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Published online 1 February 2006
Published in Crop Sci 46:655-661 (2006)
© 2006 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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CROP BREEDING, GENETICS & CYTOLOGY

Characterization of Amphiploid Hybrids between Bluebunch and Thickspike Wheatgrasses

Kevin B. Jensen*, Kory W. Maughan and Kay H. Asay

USDA-ARS, Forage and Range Research Lab., Utah State Univ., Logan, UT 84322-6300

* Corresponding author (kevin{at}cc.usu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
An amphiploid derivative from hybrids between bluebunch wheatgrass (2n = 2x = 14) [Pseudoroegneria spicata (Pursh) A. Love] and thickspike wheatgrass (2n = 4x = 28) [Elymus lanceolatus (Scribner and Smith) Gould] was developed to compare cytological, morphological, anatomical, and seed characteristics between the triploid F1 hybrids (2n = 3x = 21) and their colchicine induced hexaploid (2n = 6x = 42) derivative (C0). The amphiploid population produced an average of 3.1 seeds spikelet–1 and pollen stainability averaged 85.5%. Seed set under self-pollination in the amphiploids ranged from <1% to >99% with an average of 50%. Fifty-eight of the 103 plants studied were hexaploid (2n = 6x = 42). The remaining plants had chromosomes ranging from 2n = 39 to 2n = 44 and were less fertile than the euploids. Meiosis in the euploid plants was regular and typical of a segmental autoallohexaploid St1St1St2St2HH. The most common meiotic chromosome configuration was 21 bivalents. A high frequency of bivalents and a low frequency of quadrivalents indicate that the St genomes of the two parental species have diverged. The amphiploids had thicker culms, more leaves per culm, and longer and wider leaves than either parental species. Multivariate analysis demonstrated that the amphiploids, though morphologically similar to the parental species, was distinct. In general, C0 hybrids were less vigorous than the F1 from which they were derived. Through amphiploidy, genetic introgression between the two species is possible.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
BLUEBUNCH WHEATGRASS is one of the most important native bunch grasses in the Great Basin region and the Pacific Northwest. It inhabits dry mountain slopes at middle elevations with sagebrush and pinyon–juniper vegetation (Holmgren and Holmgren, 1977). Stands of bluebunch wheatgrass are often difficult to establish and, because of high palatability, become depleted under moderate to heavy grazing pressure, particularly at the boot stage (Miller et al., 1987). Bluebunch wheatgrass is comprised of diploid (2n = 2x = 14) and autotetraploid (2n = 4x = 28) forms containing the St genome (Stebbins and Snyder, 1956). The diploids are adapted to the more semiarid rangelands within the Great Basin while the tetraploids are restricted to the Pacific Northwest. Bluebunch wheatgrass is predominantly cross-pollinated (Jensen et al., 1990).

Thickspike wheatgrass is a cool-season perennial grass that is strongly rhizomatous and frequently glaucous (Holmgren and Holmgren, 1977). Because of the presence of rhizomes, thickspike wheatgrass is more persistent under repeated grazing than bluebunch wheatgrass. Thickspike is comprised of the St and H genomes, which associate as bivalents at metaphase I (Dewey, 1969). The St genome is derived from bluebunch wheatgrass and the H genome from Hordeum (Syn = Critesion; Dewey, 1984). Thickspike wheatgrass is largely cross-pollinated (Jensen et al., 1990). The goal was to create a hybrid between two native grasses that would combine the palatability of bluebunch wheatgrass with the persistence of thickspike wheatgrass.

On the basis of genomic structure of thickspike wheatgrass, Dewey (1965) hypothesized that it evolved through independent hybridization events involving either diploid bluebunch (StSt) and diploid Hordeum species (HH) or their autotetraploid counter parts (StStStSt; HHHH). Triploid F1 hybrids between bluebunch and thickspike were meiotically irregular at metaphase I and failed to set seed under open-pollination (Dewey, 1965). However, in tetraploid hybrids (StStStH) between tetraploid bluebunch and thickspike, stainable pollen ranged from 0 to 50% and several of the hybrids produced 100 seeds plant–1 (Dewey, 1965). Chromosome pairing data in the triploid hybrid suggest that one of the two thickspike genomes was closely related with that of bluebunch wheatgrass (Dewey, 1965).

