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Reproductive Biology |
2Iowa State University, Department of Agronomy, 2104 Agronomy Hall, Ames, Iowa 50011-1011 USA; 3University of Delaware, Department of Plant and Soil Sciences, Research and Education Center, 16684 County Seat Highway, Georgetown, Delaware 19947-9575 USA
Received for publication May 14, 2006. Accepted for publication February 11, 2007.
ABSTRACT
Transfer of herbicide resistance genes between crops and weeds is relatively well documented; however, far less information exists for weed-to-weed interactions. The hybridization between the weedy diploids Conyza canadensis (2n = 18) and C. ramosissima (2n = 18) was investigated by monitoring transmission of the allele conferring resistance to N-phosphonomethyl glycine (glyphosate). In a multivariate quantitative trait analysis, we described the phylogenic relationship of the plants, whereas we tested seed viability to assess potential postzygotic reproductive barriers (PZRB) thus affecting the potential establishment of hybrid populations in the wild. When inflorescences were allowed to interact freely, approximately 3% of C. ramosissima or C. canadensis ova were fertilized by pollen of the opposing species and produced viable seeds; >95% of the ova were fertilized under no-pollen competition conditions (emasculation). The interspecific Conyza hybrid (
Key Words: allogamy • Conyza canadensis • Conyza ramosissima • gene flow • herbicide resistance • interspecific hybridization • shikimic acid • transgressive segregation
Interspecific hybridization refers to the cross-fertilization between two species that produces a fertile or infertile progeny with phenotypic traits of both parents; this process of interspecific gene transfer promotes genetic diversity and genome evolution (Abbott, 1992
; Barton, 2001
). This natural process has been utilized in breeding efforts to improve crop traits (Anamthawat–Jónsson, 2001
). This process, however, may also facilitate the rapid evolution and adaptation of introduced plant pathogens and contribute to the genetic diversity of crop pests, and thus it may increase production difficulties in current agroecosystems (Teal and Oostendorp, 1995
; Schardl and Craven, 2003
). Furthermore, interspecific hybridization may adversely affect crop production and weed management, as interspecific transfer of herbicide resistance and genetically engineered genes has been documented (Owen and Zelaya, 2005
).
Much information exists regarding the transgene flow and transfer of herbicide resistance genes between crops and their wild relatives (Kwon and Kim, 2001
; Ellstrand, 2003
; Légère, 2005
; Guadagnuolo et al., 2006
; Reichman et al., 2006
). However, far less attention has been focused on gene flow between weed species and the impact on dissemination of herbicide resistance alleles or the evolution of novel taxa with diverse "weedy" traits. Current estimates of weed-to-weed herbicide resistance transfer based on four genera vary from 0.15% to 85% as predicted by the frequency of resistant individuals in the interspecific hybrid progeny (Table 1). To address this disparity in knowledge, we investigated hybridization between the weedy diploids Conyza canadensis (L.) Cronq. (2n = 18) and C. ramosissima Cronq. (2n = 18) by monitoring transmission of the allele conferring resistance to the herbicide N-phosphonomethyl glycine (glyphosate).
|
). The genus Conyza Less., represented by approximately 60 species, is composed of annual herbaceous plants that prosper chiefly in the tropical and subtropical regions of the globe (Nesom, 1990
). Species endemic to the United States include the winter or summer annual forbs C. ramosissima and C. canadensis (Cronquist, 1980
).
Conzya ramosissima was originally described in Illinois as Erigeron divaricatus (Michaux, 1803
); the species was later annexed to the genus Conyza based on the eligulate character of its multiseriate pistillate florets (Cronquist, 1980
). Both C. ramosissima and C. canadensis are ubiquitous in disturbed habitats and play important roles in primary ecological successions; however, only C. canadensis is reported to reduce yields in row crops, serve as an alternate host for diverse pests, and limit grazing by reducing the palatability of pasture forages (Steyermark, 1963
; Cronquist, 1980
). Importantly, C. canadensis has evolved resistance to amide, bipyridilium, glycine, imidazolinone, sulfonylurea, and triazine herbicides in more than 10 countries worldwide and thus is considered one of the 10 most important herbicide-resistant weeds (Heap, 2006
). Conzya ramosissima is found from North Dakota to Pennsylvania and south from New Mexico to Alabama in the United States, in southern Canada, and northern Mexico, while the more cosmopolitan C. canadensis is distributed throughout the Americas, West Indies, Europe, and Africa (Steyermark, 1963
). Interestingly, the Conyza taxon represents the most successful case of intercontinental colonization of the Americas to the Old World, to the extent that C. canadensis and C. floribunda H.B.K. are probably the most widely distributed species throughout the world (Thébaud and Abbott, 1995
; Pruski and Sancho, 2006
).
