Crop/Weed gene flow:Cucurbita argyrosperma Huber andC. fraterna L. H. Bailey (Cucurbitaceae)

A mixed population of Cucurbita at Vado El Mow in northern Tamaulipas, Mexico showed an anomalous pattern of fruit bitterness. Some domesticated plants (C. argyrosperma andC. moschata) expressed cucurbitacin bitterness whereas some sympatric free-living plants produced non-bitter fruits. This reversal of typical cucurbitacin expression suggested gene flow between crop and weed at the site. Isozyme analysis provided little insight as to taxa involved in gene exchange, although progeny from a single free-living plant carried IDH allozymes that are associated with Mexican landraces ofC. pepo. Synthetic hybridization revealed that fertile F, hybrids are produced from crosses involvingC. fraterna as the pistillate parent andC. argyrosperma as the staminate parent. Interspecific crop/weed hybrids can produce viable progeny upon self-pollination or backcrossingto either parent, andF₂ families display normal allozyme segregation. Hybrid fertility, as indicated by pollen stainability, increases in progeny produced by backcrossingfrom theC. argyrosperma parent. Interspecific hybridfertility represents a potential for crop/weed gene flow that would be realized under natural conditions if pollen flow occurs betweenC. fraterna andC. argyrosperma in the fields of Tamaulipas. Oligolectic “squash bees” (Teponapis), efficientCucurbita pollen vectors, are present at the site. Thus, it is likely that natural interspecific crop/weed hybridization has occurred at Vado El Moro and this might at least partially explain the anomalous distribution of fruit bitterness among extant populations at the site.     

Crop/weed gene flow:Chenopodium quinoa Willd. andC. berlandieri Moq.

Introduction of the Andean grain chenopod (Chenopodium quinoa) into North America placed this crop within the distributional range of a related wild species,C. berlandieri. This wild species, native to the North American flora, is cross-compatible withC. Quinoa. Isozyme analysis of progeny fromC. berlandieri plants growing within and at the periphery of theC. Quinoa fields, combined with fertility assessment and phenetic comparison among putative hybrids and parental types, indicates that over 30% of progeny from wild plants growing as weeds withC. quinoa in 1987 were crop/weed hybrids. This high incidence of interspecific gene flow from crop to weed appears to be the result of asymmetric pollen flow to free-living plants from high-density cultivated populations. The observed level of crop/weed hybridization, combined with heterosis and partial fertility of F₁ crop/weed hybrids, suggests that repeated annual cycles ofC. quinoa cultivation within the North American range ofC. berlandieri could produce introgressive change among sympatric wild populations. In terms of risk assessment for biotechnology, these results suggest that the breeding system may not provide an accurate indication of the potential for genetic interaction among predominately self-pollinating grain crops and their free-living relatives.     

Development-related changes of protein ubiquitination in pollen from male and female kiwifruit (Actinidia deliciosa)

Kiwifruit (Actinidia deliciosa) is a dioecious vine whose staminate and pistillate flowers nonetheless develop non-functional reproductive structures of the ompposite sex. Ubiquitin is a small, highly conserved protein found in all eucaryotes: a covalent ATP-dependent attachment of ubiquitin marks proteins for degradation. In the present paper, we used immunoblotting to investigate the presence of free ubiquitin and ubiquitin conjugates during pollen development in male (androfertile) and in female (androsterile) genotypes of kiwifruit. In the male, several high molecular mass protein conjugates were present throughout development. On the contrary, such a pattern characterized only early stages of pollen from the female genotype, where conjugates progressively disamppeared, until they were detectable only in trace amounts at anthesis. The highest content of conjugates in the male genotype was observed when microspores were ampproaching the first mitosis. Free ubiquitin increased continuously during development of the male microgametophyte so that mature pollen contained considerable amounts of the ubiquitin monomer at the time of its release from the anther. By contrast, only low levels were detectable in the degenerating microspores in the pistillate flowers. In vitro experiments using labeled ubiquitin indicated that early-uninucleate microspores of the female genotype had a much higher conjugation rate than those of the male genotype at the same stage. However, after feeding α-lactalbumin as exogenous substrate, the rate of ubiquitin conjugation strongly increased and was quite similar in both sexes. Nuclear features of pollen development in both genotypes are also described. The nucleus progressively degenerated in the microspores of the pistillate flowers starting from the early-uninucleate stage, in parallel with the progressive decrease in ubiquitin content and activity. At anthesis, the microspores in the pistillate flowers either had no nucleus or showed only traces of chromatin. Thus, the ubiquitin system seems to play an important role in protein turnover occurring during the normal developmental pathway of the kiwifruit microgametophyte, while it was mainly involved in regressive events related to microspore degeneration in the female genotype.     

