R EPORTS warming, this floral diversification also coin- 17. R. G. Raynolds et al., “The Kiowa Core, a Continuous 28. P. Wilf, Paleobiology 23, 373 (1997). cides with the uplift of the Bighorn and Drill Core Through the Denver Basin Bedrock Aquifers 29. P. Wilf, S. L. Wing, D. R. Greenwood, C. L. Greenwood, at Kiowa, Elbert County, Colorado,” U. S. Geol. Surv. Geology 26, 203 (1998). Beartooth mountain ranges (38). The results Open-File Rep. 01-0185 (2001). 30. S. L. Wing, in Late Paleocene-Early Eocene Climatic and from Castle Rock suggest that local topography 18. D. J. Nichols, R. F. Fleming, Rocky Mountain Geol.,in Biotic Events in the Marine and Terrestrial Records, M.-P. may be related to floral diversification. Con- press. Aubry, S. P. Lucas, W. A. Berggren, Eds. (Columbia Univ. versely, the Castle Rock flora may represent a 19. J. F. Hicks, K. R. Johnson, L. Tauxe, Rocky Mountain Press, New York, 1998), pp. 380–400. Geol., in press. 31. P. Wilf, GSA Bull. 112, 292 (2000). previously unidentified warm interval in the 20. R. G. Raynolds, in Geologic History of the Colorado Front 32. The formula used was [2(number of species in com- early Paleocene. Range, 1997 RMS-AAPG Field Trip #7, D. W. Bolyard, mon between the two samples)]/(number ofspecies Our results imply that, in certain set- S. A. Sonnenberg, Eds. (Rocky Mountain Association of in sample 1 ϩ number ofspecies in sample 2). Geologists, Denver, CO, 1997), pp. 43–48. 33. R. J. Burnham, in Manu: The Biodiversity of Southeast- tings, floral diversity recovered soon after 21. S. G. Robson, E. R. Banta, U.S. Geol. Surv. Open-File ern Peru, D. E. Wilson, A. Sandoval, Eds. (Smithsonian the mass extinction at the K-T boundary in Rep. 93-442 (1993). Institution, Washington, DC, 1996), pp. 127–140. North America. Another possibility is that 22. P. D. Warwick, R. M. Flores, D. J. Nichols, E. C. 34. R. M. Kirkham, L. R. Ladwig, Coal Resources of the the Laramide Front Range provided a K-T Murphy, in Coal-Bearing Strata: Sequence Stratig- Denver and Cheyenne Basins, Colorado (Resource Se- raphy, Paleoclimate, and Tectonics, J. C. Pashin, ries 5, Colorado Geological Survey, Denver, CO, refugium for Cretaceous plants and that the R. A. Gastaldo, Eds. (American Association ofPe- 1979). early Paleocene rainforest was stocked with troleum Geology Studies in Geology, Tulsa, OK, in 35. V. B. Cherven, A. R. Jacob, in Cenozoic Paleogeogra- Cretaceous survivors from the uplands. We press). phy of the West-Central United States, R. M. Flores, 23. J. F. Hicks, K. R. Johnson, L. Tauxe, D. Clark, J. D. S. S. Kaplan, Eds. (Rocky Mountain Section, Society of do not favor this hypothesis for four rea- Obradovich, in The Hell Creek Formation and the Economic Paleontologists and Mineralogists, Denver, sons: (i) The Castle Rock palynoflora, al- Cretaceous-Tertiary Boundary in the Northern Great CO, 1985), pp. 127–170. though unusually rich, is more similar to Plains: An Integrated Continental Record of the End of 36. W. R. Dickinson et al., Geol. Soc. Am. Bull. 100, 1023 typical Paleocene palynofloras than to Cre- the Cretaceous, J. Hartman, K. R. Johnson, D. J. Ni- (1988). chols, Eds. (Geological Society ofAmerica, Boulder, 37. J. A. Wolfe, G. R. Upchurch Jr., Proc. Natl. Acad. Sci. taceous ones; (ii) earliest Paleocene floras CO, 2002, in press), vol. 361. U.S.A. 84, 5096 (1987). (pre–Castle Rock floras) from the Denver 24. J. D. Archibald et al.,inCenozoic Mammals of North 38. G. I. Omar, T. M. Lutz, R. Giegengack, Geol. Soc. Am. Basin are of low diversity (Fig. 4B) and America, M. O. Woodburne, Ed. (Univ. ofCalifornia Bull. 106, 74 (1994). Press, Berkeley, CA, 1987), pp. 24–76. resemble coeval post–K-T boundary recov- 39. R. S. Barclay, D. L. Dilcher, K. R. Johnson, Geol. Soc. 25. Quarry 1200A produced 33 morphotypes from 176 Am. Abstr. Program 33, A-198 (2001). ery floras to the north (2, 5–7, 39, 40); (iii) identified specimens; 1200B produced 36 from 40. R. S. Barclay, thesis, Univ. ofFlorida, Gainesville, FL Cretaceous floras in the Denver Basin share 180; 1200C produced 50 from 193; 1200D pro- (2002). duced 39 from 185; and 1200Z produced 50 from 41. R. J. Burnham, N. C. A. Pitman, K. R. Johnson, P. Wilf, only a few