Glyphosate is one of the world's most important herbicides; glyphosate-resistant crops provide farmers with a simple, economical, and effective tool to manage a diverse weed flora, hence favoring the rapid adoption of this technology in many United States crop production systems (Owen and Zelaya, 2005
). The mechanism of glyphosate action is the competitive inhibition with respect to the phosphate moiety of phosphoenolpyruvate in the reaction mediated by 3-phosphoshikimate 1-carboxyvinyltransferase (EPSPS; EC 2.5.1.19) (Steinrücken and Amrhein, 1980
; Holländer-Czytko and Amrhein, 1983
). The unique mode of action and limited metabolism in plants are purported reasons for the low frequency of evolved glyphosate resistance compared to other herbicide chemistries (Jasieniuk, 1985
; Bradshaw et al., 1997
). Since the commercial introduction of glyphosate-resistant crops in 1996 and the accompanying ubiquitous glyphosate use in these production systems, 12 weed species resistant to glyphosate have been identified worldwide, including confirmation in 15 independent C. canadensis populations within the United States and populations in Brazil and China (Heap, 2006
).
Transmission of glyphosate resistance via gene flow has been documented between several crop and weed systems (Watrud et al., 2004
; Légère, 2005
; Guadagnuolo et al., 2006
; Reichman et al., 2006
). Presently, however, limited information exists regarding the level of within-species glyphosate resistance transfer in self-incompatible, wind-pollinated weed species as Plantago lanceolata, Amaranthus palmeri, and A. tuberculatus or the between-species transfer in the interfertile Ambrosia artemisiifolia and A. trifida or Lolium rigidum and L. perenne (Vincent and Cappadocia, 1987
; Tonsor, 1990
; Balfourier et al., 1998
; Wetzel et al., 1999
). We previously reported that the within-species glyphosate resistance transfer, based on the proportion of resistant individuals in the progeny of C. canadensis plants, ranged from 0% to 14% and 92% to 100% under pollen competition and no-pollen competition studies, respectively (Zelaya et al., 2004
). Introgressive hybridization (introgression) in Conyza is well documented in the European species; however, the existence of Conyza hybrid zones in the Americas is unknown (Knobloch, 1972
; Stace, 1975
; McClintock and Marshall, 1988
; Thébaud and Abbott, 1995
). Considering the importance of glyphosate as a global herbicide and the pervasive nature of Conyza worldwide, an investigation was undertaken to assess the potential for glyphosate resistance transfer from C. canadensis to C. ramosissima through hybridization. Postzygotic reproductive barriers and phenotypic characterization of the interspecific Conyza hybrid (
MATERIALS AND METHODS
Plant materials
The glyphosate-susceptible C. ramosissima population (
). We previously reported that glyphosate resistance in this population was governed by an incompletely dominant, single nuclear gene (R allele) (Zelaya et al., 2004
). A stable, near-homozygous glyphosate-resistant C. canadensis population (RS2) was isolated in the greenhouse through two cycles of recurrent selection on 
). Complete specimen sets of 
Growth conditions
Twenty
Conyza interspecific hybridization studies
The gynomonoecious C. canadensis and C. ramosissima possess white pistillate ray florets in the capitulum periphery and yellow perfect disk florets in the capitulum core. Pollen release may occur prior to capitula opening; therefore, both pollen competition and no-pollen competition studies were conducted pre-anthesis.