Pollen competition and seed-siring success in Picea abies

The aim of the present work was to study pollen-tube competition in Picea abies. Controlled crossings were performed with pollen mixtures including pairs of pollen lots with fast and slowly elongating pollen-tubes. Paternity analysis using isozyme markers was performed on the progenies in order to study whether the in vitro pollen-germination vigour corresponds to the proportion of seeds sired by the pollen donor. Paternal success was found to be unequal, 15 out of 23 crossings producing progeny that differed significantly from the hypothetical ratio of 1:1. The paternal contribution in the majority of the crossings was as expected: the pollen parent with more-vigorous in vitro germination sired more seeds than the less-vigorous pollen. In the case of two pollen mixtures, however, the seed-siring success summed over the maternal trees was the opposite to the expected value. Despite these aberrations, the results support the hypothesis that pollen-tube competition is one of the factors contributing to male fitness in P. abies. However, when all the other factors affecting pollination and seed set under natural conditions are taken into account, it is clear that the seed-siring success of a particular paternal genotype cannot be predicted reliably by measuring only the in vitro pollen vigour. Received: 2 July 2001 / Accepted: 7 August 2001     

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.     

POLLEN COMPETITION AND RELATEDNESS IN HAPLOPAPPUS SECTION ISOPAPPUS (COMPOSITAE). II.

Pollen competition in mixed pollinations involving a time handicap was studied among the five aneuploid members of section Isopappus of Haplopappus. Foreign pollen was applied to the stigmas of a plant followed after a delay of 0–180 min by domestic pollen. Domestic pollen was prepotent over foreign pollen in all cases, but with increasing delay an increasing percentage of hybrids appeared in the progeny. The time interval required to produce 50% of hybrids in the progeny (at which time the two pollen types were competitively matched in potency) was determined for each cross. The interval is correlated with relatedness, with chromosome number, and with length of the style of the pollen parent. It can be used as an estimate of the potential frequency of hybridization in nature, with all other factors being equal.     

POLLEN LOADS AND PROGENY VIGOR IN CUCURBITA PEPO: THE NEXT GENERATION

Previous research on the Black Beauty bush cv. of zucchini has documented a strong positive relationship between the size of the pollen load and the vigor (performance) of the progeny. Here we report the results of three studies designed to test the hypothesis that the previously observed differences in progeny vigor are heritable. Two studies examined the transmission of the pollen load effect to subsequent generations through the ovules (female role). The third study determined if there is genetic variation for pollen performance and if the pollen load effect could be transmitted to a subsequent generation through the pollen (male role). In each of these studies the vigor of the progeny from the subsequent generation was evaluated in the greenhouse and/or the field. The results of these studies reveal (1) that the ability to sire seeds does respond to selection imposed by high pollen loads, (2) that only 23 of the 35 total traits that we measured in the three studies of transmission to subsequent generations changed in the direction predicted by the pollen competition hypothesis, (3) that only 5 of the 35 traits were significantly affected by the size of the pollen load that produced the previous generation (but all 5 were in the direction predicted by the pollen competition hypothesis), and (4) that only one study produced an overall significant difference (MANOVA) attributable to the size of the pollen load that produced the previous generation (but it too was in the direction predicted by the pollen competition hypothesis). From these experiments we conclude that pollen competition appears to play a real but minor role in the production of differences in vigor between progeny arising from low versus high pollen loads. In Black Beauty bush cv. of zucchini, maternal effects, pollen-pistil interactions, or nonrandom patterns of seed abortion must play important roles as well.     

