technical gene transfer article
Daniel D. Worley (email@example.com)
Mon, 16 Feb 1998 10:11:48 -0400
>Date: Sun, 15 Feb 1998 11:16:08 -0500
>From: Richard Wolfson <firstname.lastname@example.org>
>Subject: technical gene transfer article
>X-MIME-Autoconverted: from quoted-printable to 8bit by ice.icepr.com id
>Here is a technical scientific article. AS some people on this email list
>have specifically asked for scientific articles, here it is. The main
>point of this article is genes from genetically engineered (herbicide
>resistant) varieties of canola could be carried into weeds, creating
>super-weeds that are herbicide resistant and therefore a menace to the
>ecosystem and to farmers.
>GENE TRANSFER BETWEEN CANOLA (BRASSICA NAPUS L.) AND RELATED WEED SPECIES.
>J. Brown, D.C. Thill, A.P. Brown, C. Mallory-Smith, T.A. Brammer and H.S. Nair
>Department of Plant, Soil, and Entomological Sciences, University of Idaho,
>Moscow, ID 83844-2339.
>Brassica species are particularly receptive to gene transformation
>techniques. There now exist genetically engineered canola genotypes with
>resistance to glyphosate, sulfonylurea or glufosinate herbicides. The main
>concerns of introducing herbicide resistant cultivars into commercial
>agriculture are: i. these crops will become difficult to handle volunteer
>weeds in following crops and ii. introgression of the engineered gene into
>related weed species. A survey of northern Idahoand eastern Washington
>showed that a number of weeds closely related to cultivated canola, wild
>mustard (B. kabel (DC) L.C. Wheeler), tumble mustard (Sisymbrium altissi,u,
>L.), birdsrape mustard (B. rapa L.), flixweed (Descurainia sophia L.),
>black mustard (B. nigra L.) and field pennycress (Thiaspi arvense L.) all
>bloom simultaneously with canola crops.
>In 1993 and 1994, a field study to investigate pollen movement and
>cross-pollination between herbicide resistant canola and non-resistant
>canola was carried out at two locations in Northern Idaho. In these
>studies, glufosinate resistant canola lines were planted in the center of a
>62 m, square border, of non-resistant canola. The non-resistant border was
>made up with a mixture of three canola cultivars (`Westar', `Legend' and
>`Helios') each with a different flowering time in order to ensure synchrony
>of flowering with the resistant center and non-resistant border. Seed was
>collected from the susceptible border every 1.5 m along 16 rays spaced
>22.5° apart. Each ray was 26 mlong. Each sample was threshed separately
>and the seedling progeny screened for herbicid eresistance (any resistant
>plants being the result of hybridization with resistant plants from the
>A second field study was carried out at two locations to determine the
>frequency of natural pollination that may occur when herbicide
>(glufosinate) resistant canola plants are grownin close proximity to local
>weeds. Each of three weed species (wild mustard, black mustard and
>birdsrape mustard) were examined in plots with a 1:1 mixture of weed and
>transgenic herbicide resistant canola. At harvest, weed plants and canola
>plants were separated by hand and the seed from each threshed separately.
>All weed seeds collected were screened for herbicide resistance (resulting
>from interspecific crossing). In a greenhouse study, canola was used as
>both male and female parents in crosses to the same three related weed
>species. After pollination, pollen grain germination, pollen tube
>development, fertilization, embryo and endosperm development were monitored
>on each of the 64 possible cross combinations (including selfs) over time.
>The potential of backcrossing, after initial hybrids are created, was also
>investigated in the greenhouse.
>Findings from these studies will be used in conjunction with field studies
>to develop simulation models of what may happen in nature. From the
>preliminary results it is difficult to make strong conclusions. However
>some indications already observed are: i. canola seed can be readily
>transported throughout a region and therefore there is a risk that these
>crops will become volunteer weeds; ii. canola pollen can move more than 26
>m and movement is affected by wind direction; iii. canola and some related
>weeds can combine to produce hybrid plants under glass house and possibly
>field conditions; iv. herbicide resistance is expressed in the hybrid; and
>v.bridge crosses could play a major role in the movement of herbicide
>resistant genes into the natural weed population.