Response of parents to glyphosate
Prior to conducting crosses, the phenotype of parents was assessed to verify that RS2 and
Pollen competition studies
Reciprocal crosses were performed between C. canadensis and C. ramosissima plants (
No-pollen competition studies
Disk florets of
Assessment of emasculation efficiency
Ten capitula per
Crossing scheme
Ten RS2 and
60% visual injuries but did not kill 
Postzygotic reproductive barriers
Seed viability was tested according to the Association of Official Seed Analysts (AOSA) standard germination procedure recommended for Asteraceae (AOSA, 2003
). One hundred seeds per each of the 10 parents (RS2 and
). Seed viability experiments were repeated in time 1 wk after the initial assessment (n = 2000). For postzygotic reproductive barriers, hybrid inviability was tested by assessing viability of 
Characterization of interspecific hybrids
Ten randomly selected RS2 and
Response of interspecific Conyza hybrids to glyphosate
The recommended glyphosate rate for Conyza at the 10-cm diameter rosette stage is 0.85 kg AE/ha (Roundup UltraMAX, Monsanto, St. Louis, Missouri, USA) (Anonymous, 2004
). The resistant (R) and intermediate-resistant (IR) phenotypes comprised those rosettes that developed 30% and 31–69% visual injury 20 d AT, respectively, when treated with 2.0 kg AE of glyphosate/ha (Zelaya et al., 2004
). After treatment with glyphosate, both R and IR phenotypes reached reproductive stage; however, only the R phenotype had visual growth rates equivalent to those observed in untreated Conyza plants. The susceptible (S) phenotype had 
70% visual injury and was killed at the 2.0 kg AE/ha glyphosate rate.
Whole-plant rate response
Plants were treated with deionized water (dH2O; control) or 0.5, 1, 2, 4, 8, or 16 times the recommended glyphosate rate of 0.85 kg AE/ha. Treatments were applied 30 cm above the plant canopy through an 80015-E nozzle (TeeJet Spraying Systems, Wheaton, Illinois, USA) in a CO2-powered spray chamber (SB5–66, DeVries Manufacturing, Hollandale, Minnesota, USA) delivering 187 L/ha at a pressure of 2.8 kg/cm2. Treatments had four replications per rate and were repeated once in time (n = 8). Glyphosate phytotoxicity symptoms included plant stunting, leaf chlorosis, and necrosis that developed from the meristems and leaf tips to the rest of the plant. At 20 d AT, glyphosate efficacy was visually estimated by comparing glyphosate-treated Conyza plants with the dH2O-treated control plants (0% = non-injured; 100% = completely necrotic). Plants were then cut at the soil surface, placed in paper bags, and dried at 80°C for 48 h. Biomass was estimated by weighing the individual Conyza sample per 12 cm diameter pot and used to determine the glyphosate rate that inhibited plant growth by 50% (GR50); meristems subsamples were also taken for shikimic acid determination based on a method previously reported for Conyza (Zelaya et al., 2004
).
Statistical analysis
Statistical Analysis Software (SAS, 2000
) was utilized to conduct data analyses. Seed viability tests, arranged in a complete randomized design (CRD), were subjected to analysis of variance (ANOVA; PROC GLM) as well as mean separation by Fisher's least significant difference (LSD
=0.05) when ANOVA identified significant taxon effects. Glyphosate rate response tests were analyzed as a randomized complete block (RCB). GR50, I50 (glyphosate rate resulting in 50% accumulation of the maximum estimable shikimic acid), LD50 (glyphosate rate inflicting 50% mortality within the population), |
50| (the absolute difference between two estimated GR50 values), and 
2 goodness-of-fit estimates were done as previously reported (Zelaya et al., 2004
).
Quantitative trait data were tested for normality based on the univariate Shapiro-Wilk test and accepted if the P value for W100 was 0.05; otherwise, the variance (
2) of the data were normalized by natural-log transformation (Shapiro and Wilk, 1965
). ANOVA was done on individual quantitative traits considering taxon and family nested within taxon as fixed and random effects, respectively. Fisher's LSD=0.05 tested for differences between taxa means. When the normality assumption was not met, Kruskal-Wallis analyses (PROC NPAR1WAY) were conducted, and post hoc non-parametric mean separation was performed by Dunn's test (Bonferroni's method) (Conover, 1999
). In addition, PROC VARCOMP was used to partition the total phenotypic 2 into the different ANOVA components for traits that converged to Shapiro-Wilk's normality assumption; these 2 components were then used to calculate the intraclass correlation coefficient (t):
|
| (1) |
2 between and within taxon, respectively.