species in common with Castle 203. A total of937 specimens were identified from Rock; and (iv) no known Cretaceous floras Am. J. Bot. 88, 1096-1102 (2001). a collection of1490 (the remaining leaves are too 42. We acknowledge funding from NSF (grant exhibit rainforest physiognomy. poorly preserved to be definitively identified). Al- EAR-9805474) and in-kind support from the Colo- though most ofthe well-preserved and identifiable rado Department ofTransportation. R. Burnham, P. The presence of the Castle Rock flora specimens were collected, poorly preserved and argues that orographic effects on local cli- Wilf, D. Nichols, S. Wallace, R. Barclay, K. Benson, R. unidentifiable specimens were common and many Dunn, R. Ellis, S. Manchester, R. Raynolds, M. Reyn- mate can be recognized in the fossil record were not collected, suggesting caution when quan- olds, and A. Kropp provided useful comments and and that the recovery of plant diversity after titatively comparing this flora with other unbiased help on various aspects ofthe project. samples. the K-T boundary occurred at different rates, 26. LAWG, Manual of Leaf Architecture—Morphological Supporting Online Material depending on physiographic location. Description and Categorization of Dicotyledonous and www.sciencemag.org/cgi/content/full/296/5577/2379/ Net-Veined Monocotyledonous Angiosperms (Smith- DC1 sonian Institution Press, Washington, DC, 1999). Tables S1 to S9 References and Notes 27. S. L. Wing, D. R. Greenwood, Philos. Trans. R. Soc. 1. R. Sweet, Geosci. Canada 28, 127 (2001). London Ser. B 341, 243 (1993). 21 March 2002; accepted 24 May 2002 2. K. R. Johnson, Cretaceous Res. 13, 91 (1992). 3. C. C. Labandeira, K. R. Johnson, P. Wilf, Proc. Natl. Acad. Sci. U.S.A. 99, 2061 (2002). 4. D. A. Pearson, T. Schaefer, K. R. Johnson, D. J. Nichols, Geology 29, 36 (2001). Proterozoic Modular 5. R. W. Brown, U. S. Geol. Surv. Prof. Pap. 375 (1962), p. 119. 6. L. J. Hickey, in Early Cenozoic Paleontology and Stra- Biomineralized Metazoan from tigraphy of the Bighorn Basin, Wyoming, P. D. Gin- gerich, Ed. (University ofMichigan Pap. Paleontol. Ann Arbor, MI, 1980), vol. 24, pp. 33–49. the Nama Group, Namibia 7. K. R. Johnson, L. J. Hickey, Geol. Soc. Am. Spec. Paper 247 (1990), p. 433. Rachel A. Wood,1,2* John P. Grotzinger,3 J. A. D. Dickson2 8. S. L. Wing, J. Alroy, L. J. Hickey, Palaeogeogr. Paleo- climatol. Paleoecol. 115, 117 (1995). 9. K. R. Johnson, thesis, Yale University, New Haven, CT We describe a Proterozoic, fully biomineralized metazoan from the Omkyk (1989). Member (ϳ549 million years before the present) of the northern Nama 10. J. A. Wolfe, Paleobiology 13, 215 (1987). Group, Namibia. Namapoikia rietoogensis gen. et sp. nov. is up to 1 meter 11. P. W. Richards, The Tropical Rainforest: an Ecological Study (Cambridge Univ. Press, Cambridge, ed. 2, in diameter and bears a complex and robust biomineralized skeleton; it 1996). probably represents a cnidarian or poriferan. Namapoikia encrusts perpen- 12. J. A. Wolfe, U. S. Geol. Surv. Prof. Pap. 1106 (1979), dicular to the walls of vertical synsedimentary fissures in microbial reefs. p. 37. This finding implies that large, modular metazoans with biologically con- 13. R. J. Morley, Origin and Evolution of Tropical Rainfor- ests ( Wiley, Chichester, UK, 2000). trolled mineralization appeared some 15 million years earlier than previ- 14. R. J. Burnham, Rev. Palaeobot. Palynol. 81, 99 (1994). ously documented. 15. The fossil leaves are preserved as cuticle-bearing com- pressions in a medium gray andesitic mudstone with The appearance and rapid diversification of calcified taxa have been recorded from ter- relictual ripple cross-stratification, suggesting that the matrix was a muddy sand at the time ofdeposition. metazoans with fossilizable hard parts minal Proterozoic (Vendian) strata. These Two 5-cm-thick leaf-bearing layers, separated by 10 cm around the Precambrian-Cambrian bound- taxa are solitary, weakly biomineralized, of ofmassive mudstone, lie directly above a rooted mud- ary [ϳ543 million years (My) before the uncertain affinity, and with only general- stone interpreted as a gley paleosol. 16. D. J. Hopkins Jr., K. R. Johnson, Am. J. Bot. 84, 135 present (B.P.)] marks one of the most dra- ly constrained ecological preferences (4– (1997). matic events of evolution (1–3). Only five 7). Here, we describe a Late Proterozoic