>Key words: Brassica species, transgenic herbicide resistance, gene transfer
>Over the past decade, plant genetic engineering techniques have been
>developed where specific characters (genes) can be introduced into a plant
>in a relatively straight forward manner, provided the genes coding for the
>character have been identified. Brassica species are particularly
>receptive to gene transformation techniques and several reports have been
>presented where canola (Brassica napus L. and B. campestris L.) has been
>genetically transformed for specific genes (DeBlock et al., 1989; Moloney
>et al., 1989; Thomzik and Hain, 1990).
>Effective weed control is a major problem in canola production. Although
>no information isavailable regarding the situation within the USA, yield
>losses due to weeds in Canadian canola were about 10% and have been
>estimated to cost Canadian farmers over $306 million each year (Chandler et
>al., 1984). Brassica ceae weeds also can affect canola oil and meal
>quality (Thomas,1984). It is no surprise therefore, that a major objective
>of molecular biologists has been toproduce transgenic canola lines with
>herbicide resistance. There now exist canola genotypes withtransgenic
>resistance genes for glyphosate, sulfonylurea and glufosinate herbicides.
>A major concern of introducing transgenic herbicide resistant crops into
>agriculture is the spread of the engineered gene(s), particularly by
>pollen, to related weed species (Keeler, 1989; Williamson, 1991). New
>weeds could potentially be produced from backcrossing to a hybrid and their
>invasion into the natural ecosystem would cause change (Keeler, 1989).
>Indeed, some researchers believe that producing genetically engineered
>herbicide resistant crop species willultimately lead to an increase in
>herbicide use (Hoffman, 1990; Ellstrand and Hoffman, 1990;Williamson,
>1991). In addition to hybridization with weed species, herbicide resistant
>plants could flourish as volunteer weeds on neighboring farms (Botterman
>and Leemans, 1988;Williamson, 1991).
>Preliminary surveys of grass and broadleaf weeds that infect canola fields
>in the Inland Northwestand Montana show that the same weeds that commonly
>infest small grain cereal fields also infest canola crops (Johnston, 1992;
>Brennan and Thill, 1993). These include several Brassica ceaegenera
>including wild mustard (B. kaber (DC.) L.C. Wheeler), black mustard (B.
>nigra (L.)W.J.D. Koch), birdsrape mustard (B. rapa L.), shepherdspurse
>(Capsella bursa-pastoris (L).Medik.) tumble mustard (Sisymbrium altissimim
>L.) and field pennycress (Thlaspi arvense L.). Preliminary studies under
>controlled conditions (Brown and Brown, 1995) have suggested a strong
>possibility of hybridization between some of these weed species and
>One of the major pathways in plant evolution is through hybridization
>between related species (Williamson, 1991). Many crop-weed comparisons
>show that plants can evolve into invasive genotypes based on a few gene
>polymorphisms (Keeler, 1989; Hoffman, 1990). Herbicide resistance in many
>cases can be achieved by the transfer of a single gene (Schulz et al.,
>1990). Gene expression varies with genetic background, due to epistasis,
>linkage and pleiotropy. Therefore, it can be difficult to predict how the
>genetically engineered gene(s) will be expressed in a related weed species
>(Colwell et al., 1985; Tiedje et al., 1989).
>There is substantial recent literature on other intergeneric crosses within
>the Brassicaceae family,and an even larger number on hybridization between
>different Brassica species. B. napus has been combined with several
>species of Diplotaxis and Eruca by sexual crosses and embryo rescue
>(Ringdahl et al., 1987; Batra et al., 1990), as well with various other
>Brassica species (Jourdan etal., 1989; Sjodin and Glimelius, 1989;
>Glimelius et al., 1990). Fusions of B. napus with bothdiploid Brassicas
>(e.g. B. oleracea, B. nigra) and with amphidiploid ones (e.g. B. juncea,
>B.carinata) have yielded viable plants.