Phenotypic intermediacy of the
). Multivariate normality was tested by Mardia's kurtosis (
2) analysis (Mardia, 1970
). PROC PRINCOMP was invoked to test whether the data covariance matrix was singular. If rejected, squared Mahalanobis distances ( 
2) quantile–quantile (Q–Q) comparisons of multivariate normal data; 75% confidence intervals were constructed from standard deviation () estimates of g(z), as indicated by Chambers et al. (1983)
. Quantitative traits that converged to Mardia's assumption were then subjected to canonical discriminant function (CDF) analysis utilizing PROC CANDISC (Thompson, 1984
). Taxon clustering of group averages used Mahalanobis' distance matrix based on the unweighted pair-group method with arithmetic mean (UPGMA) (PROC CLUSTER); concomitantly, PROC TREE was used to display the phylogenic relationship between taxon. RESULTS
The Conyza parental populations differed in their response to glyphosate
Nonsignificant lack-of-fit (LOF) tests and coefficients of determination estimates for biomass (F = 0.33; P = 0.98;
50| value of 1.60 (Fobs = 1.77; P < 0.01) was observed, reaffirming that the parental populations differed in their response to glyphosate. Shikimic acid determinations, which indirectly estimate the level of EPSPS inhibition by glyphosate (Harring et al., 1998
), suggested that twice the rate of glyphosate was required to inhibit 50% of EPSPS in RS2 plants compared to 
|
|
|
). Under greenhouse environments, approximately 60% of the total achenes on 
|
|
Phenotypic variance of parents and hybrid progenies
Differences among families within taxa were significant for approximately half of the quantitative traits evaluated (Table 4). Maternal family effects were significant (P < 0.05) in nine of 27 and 10 of 27 quantitative traits within the
2) was accounted for differences between the studied Conyza taxa. Furthermore, we assumed that this 2 was primarily attributable to genetic differences between the studied taxa rather than to the interaction with the environment because the Conyza populations developed under controlled greenhouse conditions (Appendix S1, see Supplemental Data accompanying online version of this article).
|
|
5), normal approximation was used for probability estimation rather than a binomial distribution. The calculated one-tailed sign test value for phenotypic intermediacy was significant (z+ = 3.27; P = 0.001) and thus prompted rejection of the null hypothesis (H0) for equal sign proportions of the evaluated quantitative traits. The collective phenotypic data therefore suggested that 
Intermediacy of 
Multivariate normality analysis determined that only 17 of the 27 evaluated morphometric traits converged to Mardia's statistics (2 = 328.2; P = 0.08) and thus were combined for multivariate discriminant analysis; the pistillate florets trait was eliminated from the analysis as inclusion instigated a singular covariance matrix. The H0 that data arose from populations with a common distribution was tested by Q–Q plots; linearity of data points along the expected mean vectors and allocation within the estimated 75% confidence intervals suggested nondeparture from normality (Fig. 4). Most (81%) of intraclass correlation coefficients (t) in the multivariate normally distributed morphometric traits possessed values greater than 0.70, hence suggesting negligible interference of correlations among families that could potentially distort interpretation of the canonical discriminant function (CDF) analysis (Table 4).
|
= 0.004; F = 229.9; P < 0.0001). The CDF analysis estimated that the first (Can1) and second (Can2) canonical variates accounted for 97% and 82% of the total quantitative trait 2, respectively. Taxa clustered in discrete sections within the canonical graph; 
Taxa separation was also discernable by group averages based on the UPGMA. Divergence in Mahalanobis distances was greatest among the RS2 and
Postzygotic reproductive barriers
Partial correlation estimates (r2 = 0.76; P = 0.001) associated with the sums of squares and crossproducts (SSCP) matrix suggested a strong relationship between the seed viability experiments repeated in time. Univariate (F = 0.14; P = 0.71) and multivariate (Wilks' = 0.99; P = 0.71) tests for the between-time effects were not significant, therefore no difference in experiments repeated in time was inferred and data for the seed viability experiments were combined in further analyzes. Previous research reported near 100% C. canadensis germination under light and constant 28°C conditions (Shontz and Oosting, 1970
). Viability of RS2 seeds under our conditions ranged from 17% to 71% within families with a mean of 46%, which was not statistically different (LSD=0.05 = 12%) from the 57% mean viability estimated for
Evidence for hybrid inviability was apparent in the 23% increase in nonviable
The presence of a hybrid sterility reproductive barrier was disregarded because the
). While the germination of 
Transgressive segregation in 
Most hybrid progenies demonstrate marked traits of transgression (Rieseberg et al., 1999
). The majority of
2 = 0.67), 
2 = 0.42), or the 
2 = 0.87), 
2 = 1.17) in response to glyphosate. Similarly, greater variations in the level of endogenous shikimic acid accumulation and visual injury were observed in the 
Phenotype of backcrosses
Visual observations confirmed that the progeny of
Inheritance of glyphosate resistance
Application of 0.40 kg AE of glyphosate/ha to
).
