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CROP BREEDING, GENETICS & CYTOLOGY

Jointed Goatgrass (Aegilops cylindrica Host) x Wheat (Triticum aestivum L.) Hybrids

Hybridization Dynamics in Oregon Wheat Fields

^a Herbarium, Dep. of Botany and Plant Pathology, Oregon State Univ., Corvallis, OR 97331-2902
^b Dep. of Crop and Soil Science, Oregon St. Univ., Corvallis, OR 97331-3002

* Corresponding author (carol.mallory-smith{at}orst.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
The ability of the weedy species jointed goatgrass (Aegilops cylindrica Host) to form seed-bearing hybrids with wheat (Triticum aestivum L.) raises questions concerning the potential movement of herbicide-resistance genes from commercial wheat cultivars into the weed population. As a preliminary step for future gene-flow risk assessments, a study of jointed goatgrass x wheat hybrids collected from infested wheat fields in 1998 and 1999 was undertaken in Oregon. Jointed goatgrass accessions representing the range of variation in its worldwide distribution also were included in this study. The high molecular weight (HMW) glutenins, a group of wheat endosperm storage proteins, were used as genetic markers for characterizing this material. In the Oregon jointed goatgrass accessions, the seed protein analysis identified F₁ hybrid seed that was formed at a rate of 0 to 8% on a per field basis. The HMW glutenin patterns in the backcross seed threshed from Oregon hybrids showed a higher proportion of seeds formed from pollination by wheat than by jointed goatgrass. Analysis of the roots for remains of the maternal seed or spikelet indicated that most hybrid plants were of the F₁ generation and that either jointed goatgrass or wheat could be the female parent. These analyses suggested a hybridization dynamics in which jointed goatgrass serves as the predominant F₁ female parent and wheat as the predominant backcross male parent. Development of introgressed jointed goatgrass forms carrying wheat genes would be dependent on the presence of a continuous hybrid zone located near or within a persistent jointed goatgrass population.

Abbreviations: HMW, high molecular weight

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
JOINTED GOATGRASS (CD genomes), a wild, weedy relative of domesticated bread wheat (ABD genomes) currently infests over 3 million hectares of winter wheat cropland in the USA (Anonymous, 2002). According to Johnston and Heyne (1960), it was introduced into the USA in multiple introductions by the United States Department of Agriculture (USDA) and private entrepreneurs as a seed contaminant of a wheat cultivar group collectively known as ‘Turkey’ which was imported from southern Russia during the early years of the 20th Century (Carleton, 1915). For the dryland winter wheat region of eastern Oregon where jointed goatgrass is a serious weed pest (Karow et al., 1995), the first recorded identification is for a 1926 collection from a roadside population in Gilliam County (Rydrych, 1984; Fig. 1) .

View larger version (32K): [in this window] [in a new window]   Fig. 1. Eastern Oregon counties (shaded) comprising the primary area of jointed goatgrass infestation. Dots indicate areas for sites where hybrids were collected.

Challenges to the hybrid sterility view are beginning to build. Naturally formed seed-bearing hybrids have been collected from wheat fields in Idaho and Oregon (Mallory-Smith et al., 1996; Morrison et al., 2002) and in herbicide-resistant wheat research plots in Washington (Seefeldt et al., 1998). Experimental work has established that seed will form on F₁ and early generation backcross hybrid plants under conditions of partial-female fertility (Zemetra et al., 1998; Wang et al., 2000, 2001, 2002) and that there is a rapid move toward self-fertility as early as the BC₂ generation (Zemetra et al., 1998; Wang et al., 2000, 2001; Snyder et al., 2000; Crémieux, 2001). Hybrid seed production under field conditions also has been experimentally documented in the USA (Snyder et al., 2000) and Switzerland (Guadagnuolo et al., 2001).