Effects of genotype on pollen performance in Cucurbita pepo

Summary This study examines the assumption of the pollen competition hypothesis that genetic differences among microgametophytes lead to differences in pollen performance and result in non-random fertilization. In addition, we examined the assumption that pollen performance is genetically correlated with sporophyte vigor due to an overlap in gene expression between the two stages of the life cycle. The results from a pollen mixture experiment in which two cultivars of common zucchini were used show that the ability to sire seeds is nonrandom with respect to the cultivar of the pollen donor plant. The proportion of the progeny sired by the two cultivars is not independent of the region of the fruit where the seeds are produced. The progeny sired by the yellow cultivar outperformed the progeny sired by the green cultivar in a greenhouse study. In addition, the progeny sired by the yellow cultivar from the stylar region of the fruit germinated faster and had more leaf area than the progeny sired by the same cultivar from the peduncular end of the fruit. Thus, the most vigorous progeny are obtained from the stylar region of the fruit where the ovules are fertilized by the most vigorous microgametophytes.     

Effects of pollination intensity on the vigor of the sporophytic and gametophytic generation of Cucurbita texana

 We examined the effects of pollen competition (pollen load size) on sporophytic vigor and gametophytic performance in Cucurbita texana, a wild gourd, while controlling for alternative interpretations of the data. Under field conditions we compared the vigor of progeny produced from large and small pollen loads and examined the in vitro performance of the pollen produced by the progeny. We found that the progeny from large pollen loads germinated faster and had a greater reproductive output (male flowers and fruits) than progeny produced from small pollen loads. In addition, we found that the pollen produced on plants derived from large pollen loads grew faster in vitro than the pollen produced on plants derived from small pollen loads. These findings indicate that pollen competition affects the performance of the resulting sporophytic generation and the microgametophytes they produce. Received: 26 January 1997 / Revision accepted: 25 June 1997     

FITNESS CONSEQUENCES OF MIXED-DONOR POLLEN LOADS IN THE ANNUAL LEGUME CHAMAECRISTA FASCICULATA

We experimentally examined the effects of pollen composition on progeny fitness in the self-compatible, annual plant Chamaecrista fasciculata. Plants were hand-pollinated with single- and mixed-donor pollen loads and with various combinations of self- and outcross pollen. For outcrosses, pollen was obtained from two plants at each of two different distances within the same subpopulation as the female parent. Seedlings from all crosses were planted back into the maternal site. For single-donor crosses, seed weight, progeny fruit production, and overall relative fitness were significantly higher for outcross, as compared to self-treatments, but we found no significant differences among outcross sources. For all fitness components, the value observed for crosses derived from mixed loads was intermediate between the values for the singledonor crosses that comprised the mixed load. In a parallel experiment, an analysis of seed paternity of progeny which resulted from pollen mixtures of self- and outcross pollen showed random paternity in two maternal families, and significant excess of outcross in one family. Our results demonstrate that mixed pollen loads do not confer a fitness advantage to the maternal plant in this species, and that the fitness observed for progeny derived from mixed loads is generally consistent with a hypothesis of random paternity.     

DEVELOPMENTAL SELECTION IN RESPONSE TO ENVIRONMENTAL CONDITIONS OF THE MATERNAL PARENT IN MIMULUS GUTTATUS

Copper tolerance is expressed in the diploid sporophyte as well as the microgametophyte of Mimulus guttatus. Previous studies, based on reproductive output, suggested that selection for copper tolerance could occur within the pistil. The objective of this study was to determine if selection within the pistil can increase sporophytic tolerance to copper and to determine whether this selection occurs pre- or postzygotically. Mixtures of pollen from copper tolerant or sensitive sources or from plants heterozygous for tolerance to copper were applied at two intensities to plants cloned and grown in control or copper supplemented solutions. The proportion of copper tolerant progeny showed a small, 7%, but significant increase when pollen recipients were grown with added copper. Comparisons of the numbers of tolerant progeny, as well as various components of reproduction, following light and heavy pollinations suggested that microgametophytic selection was unlikely to account for this increase. However, the 8 to 10% decrease in the seed/zygote ratio, compared to control values, was sufficient to account for the difference in proportion of copper tolerant progeny from control and copper treated plants. Thus, it appeared likely that selection for copper tolerance could occur within the pistil, and that much of this selection occurred postzygotically through the early failure of developing seeds.     