Overdominance of 
The
50| values for comparisons of 
).
Segregation of the R allele
Glyphosate resistance in RS2 is conferred by the incompletely dominant, nuclear R allele (Zelaya et al., 2004
). This model for glyphosate resistance in the Conyza hybrid was tested by monitoring the segregation ratios of
2 values for all 
2 = 5.72; P = 0.77) confirmed that the combined 
|
2 = 4.40; P = 0.88), and the collective data were consistent with the expected 1 : 1 (R : IR) ratio. Similarly, analysis of 
2 = 5.81; P = 0.76) that obeyed the partially dominant model (Table 6). Because the backcross data followed the monofactorial model of inheritance, results corroborated our previous assertion that RS2 and 
Further substantiation of the incompletely dominant, monogenic model was obtained graphically from the pattern of observed
: Y = WR (0.25) + WIR (0.50) + WS (0.25). Three distinct response phases were predicted based on partially dominant, monofactorial inheritance (Fig. 2). No mortality was recorded from 0.0 to 0.42 kg AE/ha, suggesting that both homozygous (RR and rr) and the heterozygous (Rr) genotypes were present at these glyphosate rates. A second segment was discernable at glyphosate rates of 0.85–3.38 kg AE/ha, which corresponded to approximately one-fourth (23–37%) mortality of the putative homozygous susceptible genotype (Fig. 2). The heterozygous (71%) and homozygous (100%) resistant genotypes were controlled at glyphosate rates of 6.77 and 13.54 kg AE/ha, respectively. DISCUSSION
Hybridization of Conyza in nature
The phylogenic boundaries in Conyza are not clearly understood and considerable phenotypic variation has been reported, particularly in response to adverse environmental stimuli (Nesom, 1990
). We initiated a project to assess potential hybridization of Conyza species and better understand the phylogenic relationship between C. canadensis and C. ramosissima, two weedy species in United States agroecosystems despite nothing described in the literature. Our work herein suggests that the studied taxa are genetically compatible, capable of transferring the R allele, and producing interspecific hybrid progenies that are vigorous and fertile. Given that C. canadensis has become a major economic weed problem in the Midwestern United States and the apparent vigor of the hybrid identified in this research, the ramifications of an interspecific Conyza hybrid that has resistance to glyphosate are potentially significant. Glyphosate-resistant crop systems are suggested to be simple and without great environmental consequences. However, we have demonstrated that there are major ecological and economic consequences from these presumed simple systems. New weeds typically evolve over a long period of time, and existing weeds tend to adapt to new agroecosystems slowly. We propose that if hybridization of new taxa with glyphosate resistance as a semi-dominant trait can occur with relative ease, the current agroecosystem is at considerable jeopardy. While the hybrid demonstrated in this research has not been specifically identified in the field, we have illustrated the potential for the occurrence.
We speculate that the relatively high genetic compatibility among the studied taxa is associated with the common diploidy structure (2n = 18) that allows for successful chromosome pairing during meiosis. Both C. canadensis and C. ramosissima represent sibling species as ascertained by nrDNA internal transcribed spacers (ITS) analysis, which clustered both taxa in a single branch within group VI of the Erigeron and allied Asteraceae cladogram (Noyes, 2000
). This phylogenic analysis estimated a recent speciation event between the ramosissima and canadensis epithets and provides further support to our thesis of genetic compatibility between the species.