The research reported here was undertaken as a preliminary investigation to characterize hybridization dynamics in wheat fields with existing jointed goatgrass infestations, a research approach that has not yet been taken. Jointed goatgrass and hybrids that were collected from infested fields located in eastern Oregon (Fig. 1) were evaluated by means of high molecular weight (HMW) glutenin seed storage proteins as diagnostic markers. The HMW glutenins are coded by two tightly linked genes, Glu-1-1 and Glu-1-2, located on the long arm of the homoeologous group 1 chromosomes (Payne et al., 1981; Thompson et al., 1985). They express as paired x- and y-subunits which produce a two-band pattern in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) gels. Positioning of the subunit pairs is genome specific. In diploid species, both subunits are expressed, whereas in polyploid species such as wheat and jointed goatgrass silencing of one or more subunits is observed (Payne et al., 1981; Galili and Feldman, 1983). Because wheat and jointed goatgrass have unique SDS-PAGE banding patterns for their respective HMW glutenin subunits, these markers can be used to determine parental contribution in hybrids.

Aegilops markgrafii (Greuter) K.Hammer and Ae. tauschii Cosson, the respective C- and D-genome parents of jointed goatgrass (Dubcovsky and Dvorak, 1994) each have a characteristic two-band HMW glutenin pattern (William et al., 1993; Wan et al., 2000). Jointed goatgrass has a three-band pattern because of nonexpression of the Glu-D1 x-subunit (Johnson, 1967; Wan et al., 2000). In the case of the soft-white wheat cultivars grown in Oregon, HMW patterns range from three to seven bands (Lookhart et al., 1993; Carter, 1999). At the Glu-A1 locus, the x-subunit is occasionally expressed and the y-subunit is always absent. The respective x- and y-subunits coded by the Glu-B1 and Glu-D1 loci are nearly always expressed, i.e., the Glu-B1 y-subunit can be absent. Genetic diversity in the Glu-1 loci, which is expressed as small changes in subunit mobility, is responsible for the multiple allelic forms found in wheat (Payne et al., 1981).

HMW glutenin diversity in jointed goatgrass has not yet been described. Thus, the initial step in this study was to establish a baseline for evaluating the jointed goatgrass parental contribution to hybrid protein patterns by analyzing material representative of jointed goatgrass geographic distributions across Asia, Eastern Europe, the USA, and eastern Oregon. The overall goal of this research was to use HMW glutenins as diagnostic markers to trace the respective parental contributions of jointed goatgrass and wheat to naturally formed seed-bearing hybrids collected in infested wheat fields located in eastern Oregon.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Plant Material
Accessions of jointed goatgrass were selected to represent the ecogeographic diversity in its native and adventive ranges in Europe and Asia, the USA, and Oregon (Table 1 ; Fig. 1). Study material of jointed goatgrass and its progenitor parents, Ae. markgrafii and Ae. tauschii, was obtained from ex situ germplasm collections held by the USDA National Small Grains Collection in Aberdeen, ID, the Genetic Resources Unit of the International Center for Agricultural Research in Dry Areas (ICARDA), and the University of California, Riverside. Ex situ USA collections of jointed goatgrass were obtained from Dr. Philip Westra of Colorado State University. An in situ collection of jointed goatgrass from Cache County, UT, was provided by Dr. Mary E. Barkworth of Utah State University.

View larger version (95K): [in this window] [in a new window]   Fig. 2. (a) Spikes of F1 hybrid and parents. From left to right: wheat, F1 (jointed goatgrass = female parent), F1 (wheat = female parent), and jointed goatgrass. (b) Roots of F1 plants with maternal jointed goatgrass spikelet attached at the crown (left) and maternal wheat seed attached at planting depth (right). (c) Spikes of hybrids grown from Oregon backcross seed showing range of variation from jointed goatgrass-like (left) to wheat-like (right). (d) Spike examples illustrating potential steps of introgression from F1 hybrid (first and second to right) via a jointed goatgrass-like backcross hybrid (center) toward jointed goatgrass (left).