www.sciencemag.org SCIENCE VOL 296 28 JUNE 2002 2383 R EPORTS fully biomineralized metazoan from Namibia. to D). Several Namapoikia specimens are some characteristics with tetradiids, a group The fossil is from the Omkyk member of upto1minwidth and 0.25 m in height otherwise known from the Ordovician, but the terminal Proterozoic–early Cambrian (Fig. 1D). The skeleton is modular and lack the distinctive quadripartite longitudinal Nama Group (Ͼ3000 m) in southern Nami- consists of multiple, incomplete, continu- fission of corallites. Tetradiids are aragonitic bia (fig. S1A). These host rocks are fossil- ously conjoined tubules ranging from 1.5 to coralomorphs, but their precise position with- iferous shallow marine carbonate and sili- 5 mm in diameter (Fig. 1, C and D), which in the Cnidaria is unclear (12). ciclastic rocks (8, 9). An ash bed that im- in transverse section appear labyrinthine to Molecular phylogenies for the divergence of mediately overlies the Omkyk Member has occasionally polygonal (Fig. 1F). The tu- the phyla Porifera and Cnidaria differ widely been dated as 548.8 Ϯ 1 My B.P. (10). bules do not appear to have expanded with but can be conservatively placed at ϳ670 My Many specimens of Namapoikia gen. growth. Skeletal elements are 0.5 to 3.5 B.P. or earlier (17). The oldest sponge body nov. were found associated with thrombo- mm in diameter. Longitudinal partition fossils (ϳ560 My B.P.) are known from both litic and stromatolitic bioherms within the walls are present, and tubules grew by lon- soft-bodied Ediacaran biota (18) and spicule Driedoornvlagte pinnacle reef complex, gitudinal fission. Growth annulae occur clusters (19). What appear to be soft-bodied near Rietoog (23° S, 51.50Ј;16° E, 39.38Ј) with a spacing of 0.5 to 2.5 mm. Skeletal cnidarians are known from phosphatized mate- (fig. S1B). The reef complex is over 300 m filling tissue, such as tabulae or dissepi- rial in the Doushantuo Formation of China (20), thick and at least 7 km long (6, 11). Bio- ments, is absent, although some structures and more generally in the Ediacaran biotas (21). herms individually form elliptical mounds resemble incomplete tabulae. The skeleton Unequivocal body fossils of actiniarian and cor- that reach up to 20 m in diameter, 5 to 10 m of all collected specimens is totally recrys- allomorphariid sea anemones are described in width, and5minheight, but coalesce to tallized to blocky calcite (ϳ20 ␮mindi- from the Ordovician (22), but supposed trace produce near-continuous structures with ameter). The distinction of skeletal from fossils of resting sites are known from the early their long axes displaying a strong orienta- pore cement calcite is difficult in transmit- Cambrian (23). tion parallel to the inferred paleoshoreline ted light (fig. S2A). We examined several Calcified skeletons have evolved multiple (now about northeast-southwest). Bioherm sections using cathodoluminescence but times in the history of the Porifera (24), and cores were constructed by massive throm- could not identify any original internal mi- molecular data suggest that, for example, the bolites, often with entrapped Namacal- crostructure (fig. S2, B and C). Systematic scleractinian coral skeleton alone may have athus; the outer (younger) layers consist of paleontology is given in Appendix S1. evolved at least four times (25). Poriferans and stromatolites up to 0.75 m thick. Nama- Diverse skeletal fossils variously de- Cnidarians can acquire or lose hard parts with poikia gen. nov. encrusts the walls of ver- scribed as coralomorphs or corals are relative ease (24, 26). Many of the putative tical synsedimentary fissures, which known from the Lower Cambrian (12, 13). Lower Cambrian calcified cnidarians show no formed perpendicular to bedding (Fig. 1, B Namapoikia shows similarities to the small obvious affinity to the two main groups of and C), and more rarely open reef surfaces. Lower Cambrian (Botomian) cerioid cor- corals (the orders Tabulata and Rugosa) that Fissures are abundant and occur with a