The likelihood of native hybridization in Conyza is probably low given the enclosed involucre arrangement in Conyza and the autogamous nature of the genus. Entomophily was reported in C. canadensis, although the relative importance to interspecific gene transfer remains unknown (Weaver, 2001
). To our knowledge, there is no prior documentation of hybridization between C. canadensis and C. ramosissima. However, since early descriptions, plants with common traits between both species have been cited (Michaux, 1803
).
Voss (1996)
reported depauperate C. canadensis biotypes in the upper peninsula of Michigan with glabrous to pubescent patterns and ramification near or below the main stem, characteristics that we observed in the
described profusely branched C. canadensis biotypes that resembled our 
).
Documentation of hybridization in Conyzinae is at present restricted to Europe. For example, in the British Isles hybridization between C. canadensis and Erigeron acer L. was reported to produce weak and apparently sterile plants with few capitula; the nothotaxa was classified as Conyzigeron x huelsenii (Vatke) Rauschert (Stace, 1975
). Furthermore, hybrids of unknown fecundity, namely Conyza x flahaultiana (Thell.) Sennen and Conyza x daveauiana Sennen in Spain and France, originated from crosses between C. canadensis and C. bonariensis (L.) Cronq. and between C. bonariensis and C. sumatrensis (Retz.) E. Walker, respectively (McClintock and Marshall, 1988
). In the Iberian Peninsula, Conyza x rouyana Sennen arose from the hybridization between C. albida Willd. ex Spreng. and C. canadensis (J. L. Carretero, Universidad Politécnica de Valencia, personal communication). Additionally, Thébaud and Abbott (1995)
stated that in France, C. sumatrensis and C. blakei Cabr. cross under natural environments and produce hybrid progenies with moderate (30%) fertility. Importantly, significant phenotypic variability exists in C. blakei (Laínz, 2001
). In Belgium, Verloove and Boullet (2001)
reported that some xenophyte Conyza populations identified as C. floribunda were in fact C. canadensis x C. sumatrensis hybrids. Furthermore, Conyza x mixta Fouc. & Neyr. reportedly arose from crosses between C. canadensis and C. bonariensis in Belgium, France, Great Britain, and Portugal (F. Verloove, National Botanic Garden of Belgium, personal communication). More recently, 
ída (2003)
reported a putative hybrid between C. bonariensis and C. triloba Decne. in the Czech Republic.
Loss of vigor is apparently a common trait to European Conyza hybrids. We speculate that ploidy differences may be a significant barrier determining successful hybridization in Conyzinae. For example, more compatible and vigorous hybrids would be expected from crosses between the allopolyploids (2n = 54) C. sumatrensis, C. floribunda, and C. bonariensis compared to crosses with the diploid (2n = 18) C. canadensis. Just as geography delimits the major groups in Asteraceae, spatial isolation is probably the principal species barrier within Asteraceae (Nesom, 1990
). Therefore, disturbances of these barriers will likely stimulate interactions between phylogenetically related taxa separated by geography (allopatry) and allow for the exchange of genetic material between unstructured random-mating (panmictic) Asteraceae populations.
Ploidy of the Conyza hybrid
Hybridization represents an important mechanism for plant speciation whereby fertile and stable taxa can arise by either chromosome number doubling (allopolyploidy) or recombinational speciation without polyploidization (homoploidy) (Rieseberg et al., 2000
). In the absence of karyological studies, the ploidy of the C. canadensis x C. ramosissima hybrid remains unknown. Nonetheless, we propose that the segregation and compatibility data allude to the formation of an interspecific hybrid without an increase in ploidy.