Although genetic introgression between the two species apparently occurs most frequently from the diploid to tetraploid level, rhizomatous forms of bluebunch occasionally occur on range sites where bluebunch and thickspike wheatgrasses grow sympatrically (Holmgren and Holmgren, 1977). They further concluded that these plants resulted from gene flow from thickspike to diploid bluebunch wheatgrass. Bowden (1965) suggested that Montana wheatgrass [Agropyron ablicans Scribner & Smith] originated from back crosses between thickspike x bluebunch hybrids with the thickspike parent. Wyoming wheatgrass [A. griffithsii Scribner and Smith], which is similar to Montana wheatgrass with the exception of its pubescent glumes and lemmas, apparently arose from crosses involving the pubescent form of thickspike. These hypotheses were supported by Dewey (1965, 1970) who obtained progeny and backcross derivatives from thickspike x bluebunch hybrids that closely resembled Montana and Wyoming wheatgrasses.

Crosses between species with different genomic compositions often result in sterile F1 hybrids, which frequently leads to cytological instability and low fertility in newly formed amphiploids (Ashman and Boyle, 1955; Hill and Buckner, 1962). Dewey (1968) concluded that sterility, cytological irregularities, and lack of vegetative vigor in early amphiploid generations must be overcome if newly developed amphiploids are to have an impact in nature as new species or as newly developed cultivars. Within the wheatgrasses, breeders have overcome sterility barriers by doubling the chromosomes of sterile F1 hybrids. Colchicine-induced tetraploids of diploid crested wheatgrass [A. cristatum (L.) Gaertn.] and diploid Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski] were used to introgress diploid and tetraploid germplasm (Asay et al., 1985, Jensen et al., 2005). The effect of chromosome doubling from diploid to autotetraploid often results in larger vegetative and floral parts (Eigsti and Dustin, 1955; Randolph, 1941; Stebbins, 1947). Induced tetraploids of Italian (Lolium multiflorum Lam.) and perennial ryegrasses (L. perenne L.) had greater seedling vigor than their diploid counterparts (Sjödin and Ellerström, 1986). Plants of diploid Russian wildrye tend to be shorter, finer stemmed, leafier, and generally higher in forage production than tetraploids (Asay et al., 1996). Tetraploids (2n = 4x = 28) have larger seeds and superior seedling vigor (Berdahl and Barker, 1991; Lawrence et al., 1990). Bingham et al. (1994) proposed that increased vigor of tetraploid alfalfa compared with diploid forms might be associated with greater complementary gene interaction at the tetraploid level. Tetraploid rye (Secale cereale L.) was 15% taller and had 12% fewer tillers than diploid rye (Muntzing, 1951). Kernel weight of rye increased 50% in the tetraploids (Muntzing, 1951). Guard cells and pollen grains are often significantly larger in polyploids than in their diploid progenitors. It has been speculated that polyploid species are more stable over a broader range of environments (Ellerston, 1959). Polyploid species of Aegilops are reported to be broadly adapted to a wider range of environments than are their diploid relatives (Zohary, 1965).

This paper cytologically and morphologically characterizes the bluebunch and thickspike amphiploid population and compares it to the triploid F1 hybrid and parents. This population was previously released as a germplasm, SL-hybrid (Asay et al., 1991).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Plant Materials and Populations
Plant nomenclature follows the "genomic system of classification" (Dewey, 1984) and genome designations are after Wang et al. (1995). Parental origin, hybrid development, chromosome pairing, and fertility of the parents, triploid, and tetraploid F1 hybrids were reported by Dewey (1965). Populations representing thickspike, diploid bluebunch, the F1 hybrid between bluebunch x thickspike, the original amphiploid (C0), and the amphiploid breeding population were included in this study.

The original bluebunch (PI 232132) x thickspike (PI 236663) cross resulted in several triploid hybrids, from which the advanced generation breeding population was derived, designed to test the hypothesis that genetic introgression can occur between bluebunch and thickspike (Dewey, 1965). Both parents used in the initial cross were meiotically regular, forming predominately bivalents during metaphase I of meiosis (Dewey, 1965).

Colchicine-treated F1 hybrids resulted in the production of relatively fertile hexaploid amphiploids, designated as the C0 generation. The most fertile and vigorous plants from each succeeding generation were intercrossed to form the C1 (first generation past chromosome doubling), C2, and C3 generations. Plants from the C1, C2, and C3 generations were combined to form the initial amphiploid breeding population, which underwent four cycles of mass selection. During each cycle, approximately 65 plants were selected from a 3000-plant population on the basis of improved fertility, vegetative vigor, forage and seed yield, leafiness, and drought tolerance. The plants included in the present study were from the fourth breeding cycle.

Cytological Samples and Squash Preparations
Pollen mother cells from the hybrid populations were preserved in Carnoy's fixative (6:3:1 absolute alcohol–chloroform–glacial acetic acid) for 24 to 48 h, transferred to 70% ethanol, and stored in a refrigerator until analyzed. Squashed preparations of the pollen mother cells were stained with 2% acetocarmine. The frequency of univalents, bivalents, trivalents, and quadrivalents were determined in at least 10 microspore mother cells (616 total) from each of 50 euploid plants at metaphase I. Somatic chromosome numbers were determined at anaphase I based on a minimum of 10 cells from 100 amphiploid plants. The frequency of lagging chromosomes at anaphase I was determined in 50 cells from each of 50 plants. Micronuclei were counted in 100 postmeiotic microspore tetrads from each of 50 amphiploid plants.