alomorphs, in particular Yaworipora from dominated the Paleozoic record and so have spacing of up to three per meter. Individual Siberia (13) and the cryptic, encrusting taxa been proposed to represent a series of indepen- fissures can reach at least5minlength and Labyrinthus (14) and Rosellatana (15) dently skeletalized clades of anemone (12). We 0.3 m in width (Fig. 1A). Fissure systems from eastern North America and British suggest here that Namapoikia represents a fur- form in reefs as a result of early lithifica- Columbia, respectively. Yaworipora and ther calcified, modular clade of probable cni- tion, and many became filled with cement Labyrinthus grew as conjoined, thick- darian or poriferan affinity. and sediments; such fissures are common in walled, polygonal tubes, Ͻ5 mm in diam- Terminal Proterozoic reefs have been modern and ancient reefs. Namapoikia in- eter, with an open, labyrinthine transverse thought to be ecologically simple and of low dividuals may partially or completely fill section. Rosellatana shows a more regular biodiversity (27); but the presence of Nama- the fissure void, although details of the tubular construction, with polygonal to poikia implies that there was a common original basal attachment sites have been rounded lumens up to 1.5 mm in diameter. metazoan component to these communities destroyed by stylolitization (Fig. 1C). All three genera lack tabulae. and that a differentiation of reef metazoans Some Namapoikia were subsequently en- These forms and Namapoikia show a clear into distinct open surface and cryptic inhab- crusted by thin rinds of stromatolites (now modular organization, suggesting affinity itants, so characteristic of Phanerozoic reefs, dolomitized) (Fig. 1C). Remaining void with some calcified protozoans or lower in- had taken place by the end of terminal Pro- space is filled with large neomorphosed vertebrates (Porifera and Cnidaria). Their tu- terozoic time. aragonitic botryoids up to 15 mm in radius bular construction is particularly similar to Fossilizable hard parts were thought to that represent early marine aragonite ce- that of chaetetids [a polyphyletic group of have first appeared as weakly skeletal, soli- ments (now calcite); bioclastic-rich pack- calcified sponges (16)], known from the mid- tary organisms in the terminal Proterozoic, stone with abundant Cloudina debris, Ordovician to Recent, and tabulate corals followed by an array of small, mainly calcar- which may form geopetal infills; and late- (Lower Ordovician to Permian). The tube eous, shelly fossils in the earliest Cambrian burial calcite spar (Fig. 1D). diameter of Namapoikia, however, consider- (Nemakit-Daldyn to Tommotian); large Namapoikia gen. nov. begins as nodular ably exceeds that of all described skeletal metazoans with heavily biomineralized or domal individuals that either coalesce or protozoans and algae and, to a lesser degree, skeletons were not known previously be- extend laterally, similar to a sheet (Fig. 1, B chaetetid sponges. Recrystallization of the fore the Tommotian (1–7). The skeleton of skeleton precludes identification of either a Namapoikia is notably robust, and its large 1Schlumberger Cambridge Research, High Cross, Mad- lamellar microstructure or a central wall (in- size is reminiscent of coral and poriferan ingley Road, Cambridge CB2 OEL, UK. 2Department of dicative of biomineralization through a cni- organizations that do not otherwise appear Earth Sciences, University ofCambridge, Downing darian epithelium), which would provide un- in the fossil record until the mid-Paleozoic. 3 Street, Cambridge CB2 3EQ, UK. Department of equivocal placement within the Cnidaria. Re- Namapoikia thus demonstrates that large, Earth, Atmospheric, and Planetary Sciences, Massa- chusetts Institute ofTechnology, Cambridge, MA crystallization of the original microstructure modular, skeletal metazoans appeared some 02139, USA. is, however, suggestive of an original arago- 15 million years earlier than previously *To whom correspondence should be addressed. E- nitic mineralogy for Namapoikia. Labyrin- documented (1–7), and complex reef ecol- mail: [email protected] thus, Rosellatana (12), and Namapoikia share ogies even earlier.