>Other intergeneric Brassica hybrids recently produced include various
>combinations of B. juncea, B. campestris, Diplotaxis, Eruca, Raphanus,
>Moricandia, and Trachystoma (e.g. Toriyama et al.,1987a; 1987b; Agnihotri
>et al., 1990a; 1990b; Sikdar et al., 1990; Takahata and Takeda, 1990;Kirti
>et al., 1992). These hybrids generally showed some degree of fertility in
>crosses and a range of chromosome numbers in the progeny. Preferential
>loss of chromosomes from one parent wasoccasionally observed (Fahleson et
>Most interspecific crosses do not produce mature seed due to failure of
>endosperm development,resulting in wrinkled or empty seeds (Nishiyama et
>al., 1991). However, normally incompatible interspecific hybridization can
>spontaneously produce a few seeds which usually yield true F1plants as a
>result of unexpected ploidy changes (Nashiyama et al., 1991). This
>phenomenon has been documented between B. napus and B. campestris, and
>between B. napus and B. nigra.
>The aim of this project is to determine the feasibility and frequency of
>gene flow between transgenic canola (B. napus) and related weed species.
>Experiments carried out under controlled conditions will provide data that
>can be used to model possible genetic transfer that may occur under field
>conditions. Similarly, competitive fitness of plants from hybrid
>combinations will add invaluable information concerning the potential risks
>to the environment should genetic transfer occur between cultivated and
>MATERIAL AND METHODS
>Field Pollen Movement. A field pollen movement study was carried out in the
>spring of 1993. In this experiment a plot (10 m2) of glufosinate resistant
>canola was surrounded by an 8 m border of glufosinate susceptible canola
>(`Helios'). At harvest the susceptible border was sampled in four
>directions (north, east, south and west) every 1.5 m from the herbicide
>resistant center. Four samples, each of 200 seeds, were sown from each
>sample position. When seedlings reached a 1-2 leaf stage they were sprayed
>with the recommended field rate of glufosinate. Surviving plants were
>sprayed a second time to avoid errors caused by escapes. Plants surviving
>the second spray were counted and assumed to have resulted from cross
>pollination with plants from the herbicide resistant center.
>In the following year (1994) pollen movement was examined in greater detail
>in an experiment grown at two locations in Northern Idaho (Moscow and
>Genesee). At both locations a plot of transgenic glufosinate resistant
>canola was surrounded by a 30 m wide border of herbicide susceptible
>canola. The susceptible canola border was planted using a mixture of seed
>from three canola cultivars (`Legend', `Westar' and `Helios') each with a
>different flowering time to ensure synchronous flowering with resistant
>genotypes in the center. At maturity, seeds were collected from the
>non-transgenic canola border every 1.5 m along sixteen 26 m long rays
>spaced 22.5°apart. The seed from each sample position was grown in the
>greenhouse to the 1-2 leaf stage, and sprayed with glufosinate at the 0.42
>kg ai/ha. Sprayed plants were evaluated 7 days after treatment. To ensure
>there were no escapes, all surviving plants were sprayed again with the
>herbicide dose previously mentioned and survivors counted.
>In both years of study, herbicide resistant and susceptible cultivars were
>included in the herbicide spray design as controls.
>Greenhouse Interspecific Hybridization. Interspecific and intergeneric
>hybridization between transgenic herbicide resistant canola and three
>related weed species (wild mustard, birdsrape mustard and black mustard)
>was examined in greenhouse experiments. Two genotypes of transgenic canola
>and one susceptible canola (`Cyclone') were crossed in all possible
>combinations, including selfs and reciprocals in a full diallele crossing
>design with the three weed species.
>Mercury lights were used to supplement natural lighting and provide a 16
>hour day-length. Greenhouse temperature was not effectively controlled.
>However, daytime temperatures were around 21 ± 5°C.
>Two days after pollination, 10 siliques were taken at random to examine
>pollen germination onthe stigma and pollen tube development down the style
>towards the ovary. The method used was similar to that of Martin (1959).