Band Analysis
Identification of the x- and y- type subunits of the respective C- and D- genome band contributions to the HMW glutenin pattern for jointed goatgrass was made by comparison with the band patterns of Ae. markgrafii and Ae. tauschii (Wan et al., 2000; data not shown). Wheat cultivars used for the identification of the wheat-band contribution in the hybrid material included cultivar Chinese Spring and the principal Oregon soft white wheat cultivars Madsen and Stephens. Relative band position served as the basis for the comparative analysis. The Glu-1 subunit assignment system developed by Payne and Lawrence (1983) (hereafter, Payne numbers) was used to establish the relative band positions for the Glu-A1, Glu-B1, and Glu-D-1 subunits in the development of the coding scheme for the wheat parental contribution (Fig. 3) . They could not be used in the hybrid pattern analysis because the Glu-C1 alleles are not encompassed in this band assignment system. The additive, five- or six-band F₁ hybrid band pattern identified in experimental lines (Cérmieux, 2001; Fig. 4) was used for a comparative analysis of patterns found in the hybrid seed collections.

View larger version (11K): [in this window] [in a new window]   Fig. 3. Glu-A1, Glu-B1, and Glu-D1 HMW glutenin subunits found in soft-white wheat cultivars grown in Oregon with Payne number assignments and their respective subunit coding for the male parentage analysis.

View larger version (15K): [in this window] [in a new window]   Fig. 4. (a) Coding for HMW glutenin parental patterns for jointed goatgrass (cyl) and wheat (aes) and their F1 hybrid (F1). Occasional presence of the By subunit band is indicated by "---". (b) SDS-PAGE gel of HMW glutenin patterns of jointed goatgrass (1), Madsen (2), Madsen x jointed goatgrass F1 synthetic hybrid (3,4), and molecular weight marker (M) in kilo Daltons.

View larger version (59K): [in this window] [in a new window]   Fig. 5. (a) Coding for HMW glutenin patterns in backcross hybrid seed: F1-like pattern F1-1; wheat-like pattern aes-1; jointed goatgrass like patterns cyl-1 and cyl-2. When the By subunit band as indicated by "---" is absent, the F1-like and wheat-like patterns are designated F1-1 and aes-1, respectively; when it is present, the patterns are designated F1-2 and aes-2, respectively. (b) composite SDS-PAGE gel of HMW glutenin patterns of jointed goatgrass (1); Chinese Spring (5); F1-1 (4,8); aes-1 (6); aes-2 (7); cyl-1 (3); cyl-2 (2).

In text and graphical illustrations of the HMW glutenin patterns, the Glu-1 x- and y-subunits have been represented in descending order of migration, i.e., from slowest to fastest (Fig. 3–6) . Given the difficulties in determining band assignments because of overlapping y-subunit regions as discussed above, only the Cx and Dx subunits were used to determine jointed goatgrass (Cx) versus wheat (Dx_aes) backcross pollination in the male parentage analysis. Since the Dx_aes and Bx,By contributions were always linked to the wheat parent, they were treated together in the parentage analysis.

View larger version (50K): [in this window] [in a new window]   Fig. 6. SDS-PAGE gel of HMW glutenin patterns for Oregon material from site 113b: jointed goatgrass (3); F1cyl hybrid seed (1,2); Chinese Spring (4).

	RESULTS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

A pattern distribution was constructed following the traditional Andersonian hybrid-index histogram format (Anderson, 1949) with the jointed goatgrass-like patterns at one end, the wheat-like patterns at the other end, and the F₁-like patterns in the middle (Fig. 7) . Distribution of the patterns by year, collection type (single-plant versus bulk), site, and county showed a consistent trend towards the wheat end of the distribution (Fig. 7a,b,d; Table 4) . A similar trend was found within individual plants that produced greater than 10 seeds (Table 5) .