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Fig. 1. Namapoikia rietoogensis gen. et sp. nov.; Omkyk Member, Zaris Formation, Nama Group, Driedoornvlagte; southern Namibia. (A to D) Outcrop photographs; (E and F) holotype (F 623), polished surfaces. (A) Three intersecting neptunian dykes (walls indicated by arrows) within a thrombolitic-stromatolitic bioherm. Note how the dykes truncate the digitate thrombolite fabricofthe reefframework.(B) Series ofsmall nodular individuals attached to a single fissure wall. The contact is stylolitized. (C) Nodular individual growing perpendicular to a fissure wall, encrusted by a rind ofdolomitized stromatolite (arrows). (D) Tangent ial section ofa single, extensive individual, with some early botryoidal (dark) and late burial spar (white) cements. (E) Longitudinal section, showin g open, tubular construction. (F) Transverse section showing labryrinthine tubule cross sections.

www.sciencemag.org SCIENCE VOL 296 28 JUNE 2002 2385 R EPORTS References and Notes 14. D.࿜࿜࿜࿜ R. Kobluk, Can. J. Earth Sci. 16, 2040 (1979). 27. R. Wood, Reef Evolution (Oxford Univ. Press, Oxford, 1. S. Bengston, S. Conway Morris, Top. Geobiol. 20, 447 15. , J. Paleontol. 58, 703 (1984). UK, 1999). (1992). 16. J. R. Reitner, Berliner Geowissenschaft. Abh. E. 1,1 28. Supported by the Geological Survey ofNamibia, NSF, 2. M. D. Brasier, The Precambrian-Cambrian Boundary (1992). Petroleum Development Oman, and Schlumberger. (Clarendon, Oxford, UK, 1989), pp. 117–165. 17. D. Bridge, C. W. Cunningham, R. DeSalle, L. Buss, Mol. We thank C. Husselmann for access to Driedoorn- 3. S. A. Bowring et al., Science 261, 1293 (1993). Biol. Evol. 12, 679 (1995). vlagte; D. McCormick and S. Schroeder for assistance 4. S. W. F. Grant, Am. J. Sci. 290A, 261 (1990). 18. J. H. Gehling, J. K. Rigby, J. Paleontol. 70, 185 (1996). in the field; D. Simons for photographic expertise; S. 5. G. B. H. Germs, Am. J. Sci. 272, 752 (1972). 19. M. D. Brasier, O. Green, G. Sheilds, Geology 25, 303 Conway Morris, C. Scrutton, A. Knoll, S. Bengston, M. 6. W. A. Watters, J. P. Grotzinger, Paleobiology 27, 159 (1997). Brasier, M. Droser, and an anonymous reviewer for (2001). 20. S. Xiao, X. Yuan, A. H. Knoll, Proc. Natl. Acad. Sci. comments on the manuscript; R. Duncan-Jones and 7. H. J. Hofmann, E. W. Mountjoy, Geology 29, 1091 U.S.A. 97, 13684 (2000). A. Brett for etymological expertise; and R. Swart (2001). 21. S. Conway Morris, Palaeontology 36, 593 (1993). (National Petroleum Corporation ofNamibia) for 8. B. Z. Saylor, A. J. Kaufman, J. P. Grotzinger, F. Urban, 22. C. T. Scrutton, in The Origin of Major Invertebrate permission to publish the LANDSAT image shown in J. Sediment. Res. 68, 1223 (1998). Groups, M. R. House, Ed. (Academic Press, London, fig. S1B. 9. G. M. Narbonne, B. Z. Saylor, J. P. Grotzinger, J. 1979), pp. 161–207. Supporting Online Material Paleontol. 71, 953 (1997). 23. S. Jensen, B. Z. Saylor, J. G. Gehling, G. J. Germs, www.sciencemag.org/cgi/content/full/296/5577/2383/ 10. J. P. Grotzinger, S. A. Bowring, B. Z. Saylor, A. J. Geology 28, 143 (2000). DC1 24. R. Wood, Am. Sci. 78, 224 (1990). Kaufman, Science 270, 598 (1995). Figs. S1 and S2 11. J. P. Grotzinger, Commun. Geol. Surv. Namibia,in 25. S. L. Romano, S. D. Cairns, Bull. Mar. Sci. 67, 1043 Appendix S1 press. (2000). 12. C. T. Scrutton, Proc. Yorks. Geol. Soc. 51, 177 (1997). 26. G. D. Stanley Jr., D. G. Fautin, Science 291, 1913 13. A. Yu. Zhuravlev, Palaeontol. J. 33, 502 (1999). (2001). 4 March 2002; accepted 15 May 2002