Man-made allopolyploids often display homeotic transformations and aberrant chromosomal rearrangements that result in gene silencing, hybrid instability, and lethality (Comai, 2000
). These phenomena were absent from the Conyza hybrid, as the interspecific hybrids demonstrated a vigorous growth and marginal (<5%)
). The genome of 
). Given that RS2 and 
Establishment of the Conyza hybrid in the environment
The low hybridization levels (0% to 12%) in the pollen competition studies and the estimated high genetic compatibility between C. canadensis and C. ramosissima (98%) suggest that the resultant hybrid will probably evolve in nature as a contiguous population in a common geographic range (parapatry) (Table 3). Under natural environments, hybridization may occur at different frequencies than those observed in the greenhouse, depending on insect- and wind-pollination levels and other environmental factors such as competition and resource availability, which can affect flowering time in Conyza (Thébaud et al., 1996
). Regardless, the fact that
Models for homoploid speciation suggest that superior hybrid competitiveness results in the rapid displacement of the parental taxa, while under a fitness disadvantage, either the hybrid taxa becomes extinct or it coexists with the parents, provided adequate niche differentiation in the environment (Rieseberg, 1997
). Hybrids are rarely better fit than their well-adapted congeners. Low initial frequencies, reduced fertility and viability, and competitive disadvantage with respect to parents typically lead to hybrid extinction (Wolf et al., 2001
).
Ascertaining fitness of the Conyza hybrid and the potential ecological displacement of other taxa in different environments (vicariation) would require further experimentation and is not within the scope of this study. The supposition that
; Koger and Reddy, 2005
; Dinelli et al., 2006
). Furthermore, 
We therefore suggest that the Conyza hybrid may be well fitted to agroecosystems, because evaluation of the
). Hence, the larger 
According to Barton (2001)
, reasons for higher hybrid fitness include the (1) occurrence of diverse phenotypes through transgression that may have ecological advantages under particular environments, (2) reconstruction of an "ancestral linkage" that was competitive in the past and is probably adapted to present environments, and (3) coalescence of previously disjunct gene sets that may have a positive impact on fitness. Hybridization may therefore explain some cases of niche differentiation and provide the raw materials for the adaptation of novel weeds to the environment (Rieseberg et al., 1999
). It is important to acknowledge that the transgressive fitness with respect to glyphosate resistance resulted from the combination of phenotypes of the two Conyza populations herein studied; a different fitness may be observed in hybrids originating from crosses between other Conyza species.
Implication of hybridization on herbicide resistance management
The impact of hybridization on glyphosate resistance management would depend on the native introgression levels of the R allele and the stability, fertility, and fitness of the Conyza hybrid with respect to RS2 and
Absent from this hypothesis, however, are the natural hybridization frequencies in Conyza. Native hybridization frequencies in the European Conyza approximate 50 plants in thousands of individuals, although 60% of the resultant hybrids were infertile (Thébaud and Abbott, 1995
). These hybridization frequencies are completely dependent on the ecological circumstances under which plants developed. However, we would expect higher hybridization frequencies than those reported in Europe since the parents evaluated in this study are compatible and the resultant Conyza hybrid demonstrated negligible postzygotic reproduction barriers. Even low initial hybrid frequencies within the population could increase in time because Conyza plants can produce more than 240 000 achenes per growing season, mostly viable, which are capable of dissemination to 30 m in 16 km/h wind (Muenscher, 1935
; Dauer et al., 2006
; Nandula et al., 2006
).
Interspecific hybridization may therefore affect the extinction rates of weeds in the environment and serve as a mechanism for the dissemination of transgenic and herbicide resistance genes (Owen and Zelaya, 2005
). Farmers should consider the potential for hybridization between weeds when developing programs aimed at managing herbicide-resistant weeds. Production systems that depend on a single herbicidal chemistry for weed control should be combined with alternative management tactics, thus mitigating the evolution of herbicide resistance and maintaining the sustainability of current agroecosystems. Further research is needed to monitor native gene flow levels between weeds and ascertain the potential for dissemination of herbicide resistance through introgressive hybridization.
FOOTNOTES
1 The authors thank J. Wendel for his valuable discussions on evolutionary biology; M. Graham, P. Tranel, J. Gressel, and R. Hartzler for their critical review of earlier versions of this manuscript; D. Lewis at the Ada Hayden Herbarium (ISC) for mounting the specimens submitted to ISC, the Botanical Research Institute of Texas (BRIT), and the New York Botanical Garden (NY); and P. Knosby, J. Ruhland, and R. van der Laat for assistance with greenhouse endeavors. 
4 Author for correspondence (e-mail: iazelaya{at}iastate.edu
; current address: Syngenta Ltd., Weed Control Research, Jealott's Hill International Research Centre, Bracknell, Berkshire, RG42 6EY, UK
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