>Pollinated styles were fixed in absolute alcohol: glacial acetic acid(3:1)
>48 hours after pollination and stored at 4°C until examined. The fixative
>was replaced with1N sodium hydroxide (NaOH) and the styles were left at
>room temperature. Hydrolysis was completed with a change of 1N NaOH and a
>further half hour at 60°C. After the hydrolyzed styles were rinsed with
>water and stained with a methyl blue solution [0.2% methyl blue + 2.0%K3PO4
>(w/v)] they were observed for fluorescence using wavelength of light in the
>range 350-400nm. The styles were examined for pollen germination and
>pollen tube growth in the style andovary.
>After pollination, developing siliques were harvested at 2 day intervals
>between 4 and 28 daysafter pollination and fixed in absolute
>alcohol:glacial acetic acid (3:1) and stored at 4°C until examination. The
>ovary was opened and any developing ovules were removed. Individual ovules
>were opened and a few drops of HCl-carmine (Snow, 1963) placed inside.
>After a few minutes,excess stain was rinsed away with 70% ethanol. The
>ethanol was removed using a piece of filter paper and replaced with
>Rattenburys' Fluid (45% acetic acid:glycerine, 10:1). The endosperm and
>embryo were teased from the ovular tissue, which was then discarded. The
>prepared material wasexamined using a Jenval transmitted light research
>microscope. The developmental stage of theembryo and endosperm and any
>abnormalities present were noted. A full data set was notavailable from
>this study at the time of producing this report and only a sub-set of
>observationsfrom one silique per sample are presented.
>Field Interspecific Hybridization. Transgenic, glufosinate-resistant canola
>was planted in a 1:1 mixture with three weed species (wild mustard,
>birdsrape mustard and black mustard) in 1 x 5 m plots arranged as a
>randomized complete block design with four replications. Seeding rate of
>each species was adjusted prior to planting to account for any differences
>in germination. At harvest weed plants were threshed separately from the
>canola mixtures. Weed seed collected from the canola:weed mixtures were
>grown in the greenhouse to the 1-2 leaf stage, and sprayed with glufosinate
>at 0.42 kg ai/ha. Sprayed plants were evaluated 7 days after treatment.
>To ensure there were no escapes, all surviving plants were sprayed again
>with the herbicide dose previously mentioned and survivors counted.
>Herbicide resistant and susceptible cultivars were included as mentioned
>Biological Fitness Study. A number of fertile interspecific plants were
>produced from handcrossing canola and birdsrape mustard. Most of the
>hybrid plants were sterile. However, two F1 families were found to set
>self seed. These two hybrid lines were backcrossed to each of the parent
>species. This resulted in eight families (canola, birdsrape mustard,
>F1(A), F1(A) x canola, F1(A) x birdsrape mustard, F1(B), F1(B) x canola and
>F1(B) x birdsrape mustard) which were grown in a randomized block design in
>the greenhouse with two replicates each of five single plants per family.
>Data were collected from individual plants for pre-harvest characters and
>Field Survey of Weed Populations. Twenty fields were surveyed in Latah, Nez
>Perce and Lewis counties in Idaho and Whitman county in Washington. Wild
>mustard (B. kabel (DC) L.C.Wheeler), tumble mustard (Sisymbrium altissi,u,
>L.), birdsrape mustard (B. rapa L.), flixweed (Descurainia sophia L.), and
>field pennycress (Thiaspi arvense L.) all bloomed simultaneously with
>canola (Brassica napus L.). The weeds occurred in the same field with
>canola, or in ditches next to canola fields. Among the weed populations
>observed wild mustard and birdsrape mustard occurred with greatest
>frequency. A number of canola plants were also found by road sides and
>ditches which must have volunteered from farmers fields.
>Pollen Movement and Cross Pollination Study. The percentage cross
>pollination between herbicide resistant plants and the non-resistant border
>decreased with greater distance from the herbicide resistant pollen source.