INTERSPECIFIC GENE FLOW IN CUCURBITA: C. TEXANA VS. C. PEPO

The potential for interspecific genetic exchange was examined by monitoring flowering patterns, pollinator movement, and gene flow among experimental populations of the Texas gourd (Cucurbita texana) and cultivars of Cucurbita pepo. While flowering patterns and pollinator movement tended to maximize self-pollination and local gene exchange, movement of effective pollen exceeded 1,300 m. This movement, mediated by the solitary bee Xenoglossa strenua and monitored by tracking allozyme variants, produced interspecific hybrids in 5% of the progeny from experimental plants. Interspecific gene exchange occurred in either direction with either species serving as staminate or pistillate parent. No obvious constraints to gene flow among plants representing C. texana and distinctive cultivars (vars. ovifera, medullosa, melopepo) of C. pepo were detected. Genetic exchange among different species and cultivars is enhanced by the foraging behavior of Xenoglossa. Multiple visits to either staminate (pollen carryover) or pistillate (multiple pollinations) flowers often result in the deposition of mixed pollen on receptive stigmas. The wild type (C. texana) can donate and receive effective pollen when growing under both weedy and natural conditions. The observed lack of interspecific reproductive isolation supports treatment of cultivars and wild types as a single species and, in conjunction with available data concerning temporal/geographical relationships among bees, squash, gourds, and humans in eastern North America, suggests the possibility of long-term genetic interaction between wild types and domesticates.     

Microgametophyte population sizes and plant reproductive output in the insect-pollinated Prunella grandiflora (Lamiaceae)

Intraspecific variation in pollen deposition and number of pollen tubes per style is rarely quantified, but is essential for assessing the occurrence of pollen limitation and pollen competition and their evolutionary implications. Moreover, pollen deposition, pollen tube growth, and the fate of fertilized ovules are rarely distinguished in field studies. Here we present such a study in eight natural populations of Prunella grandiflora. We quantified microgametophyte population sizes and inferred pollen limitation when the number of fertilizable ovules exceeded pollen tubes, and assessed seed set and fate after open pollination. Two and three populations had on average significantly fewer pollen grains and pollen tubes per flower, respectively, than the fixed number of fertilizable ovules per fruit, while one population experienced significant pollen competition. Style length was positively correlated with the number of pollen tubes. While pollen availability was very variable, seed abortion was significantly less frequent in denser populations, and in one population the proportion of well-developed seeds was significantly, positively correlated with the number of pollen tubes in the style. Less pollen deposition, lower numbers of pollen tubes reaching the base of the style, lower pollen quality and therefore increased abortion of fertilized ovules can all reduce seed set in natural P. grandiflora stands. Substantial intraspecific variability implies that microgametophyte competition also occurs in this species. Finally, style morphology may affect pollen receipt.     

A model of the competitive relationships between R. cochinchinensis and Z. mays

An understanding of the competitive relationship between weed and crop can be used in assessing what physiological aspects of the association can be exploited in the control of the weed. Field and greenhouse studies were conducted with Z. mays L. and Rottboellia cochinchinensis (Lour.) W D Clayton using a modified replacement series model in which the overall weed crop density in each treatment was maintained as a constant, but the proportion of the two species varied. The results indicated that the crop had greater competitive ability (K_mr) than the weed even at increasing weed densities. Under field conditions, the values of the Plant Relative Yield (PRY) indicated that severe specific competition was occurring and suggested that some environmental factor was limiting, while the Relative Yield Total (RYT) suggest that either the crop and the weed were exploiting the resources in mutual antagonism or that allelopathy was occurring. The limiting environmental factor that both species exploited in mutual antagonism in the field was light under conditions of adequate soil moisture and nutrients.     