tional variety will contain one copy of each Pollen-Mediated Movement of gene. To assess gene flow, seeds were collected Herbicide Resistance Between from 63 conventional canola fields growing near herbicide-resistant fields in New South Wales, Victoria, and South Australia. These Commercial Canola Fields three states represent over half of the canola- Mary A. Rieger,1,2* Michael Lamond,3 Christopher Preston,1,2 growing area in Australia as well as a wide and Stephen B. Powles,3 Richard T. Roush1 diverse range of environments. Source and sink fields were of similar sizes, ranging from 25 to There is considerable public and scientific debate for and against genetically 100 ha. At crop maturity, 10 stratified samples modified (GM) crops. One of the first GM crops, Brassica napus (oilseed rape totaling at least 100,000 seeds were taken from or canola) is now widely grown in North America, with proposed commercial each of three locations in each field of conven- release into Australia and Europe. Among concerns of opponents to these crops tional canola. These were parallel to the source are claims that pollen movement will cause unacceptable levels of gene flow field and taken at the edge nearest to the source from GM to non-GM crops or to related weedy species, resulting in genetic field, the middle, and the edge furthest from the pollution of the environment. Therefore, quantifying pollen-mediated gene flow source field. Collected seed samples (500 g) is vital for assessing the environmental impact of GM crops. This study quan- were planted as separate plots, in an irrigated tifies at a landscape level the gene flow that occurs from herbicide-resistant field, along with two resistant and two suscep- canola crops to nearby crops not containing herbicide resistance genes. tible canola controls. To determine whether pol- len-mediated gene flow from source to sink Data on pollen dispersal has mostly been ob- system—one herbicide resistance gene on fields had occurred, we screened the seedlings tained from small-scale field trials of limited each genome—and is homozygous for both with a lethal discriminating dose of the ALS- sample size (1–5). Canadian experiments with genes. Therefore, any crosses to a conven- inhibiting herbicide chlorsulfuron, and any sur- GM canola found less than 0.03% pollination at 30 m into conventional varieties (6). However, 0.25 Hall et al.(7) suggested GM canola pollen moved over greater distances. Therefore, we examined pollen movement between herbicide- 0.2 resistant canola and conventional varieties on a commercial scale, testing over 48 million indi- vidual plants. This was possible because canola 0.15 resistant to acetolactate synthase (ALS)–inhibit- ing herbicides was grown commercially in Aus- tralia for the first time in 2000. These first 0.1 commercial fields served as the herbicide resis- Frequency % tance gene source in an uncontaminated environment. This variety has a two-gene 0.05

1Cooperative Research Center for Australian Weed Management, 2Department ofApplied and Molecular Ecology, University ofAdelaide, PMB1, Glen Osmond 0 SA 5064, Australia. 3Western Australian Herbicide 0 1000 2000 3000 4000 5000 6000 Resistance Initiative, University ofWestern Australia, Distance (m) Nedlands WA 6907, Australia. Fig. 1. Percentage ofALS herbicide-resistant individuals in seed fromnonresistant varieties in *To whom correspondence should be addressed. E- relation to distance from the source field. Three individual samples were collected per field, with mail: [email protected] 190 individual collection locations.

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