>When grown adjacent to each other, there was 6.3% cross-pollination between
>plants. However, cross pollination was greater than 1:200 seeds when
>plants were separated by 7.5 m (Figure 1a). The percentage cross
>pollination changed according to direction from the pollen source (Figure
>1b). Greatest cross pollination occurred down wind ofthe pollen source.
>The weather conditions in 1994 were drastically different from those in the
>previous year. 1994was a hotter and dryer season which greatly reduced the
>time plants remained in flower, pollen load per flower and the number of
>insects observed in the experiment. With the exception of the adjacent
>samples, this resulted in fewer cross pollinations compared to the wetter
>and cooler conditions of the previous season. The percentage cross
>pollination according to distance from pollen source is shown in Figure 2.
>This figure shows that hybridization decreases rapidly with increasing
>distance from the transgenic canola. A very high frequency of cross
>pollination was found in the sample adjacent to the transgenic resistant
>center. This high frequency should be treated with caution as there is
>some suggestion that a proportion of these samples was taken from the
>transgenic center by error. Beyond 5 m from the pollen source the
>frequency of cross pollination is only about one fertilization in 1000,
>although there was no suggestion of the frequency reducing below that
>frequency over greater distances than was examined in the study. Indeed, a
>small number of transgenic resistant hybrids were found even at the
>furthest distance from the pollen source (i.e. up to 26 m).
>Hybridization frequency and pollen movement also varied by site and by wind
>direction. Site differences included terrain (side of hill vs. on top of
>hill), and climate (wind direction, speed). The average wind direction was
>from the southwest at both locations and the average wind speedwas 2.5 m/s
>at Genesee and 1.8 m/s at Moscow.
>Data collected on the frequency of pollination related to distance of
>pollen movement were analyzed using a maximum likelihood approach to assess
>the risk of gene escape. A general model was as follows:
>where c is the frequency of cross pollination when two plants are adjacent
>and R(x) is theprobability that pollen grains travel at least x distance
>away from the source plant.
>The best fit of possible forms of R(x) was the Weilbull model. This model
>states that pollen iscarried from the source and is deposited at a variable
>rate, which is distance dependent. Therefore:
>where x is the mean distance traveled by the pollen, and a, b and c are
>parameters estimated from the data collected.
>Table 1 shows the estimated parameters from the Weilbull model at each
>location (Genesee and Moscow) estimated from up wind and down wind
>directions. One of the most important parameters is c, which indicated a
>changing rate of hybridization between upwind and downwind directions,
>especially at the Genesee site. This suggests that wind direction was also
>one of the factors effecting the rate of pollen movement; and therefore,
>the hybridization between the transgenic and non-transgenic canola. On
>average, pollen movement (Table 1) was always less than 1 m from the source
>plant. However, as is shown in Figure 1 and Figure 2, cross pollination
>still occurs at low frequency at far greater distances.
>Greenhouse Interspecific Hybridization. Pollen grain germination and pollen
>tube developmentwas observed in all of the 12 hybrid species combinations
>(including reciprocals) studied. Some hybrid combinations, for example
>canola x black mustard, showed mostly an incompatible pollen germination
>reaction, where many short twisted pollen tubes did not penetrate the
>stigma. In these cases very few pollen tubes were observed around the
>ovary. Other hybrid combinations,for example wild mustard x canola, showed
>considerably more compatibility with a much larger presence of pollen tubes
>in the ovary. Pollen tubes were observed penetrating the micropyle in all
>Fertilization occurred in 10 of the possible 12 hybrid combinations, as
>evidenced by the onset of embryo and endosperm development. A great deal
>of variation existed in the stage of development observed 16 days after
>pollination (Table 2). Some cross combinations aborted at the early
>globular stage, very soon after fertilization. However, excluding selfed
>crosses, 30% of cross combinations studied developed to the heart stage or
>a later stage of embryo growth. Hybridizations where black mustard was used
>as the maternal parent consistently resulted in early embryo abortion.
>However, as a pollen parent in hybrid combinations, embryo development
>ofblack mustard hybrids advanced to later stages of development.