POSTPOLLINATION EFFECTS ON SEED PATERNITY: MECHANISMS IN ADDITION TO MICROGAMETOPHYTE COMPETITION OPERATE IN WILD RADISH

After pollen arrives on a stigma, the paternity of seeds may be influenced by microgametophyte competition, maternal choice, genetic complementation between parents, and embryo competition. While microgametophyte competition has been well accepted, the other mechanisms are more difficult to demonstrate, and their effects are often confounded. Here, wild radish plants were pollinated with single and mixed pollen loads, and some plants were stressed such that reproduction was reduced. Effects of pollen donors, maternal families, maternal × paternal interaction, pollen donor number, and stress on fruit abortion, seed number per fruit, seed weight, and total seed weight per fruit were measured. Maternal-plant × pollen-donor interaction effects were found for all variables, indicating that genetic complementation or maternal choice occurred. Values of the components of reproduction were generally higher for multiply sired fruits than for singly sired fruits, indicating that either competition among embryos changed under multiple paternity or maternal choice for multiply sired fruits occurred. Finally, when maternal plants were stressed, the components of reproduction were more strongly affected by seed and fruit paternity. This result indicates that either competitive regimes among embryos were affected by stress or maternal plants become more selective under stress. In both cases where embryo competition might have been an explanation of the results, variation in seed weight within fruits was unaffected, suggesting that competitive regimes were unchanged. Clearly, mechanisms in addition to microgametophyte competition are important in sorting the pollen that arrives on stigmas of wild radish. These data suggest that maternal choice is likely to be important. In addition, these processes are likely to occur in the field, since the effects are stronger in stressed than in control plants.     

CONTRIBUTION OF CRYPTIC INCOMPATIBILITY TO THE MATING SYSTEM OF EICHHORNIA PANICULA TA (PONTEDERIACEAE)

Tristylous populations of the annual aquatic Eichhornia paniculata have high levels of outcrossing and intermorph mating despite being fully self- and intramorph compatible. Experimental studies of pollen germination, ???pollen-tube growth, and pollinations with mixtures of genetically marked pollen were used to determine whether postpollination processes contribute to the observed mating patterns. Differences in pollen germination were small and did not contribute to differences in pollen siring ability. The fraction of pollen tubes first entering the ovary, however, was greater for legitimate outcross pollen than for either of the other two pollen types (self or outcross illegitimate pollen) in all recipient morphs. Moreover, legitimate pollen had higher siring success when in competition with illegitimate pollen types (self or outcross) in each recipient style morph. The ranking of pollen performance for different pollen-style combinations was the same for both the pollen-tube growth and marker-gene experiments indicating that differences in pollen-tube growth rate are the principal cause of differences in pollen siring ability. Cryptic incompatibility in E. paniculata may represent a weak heteromorphic incompatibility system because the observed patterns of pollen-tube growth parallel pollen-tube growth and seed-set patterns that occur in related species with strong trimorphic incompatibility. The ability to produce mostly outcrossed progeny when pollinators are abundant, but to reliably produce seed under a variety of environmental and demographic conditions may be favored in E. paniculata because of its colonizing life history and occurrence in ephemeral habitats. Cryptic incompatibility may be more likely to occur in species subject to wide fluctuations in population size and levels of pollinator service.     

Interspecific pollen competition as a reproductive barrier between sympatric species of Helianthus (Asteraceae)

Artificial crosses between Helianthus annuus and H. petiolaris using 1:9, 1:1, and 9:1 mixtures of intraspecific: interspecific pollen were conducted to determine the role of interspecific pollen competition as a reproductive barrier in Helianthus. Of 1,245 achenes analyzed from the pollen competition experiments, only 49 were hybrids. The number of hybrids observed was significantly less than expectations for all three pollen mixtures, regardless of the identity of maternal parent (P < 0.01). Stigma age and pollen ratio had no significant impact on hybrid frequency. However, hybrids were significantly more frequent with H. annuus than with H. petiolaris as the maternal parent (P < 0.01). Analysis of pollen tube growth rates revealed no differences in the rate of growth of intraspecific vs. interspecific pollen. Likewise, pollinations with either intraspecific or interspecific pollen or with different pollen ratios did not affect the percentage of filled achenes. Thus, the mechanism responsible for selective fertilization by intraspecific pollen in mixed pollen loads remains unclear. Nonetheless, these findings suggest that interspecific pollen competition plays an important role in controlling the formation of hybrids between H. annuus and H. petiolaris and may partially account for patterns or differential cytoplasmic vs. nuclear introgression in Helianthus.     

