was similar up to the point of micropyle penetration in both morphs but, subsequently, a pre‐zygotic failure appeared to affect male and female gametophytes in selfed pistils. A high proportion of ovules from self‐pollinated flowers showed signs of collapse and self‐pollen tubes were blocked or behaved abnormally before entering the embryo sac. Self‐incompatibility was stronger in the long‐styled morph than in the short‐styled morph. We did not find any conclusive sign of differential functioning between intra‐ and inter‐morph cross‐pollinations in any morph. These results enable us to rule out the possible effects of pollen–pistil interactions in N. papyraceus as a cause of morph‐ratio biases and confirm the exceptional nature of the self‐incompatibility mechanism in this polymorphic species. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 629–643.     

Homomorphic Self‐Incompatibility,with Stylar Rejection Site,in Luehea grandiflora Mart. and Zucc. (Tiliaceae‐Malvaceae s.l.)

Abstract: The breeding system of Luehea grandiflora (Tiliaceae‐Malvaceae s.l.) was investigated using hand pollinations and fluorescence microscopy studies of pollen tube growth. Although selfed flowers persisted for some 10 days, our study indicates that L. grandiflora is self‐incompatible, with self pollen tube inhibition in the upper style, as occurs in many taxa with homomorphic, gametophytic self‐incompatibility (GSI). L. grandiflora is only the second species reported within the Malvales with homomorphic stylar inhibition. This result is discussed within the context of a report for self‐compatibility in this species, and we also consider the phylogenetic implications for the occurrence of GSI in the family Malvaceae s.l.     

Heterostyly inPrimula. 2. Sites of pollen inhibition,and effects of pistil constituents on compatible and incompatible pollen-tube growth

Summary In incompatible (intramorph) pollinations of the heterostylousPrimula vulgaris, pollen germination or tube growth may be partially inhibited in several sites associated with the stigma or style. Blockage may occur, a) on the stigma surface through the failure of germination or of pollen tube penetration after germination, b) in the stigma head during the passage of the tube through the specialized transmitting tissue of the head, or c) in the transmitting tract of the style. None of the barriers is complete, and the prohibition of selfing or intramorph crossing depends upon the cumulative screening effect of one following upon the other. In both morphs, the germination of incompatible pollen on the stigma is enhanced in high ambient relative humidity, but many tubes still fail to penetrate the stigma. Those that do are retarded or blocked in their growth in the transmitting tissues of the stigma head and style. Crude extracts from the tissues of the stigma head and style show some differential effect on the growth of pollen tubesin vitro, and dialysates of extracts containing high molecular weight fractions show a consistent differential effect, those from thrum tissues retarding thrum tubes while having a lesser effect on pin tubes, and those from pin tissues retarding pin tubes while having lesser effect on thrum. It is suggested that the factors influencing tube growth are present in the intercellular secretions of the transmitting tract.     

Attempted pollen-mediated plant transformation employing genomic donor DNA

Summary Experiments were conducted to test the validity of previous reports of pollen-mediated plant transformation utilizing genomic donor DNA. Multiple Mendelian markers were employed in Zea mays L. and Lycopersicon esculentum Mill, to detect transformation events. Pollen from multiple recessive (recipient) lines was incubated with genomic DNA from multiple dominant (donor) lines, under various conditions. Treated pollen was subsequently used for pollinations on multiple recessive females, and resulting seeds were screened for transformation events. Over 200 crosses were made in tomato, and over 80 crosses were made in corn. Over 600 resulting seedlings were tested in tomato and over 800 seeds were screened in corn. Because multiple markers were used, 4,937 potential transformation events were screened. No clear-cut transformation events were observed. Therefore, using well-defined multiple markers, we have been unable to confirm the earlier claims of high efficiency pollen-mediated transformation employing genomic donor DNA.