>Overall, the three weed species examined combined to a relatively high
>degree in hybrid crosses with other weed species, while weed x canola
>hybrids showed the least embryo development compared to the development of
>The different species examined in this study showed different rates of
>embryo development (Table3). The rate of embryo development in canola was
>faster than any of the weed species, after self pollination. Canola
>developed globular embryos after 8 days and heart shaped embryos after 12
>days. Globular embryos were not observed in any of the three weed species
>within the first 10 days after fertilization. Heart shaped embryos were
>not observed in birdsrape mustard that had been self pollinated until 14
>days after pollination and after 16 and 18 days, respectively, for wild
>mustard and black mustard.
>The general trend of embryo developmental rates, where embryos were
>observed, tended to follow the rate of the slower developing parent,
>irrespective of which is used as the maternal parent.
>The normal pattern of endosperm development in Brassica is to progress from
>a coenocytic statein the early stages, to a cellular endosperm
>corresponding to the heart stage of embryodevelopment. The primary cause
>of early embryo abortion was failure of the endosperm todevelop. Very
>small amounts of degenerating endosperm were observed. Aborting endosperm
>ischaracterized by the presence of large nuclei and nucleoli. In this
>study, dumb-bell nuclei,anaphase bridges, lagging chromosomes, various
>widths of the metaphase plate and split spindleswere observed in dividing
>A proportion of pollinations were allowed to mature and set seed. Mature
>seed was obtainedfrom the interspecific weed hybridizations of birdsrape
>mustard x wild mustard and wild mustardx birdsrape mustard. In addition
>mature seed was also obtained from crosses between transgenic canola and
>Biological Fitness of Hybrid Combinations. As most successful hybrid cross
>combinations have been between canola and birdsrape mustard, this cross
>combination was examined with respect to the biological fitness of the
>hybrids and their progeny. Most hybrid plants produced were sterile
>(probably due to chromosome abnormalities or a lack of homologous pairing
>at meiosis). Allhybrid plants produced; however, expressed herbicide
>resistance, although to varying degrees. This observation suggests a gene
>Fertile hybrid plants were readily produced by treating with colchicine.
>However, two F1 hybrid plants produced self seed without colchicine
>treatment. These two hybrid lines were backcrossed to both parents and
>grown in a replicated greenhouse study to examine the biological fitness
>ofthe hybrids and backcrosses compared to the parental species. F1 hybrid
>plants appeared to show lower seedling vigor compared to either parent
>species. However, vigor was restored to be equalor exceed the parent
>species after backcrossing (Table 4). Similarly, F1 hybrid plants were
>shorter with fewer leaves, less leaf area and lower yield than the parent
>species. However, backcrossing to either parental species resulted in
>increased plant height, leaf number and area and also greateryield.
>Field Hybridization. Seed collected from weed plants in resistant
>canola:weed species mixtureswere all screened for herbicide resistance. No
>species hybrid seedlings (i.e. herbicide resistantseedlings) were found in
>black mustard or wild mustard. However, a few (approximately 1:1000
>seedlings examined) hybrid plants were obtained between canola and
>birdsrape mustard. These hybrid plants are presently being investigated in
>more detail in cytological studies.
>DISCUSSION AND CONCLUSIONS
>Pollen Movement. Canola pollen can move, by insect or wind, further than 26
>m. Wind direction and speed are factors in pollen movement with greatest
>frequency of pollination being down wind. In the 1994 study, on average,
>pollen moved less than 1 m from parent source. High temperatures and
>drought conditions in the 1994 season may have had a restrictive effect on
>crosspollination due to reduced flowering time, fewer insects and lower
>pollen load. However, a low frequency of cross pollination was observed up
>to 26 m from the source, the greatest distance considered in this study.
>Herbicide resistant canola fields grown adjacent to non-resistant canola
>crops will therefore have high potential of cross pollination between these
>Glasshouse Interspecific Hybridization. Mature hybrid seed was obtained
>from the crosses, birdsrape mustard x wild mustard and its' reciprocal and
>the interspecific cross canola x birdsrape mustard. The latter canola x
>weed hybrid was also found to occur under field conditions.