View larger version (23K): [in this window] [in a new window]   Fig. 7. HMW glutenin pattern distributions for backcross hybrid seed (a) by year for all seed; (b) by collection for all seed – SP (single-plant collection), Bulk (bulk plant collection); (c) by F1 female parentage—aes (wheat) or cyl (jointed goatgrass) seed-spikelet; (d) by site for the seed-spikelet subsample.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Hybrid Parentage
Studies of aneuploid wheat lines have established that irregular HMW glutenin subunit expression occurs as a consequence of intergenomic gene interactions that cause a nonlinear dosage response among the Glu-1 alleles (Galili and Feldman, 1984; Galili et al., 1986). These expected expression irregularities were evident in the HMW glutenin patterns of the Oregon backcross hybrid material. For this reason, subunit band intensity, which in a euploid seed would express as a dosage difference between the 2n female versus 1n male endosperm donation, was not used for identifying hybrid parentage. Instead, parentage was determined by a comparative analysis of the Glu-1 subunit combinations forming either jointed goatgrass-like or wheat-like patterns. Backcross seeds with the wheat-like pattern (Fig. 5) showed a loss of the Glu-C1 locus in the female and pollination by wheat. In the case of the cyl-1 pattern (Fig. 5), a gain of the Bx or By subunit in the absence of the Dx_aes subunit suggested retention of the 1B chromosome in the female gamete and a jointed goatgrass backcross pollination. Similarly, a gain of the Dx_aes subunit in the absence of the Bx subunit in the cyl-2 pattern (Fig. 5) suggested jointed goatgrass pollination of a female gamete that retained the 1D_aes chromosome.

In the absence of the seed-spikelet, an inference of parentage can be made on the basis of spike morphology. F₁ plants formed with either wheat or jointed goatgrass as the female parent produce the same intermediate spike morphology (Fig. 2a). In the backcross hybrid generation, spike morphology for plants either grown from backcross seed harvested from Oregon hybrid collections or from experimental lines shows an intergrading variation ranging from wheat-like to jointed goatgrass-like (Fig. 2c; Crémieux, 2001). Although an F₁-like morphology is found in this variation, it differs in awn development, internode length, and spikelet shape from the intermediate F₁ type (Fig. 2a,c). On spike morphology alone, most of the Oregon hybrid collections appeared to be of F₁ generation plants. The intermediate F₁ spike morphology of the six plants listed in Table 5 would suggest that they also are from the F₁ generation.

The seed-spikelet analysis verified that both jointed goatgrass and wheat could serve as the female F₁ hybrid parent in the field environment. However, on the assumption that the seed-spikelet subsample was representative of the larger population of hybrid plants, most F₁ seeds were produced on jointed goatgrass. This finding agreed with an expected higher incidence of jointed goatgrass as the F₁ female parent given its ability to cross-pollinate (L.A. Morrison, 1998–2001, data not shown) and the higher wheat pollen load in the wheat field environment. For the backcross hybrid seed, the skewed distribution of the Glu-1 patterns toward the wheat parental subunit composition (Fig. 7) again verified the effect of the higher wheat pollen load, here moving the majority of the backcross hybrid population in the direction of wheat. For the subsample of 47 backcross seed for which jointed goatgrass (four seeds) versus wheat male parentage (43 seeds) could be established (Fig. 7c), the relative rate of wheat pollination was 91%. This figure was consistent with the 90% wheat pollination rate in experimental BC₁ hybrid production under field conditions documented by Crémieux (2001). These observations would suggest a hybridization dynamics in which jointed goatgrass serves as the predominant F₁ female parent and wheat as the predominant backcross parent.

Variation in the F_1cyl hybridization rate in Oregon wheat fields encompassed the 2.7% rate found in experimental field trials by Guadagnuolo et al. (2001). Hybridization rate variation among sites is presumably due to environmental factors as well as particular features of the jointed goatgrass ecotype and population structure. Although an explanation for the relatively high 8% rate for material collected at site 113b in northeast Wallowa County (Fig. 1) is not possible without further study, the high elevation and seasonal temperature extremes found in this region may play a role. In any case, F_1cyl hybridization rate serves as a measure of the initial contribution of jointed goatgrass to gene movement between wheat and this weedy species. As such, it will be an important factor for risk assessments of potential movement of herbicide-resistant genes from wheat into jointed goatgrass.