Nonrandom mating in Clarkia gracilis (Onagraceae): a case of cryptic self-incompatibility

In plants capable of both self-fertilization and outcrossing, the selfing rate depends on the proportion of self pollen in pollen loads and on the relative postpollination success of self pollen in siring offspring. While the composition of pollen loads is subject to unpredictable variation, paternity success of self vs. outcross pollen following pollen deposition may be controlled by maternal plants. This study examined postpollination paternity success in Clarkia gracilis ssp. sonomensis, in which deposition of self pollen is common. Pure loads of self and outcross pollen produced similar numbers of mature seeds, but equal mixtures of self and outcross pollen yielded more than three times as many outcrossed offspring as selfed offspring. The finding that the paternity success of self pollen depends on whether it is in competition with outcross pollen helps to explain an earlier finding that the selfing rate in experimental populations was highest when pollinator activity was lowest. Cryptic self-incompatibility allows paternity by self pollen when outcross pollen is unavailable.     

EVOLUTION OF FLORAL TRAITS IN A HERMAPHRODITIC PLANT: FIELD MEASUREMENTS OF HERITABILITIES AND GENETIC CORRELATIONS

Genetic variances, heritabilities, and genetic correlations of floral traits were measured in the monocarpic perennial Ipomopsis aggregata (Polemoniaceae). A paternal half-sib design was employed to generate seeds in each of four years, and seeds were planted back in the field near the parental site. The progeny were followed for up to eight years to estimate quantitative genetic parameters subject to natural levels of environmental variation over the entire life cycle. Narrow-sense heritabilities of 0.2–0.8 were detected for the morphometric traits of corolla length, corolla width, stigma position, and anther position. The proportion of time spent by the protandrous flowers in the pistillate phase (“proportion pistillate”) also exhibited detectable heritability of near 0.3. In contrast, heritability estimates for nectar reward traits were low and not significantly different from zero, due to high environmental variance between and within flowering years. The estimates of genetic parameters were combined with phenotypic selection gradients to predict evolutionary responses to selection mediated by the hummingbird pollinators. One trait, corolla width, showed the potential for a rapid response to ongoing selection through male function, as it experienced both direct selection, by influencing pollen export, and relatively high heritability. Predicted responses were lower for proportion pistillate and corolla length, even though these traits also experienced direct selection. Stigma position was expected to respond positively to indirect selection of proportion pistillate but negatively to selection of corolla length, with the net effect sensitive to variation in the selection estimates. Anther position also was not directly selected but could respond to indirect selection of genetically correlated traits.     

Gene transfer between canola (Brassica napus L. and B. campestris L.) and related weed species

Brassica species are particularly receptive to gene transformation techniques. There now exists canola genotypes with transgenic herbicide resistance for glyphosate, imidazolinone, sulfonylurea and glufosinate herbicides. The main concern of introducing such herbicide resistance into commercial agriculture is the introgression of the engineered gene to related weed species. The potential of gene transfer between canola (Brassica napus and B. campestris) and related weed species was determined by hand pollination under controlled greenhouse conditions. Canola was used as both male and female parent in crosses to the related weed species collected in the Inland Northwest region of the United States. Weed species used included: field mustard (B. rapa), wild mustard (S. arvensis) and black mustard (B. nigra). Biological and cytological aspects necessary for successful hybrid seed production were investigated including: pollen germination on the stigma; pollen tube growth down the style; attraction of pollen tubes to the ovule; ovule fertilisation; embryo and endosperm developmental stages. Pollen germination was observed in all 25 hybrid combinations. Pollen tubes were found in the ovary of over 80% of combinations. About 30% of the hybrid combinations developed to the heart stage of embryo development or further. In an additional study involving transgenic glufosinate herbicide resistant B. napus and field mustard it was found that hybrids occurred with relatively high frequency, hybrids exhibited glufosinate herbicide resistance and a small proportion of hybrids produced self fertile seeds. These fertile plants were found to backcross to either canola or weed parent.