>In all hybrid combinations examined, pollen germinated and pollen tubes
>penetrated the style. Only two species cross combinations did not show any
>pollen tubes in the ovary after hybridization. It should, however, be
>noted that one of these was the self species cross wild mustard, which
>later produced mature seeds. Therefore, this suggests that the sample size
>presented in this report is too small to detect some necessary differences
>or occurrences. It also should be noted that under field conditions, there
>is the potential of millions of hybridizations that could occur rather than
>the relatively few examined here.
>A number of hybrid embryos developed to the heart stage which appears to be
>a crucial stage of embryo development (Brown, 1985). If the embryo
>develops to this stage, there is a potential of continuing development to
>Spontaneously doubled fertile F1 hybrids showed lower vigor and seed yield
>compared to either parent species. However, it was noted that the back
>crossed plants, obtained by crossing the F1 to either canola or birdsrape
>mustard, were as vigorous and high yielding as their parent species.
>Many hybrid cross combinations in Brassica result in sterile pollen due to
>cytological defects and lack of chromosome pairing in meiosis. Sterile
>plants have a greater tendency to be cross pollinated by other plants, or
>species, than fertile ones and a second cycle of hybridization may occur in
>gene flow situations. The possibility of bridge crossing is just beginning
>to be explored as part of this project.
>In order to explain potential bridge crosses, consider the hybrid
>combination birdsrape mustard x wild mustard (which produced mature seed in
>this study, in either direction). Birdsrape mustard has 10 paired
>chromosomes (2n = 20) and wild mustard has 9 paired chromosomes (2n = 18).
>A hybrid between the species would most likely be an allotetraploid with 19
>paired chromosomes (2n = 38) and hence will have the same chromosome number
>as canola. Having the same chromosome number, especially with a common
>genome (the A genome from B. rapa) may be an important factor in hybrid
>formation. A hybrid between these two weed species could there foreact as
>a bridging species with canola and could further add to the risk of gene
>flow of transgenic herbicide resistance into weed species.
>In addition, bridging and backcrossing also offers the prospect of
>combining multiple herbicide tolerance within the same weed species.
>Further studies need to be carried out to determine the possibility of gene
>flow with sterile single hybrid combinations. For example, F1 hybrids need
>to be inter-crossed to determine the possibility of bridge crossing (i.e.
>[canola x weed A] x weed B,or [weed A x weed B] x canola). In addition,
>the degree of fertility in hybrid combinations andthe potential of
>backcrossing needs further investigation.
>Overall, it is difficult to make strong conclusions as the data sets are
>only partially complete. However some indications already observed are: i.
>canola seed can be readily transported throughout a region and therefore
>there is a risk that these crops will become volunteer weeds; ii.canola
>pollen can move at least, or more than, 26 m and movement is affected by
>wind direction;iii. canola and some related weeds could combine to produce
>hybrid plants under glasshouse and field condition; iv. herbicide
>resistance is expressed in species hybrids; v. biological fitness of hybrid
>plants is less than parent species although fitness is increased by
>backcrossing; vi. bridge crosses could play a major role in the movement of
>herbicide resistant genes into natural weedpopulations.
>The authors would like to thank Plant Genetic Systems N.V. Ghent, Belgium
>for providing the transgenic herbicide genotypes used in this research.
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>Figure 1. Percentage cross pollination observed in the 1993 field study
>according to distance from pollen source (a) and percentage pollination
>according to direction from pollen source (b).
>Figure 2. Log of the percentage cross pollination according to distance
>from Genesee in 1994.
>Richard Wolfson, PhD
>Consumer Right to Know Campaign,
>for Mandatory Labelling and Long-term
>Testing of all Genetically Engineered Foods,
>500 Wilbrod Street
>Ottawa, ON Canada K1N 6N2
>Our website, http://www.natural-law.ca/genetic/geindex.html
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