The parentage analysis also revealed a sampling bias whereby hybrid plants with an intermediate F₁-like spike morphology were preferentially selected. Experimental studies have shown that variation in BC₁ spike morphology ranges across an intergrading scale with the parental types at either end (Snyder et al., 2000; Crémieux, 2001). As illustrated by the growouts of backcross seed collected in Oregon's wheat fields (Fig. 2c), this range of variation also can be expected in infested fields where backcross hybrid plants successfully establish. However, since the Oregon field collections were focused on the distinctive F₁-like spike morphology, only backcross material resembling this intermediate type was collected. Backcross material with the parental type morphology, particularly jointed goatgrass-like plants (Fig. 2c), would be difficult to recognize in the field. Obviously, future collections should be directed to finding hybrid plants with a more jointed goatgrass-like phenotype. Another related issue was the predominance of F_1cyl plants in the collections. In future studies, a comparative F_1aes versus F_1cyl hybridization rate should be calculated for assessments of the role of F₁ hybrid parentage in backcross hybrid success.

Introgression
Crémieux (2001) has documented the development of stable, self-pollinating BC_cyl lines (chromosome numbers ranging from 28 to 34) in two to three generations from a Madsen x jointed goatgrass F₁ hybrid. Genomic in situ hybridization work on these hybrid lines (unpublished data) as well as others (Wang et al., 2000) has shown that they can carry extra or translocation chromosomes between the A/B genomes of wheat and the C genome of jointed goatgrass. What is remarkable about these advanced backcross derivatives is that their spike and vegetative morphology is indistinguishable from jointed goatgrass. The jointed goatgrass-like patterns (cyl-1 and cyl-2) of the Oregon backcross hybrid seed matches the HMW glutenin band patterns of experimental BC₁ and BC₂ lines pollinated by jointed goatgrass under field conditions (Crémieux, 2001). Also, the spike morphology of a hybrid plant grown from backcross seed collected at site 226a (Fig. 2c) sufficiently resembles jointed goatgrass to suggest that it may represent a move back toward the parental type. This comparative similarity between experimental and natural hybrid material for the Glu-1 banding patterns and spike phenotype suggests the potential for advancement beyond the BC_1cyl generation into self-fertile introgressed forms that can reenter the weed population unnoticed.

The experimental evidence (Zemetra et al., 1998; Wang et al., 2000, 2001; Snyder et al., 2000; Crémeiux, 2001) and the field collections (Tables 3 and 5) show F₁ and BC₁ hybrid seed production to be relatively low on a per plant basis. On a field-wide basis, this low yield may become significant when hybrid populations are large and extensive. While the Oregon field collections suggest that seed-bearing hybrids are being produced in numbers sufficient to constitute hybrid infestations, success beyond the F₁ and BC₁ generations both in terms of fertility restoration and plant establishment is currently a matter of speculation. However, the speed with which experimental backcross hybrid lines have progressed to self-fertile jointed goatgrass forms is compelling evidence supporting the possibility for introgression in infested wheat fields.

	CONCLUSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
This research establishes that hybridization in infested wheat fields between jointed goatgrass and wheat is at least successful to the BC₁ generation. According to the parentage analysis of the material evaluated in this study, there is a higher incidence of F_1cyl versus F_1aes hybrids, and conversely BC_1aes versus BC_1cyl hybrids, in infested fields. This distribution suggests that out-crossing jointed goatgrass and F₁ flowers will be more likely pollinated by wheat than by jointed goatgrass. Although a comparatively greater wheat pollen load in a cultivated field is expected, as is a low rate of backcross pollinations by jointed goatgrass, the role of pollen load in the steps toward successful jointed goatgrass introgression has not been previously considered. For example, experimental research in the USA has focused exclusively on F_1aes hybrids and their backcross derivatives (Zemetra et al., 1998, Wang et al., 2000, 2001, 2002; Snyder et al., 2000; Crémieux, 2001). This work suggests that BC_1cyl lines derived from an F_1aes can develop into introgressed jointed goatgrass forms. It is unknown if the same is true of an F_1cyl hybrid, particularly in the wheat-field environment. In future investigations of jointed goatgrass x wheat hybridization, a more comprehensive study of the early-stage hybrid dynamics will be necessary to establish whether there is a differential success of F_1cyl versus F_1aes in advanced backcross generations.

On the basis of the findings to date, the risk of introgression of wheat genes into jointed goatgrass populations may be tied to (i) the potential for fertility restoration, (ii) the overall numbers of hybrid plants that potentially can produce seed, and (iii) the viability of the seed as well as its ability to successfully establish. Selection pressures in the form of the wheat harvest and wild-type dispersal biology will play a role in the movement of wheat genes into jointed goatgrass. Removal of competing F_1aes hybrids during wheat harvest will favor the move toward an introgressed jointed goatgrass form. Dispersal to the ground of jointed goatgrass spikelets and seed-bearing hybrid spikes will ensure that F_1cyl and backcross seed have an opportunity for survival to the next generation. Backcross plants that closely resemble jointed goatgrass and successfully move into the weed population will be potentially capable of spreading their acquired wheat genes into the population.

Presence in the Oregon material of BC_1cyl hybrids with jointed goatgrass-like HMW glutenin patterns provides preliminary evidence of natural hybridization events which may lead toward introgressed jointed goatgrass forms. Potential gene flow also is suggested by the spike morphology of the backcross hybrid illustrated in Fig. 2c and 2d, which appears to be the product of a rapid move toward a jointed goatgrass form. In this regard, inadequate control of weed infestations in and around wheat fields offers favorable conditions that supply a steady source of female parents, pollen for jointed goatgrass backcrosses, and a stable population into which introgressed forms can move. A changing agricultural environment inherent in the alternating winter wheat–fallow cultivation regime presumably offers a check on the progressive formation of fertile, introgressed forms of jointed goatgrass carrying wheat genes. However, uncontrolled jointed goatgrass populations in and around fallow or rotation-crop fields allows the continued existence of a hybrid zone. In the agricultural environment, this is a dynamic zone that can shift with annual cropping regimes or persist in the absence of effective weed control.

Presence of F₁ hybrids in wheat fields, and the BC₁ seeds produced on them, hints at the potential for development of advanced generation forms that can become part of the existing jointed goatgrass weed population. That the HMW glutenin patterns of some BC₁ seed appear to be moving toward the jointed goatgrass parental type suggests the need to evaluate seriously the possibility for natural gene flow between wheat and jointed goatgrass. Future work will focus on whether this early-generation material is capable of advancing toward introgressed jointed goatgrass forms in infested wheat fields, as is the case for experimental hybrid lines.

	ACKNOWLEDGMENTS

 
This research was supported in part by a grant from the National Research Initiative Competitive Grants Program/USDA—Grant No. 2000-00843. Our appreciation to Oregon State University Extension Agents and Seed Certification Service personnel for their assistance in identifying wheat-field sites for the jointed goatgrass and hybrid collections and to the Oregon wheat growers who kindly participated in this research. We also would like to acknowledge the assistance of Dr. Donald D. Kasarda of USDA-ARS with questions on the HMW glutenin work and helpful comments from Dr. Andrew Ross of Oregon State University and three anonymous reviewers.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 
Journal Paper no. 11873 of the Oregon St. Univ. Agric. Exp. Stn.

Received for publication October 11, 2001.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSION REFERENCES

 

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