Pollen Stainability and Seed Set
Spikes for pollen stainability were collected at anthesis from 100 amphiploid plants. The pollen grains were immersed in an I2KI (iodine–potassium iodide) solution, which stains starch found in viable pollen grains black or dark gray. Aborted pollen grains are shrunken and light amber colored in I2KI. A minimum of 1000 pollen grains were scored as viable or inviable in the amphiploid. Seed set under open-pollination for the amphiploid was determined on five spikes per plant 1 mo after anthesis. Self-fertility of the amphiploid was determined by isolating five spikes of the same 100 plants in parchment bags before anthesis. The spikes were hand threshed and seed counted to estimate plant fertility expressed as seeds spikelet–1. Percentage self-fertility was computed by comparing the seed set spikelet–1 of the self-pollinated spikes with that of the open-pollinated spikes.

Morphological Traits
All plant materials were located at the Evans Research Farm approximately 2 km south of Logan, UT (41°45'' N, 111°8'' W, 1350 m above sea level). Soil at the site is a Nibley silty clay loam series (fine, mixed, active, mesic Aquic Argixerolls). The 40-yr (1951–1999) average annual precipitation at the site was 455 mm, with approximately one-half occurring from May through October.

Morphological variation in the parents and the advanced amphiploid population were measured on 18 morphological characters (Table 1). Forty-two mature plants each of bluebunch and thickspike were randomly selected from 10 and nine accessions, respectively, in a completely randomized design for inclusion in the morphological portion of the study. Bluebunch wheatgrass accessions included ‘Whitmar’, PI 236670, D-1252, D-2839, D-2840, D-2815, D-2837, D-2838, P-739, and a collection from Yellowjacket, ID. Accessions of thickspike wheatgrass included ‘Critana’ (Stroh et al., 1972), ‘Elbee’ (Smoliak and Johnston, 1985), ‘Sodar’ (Douglas and Ensign, 1954), D-2847, D-2848, D-2845, D-2850, PI 232115, and a collection from Wyoming. Accessions designated as D- were obtained from the Triticeae Collection at the Forage and Range Research Lab, USDA-ARS, Logan, UT. Morphological samples were taken at the onset of anthesis and measurements were made on five reproductive culms selected at random from each of 42 plants. The parental nursery was adjacent to the amphiploid breeding population from which 42 plants were randomly selected.


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  		Table 1. Means for morphological characteristics for 42 plants each of bluebunch, thickspike, and the amphiploid breeding population.

 
The effect of chromosome doubling was measured on 19 morphological characters (Table 2) in the triploid F1 and C0 (doubled F1 hybrid) hybrids. Seven clones of one F1 plant and eight C0 clones, genetically identical to the F1 except for ploidy level, were included in the study. Measurements were taken on seven reproductive culms per clone.


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  		Table 2. Mean characters for F1 and C0 amphiploid plants.

 
Impressions of the adaxial stomatal apparatus were obtained as described by Sinclair and Dunn (1961) and Hilu and Randall (1984). Clear nail polish was applied to the adaxial side of five blades plant–1 that had been previously soaked in water for 12 h. After 4 h, the nail polish was peeled off and mounted on a microscope. Length and width of 20 stomatal apparatus from each leaf (100 plant–1) were measured with a micrometer at 400x.

Statistical Analysis
All data were subjected to ANOVA using GLM procedures as a fixed model with plants nested within populations. The MS associated with the variation among populations was tested for significance using the MS for variation among plants within populations as an error term. Mean separations were made on the basis of LSDs at the 0.05 probability level (SAS Institute, 1999). Correlation coefficients were computed based on entry x rep means using PROC CORR (SAS Institute, 1999). Principal components were derived using correlation matrices. Cluster analysis was performed using unweighted pair group mathematical average (UPGMA) algorithms on the distance matrices to provide a distance phenogram. The distance coefficient was defined as the average taxonomic distance computed by NT-SYS (Rohlf, 1992).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Cytology
Chromosome pairing in bluebunch-2x and thickspike was regular, forming seven and 14 bivalents per cell, respectively, as reported by Dewey (1965). Chromosome pairing in the triploid hybrid (StStH) between bluebunch and thickspike averaged 6.84 univalents, 6.95 bivalents, and 0.08 trivalents per cell (Table 3) (Dewey, 1965). The high proportion of bivalents in the triploid hybrid (6–7 per cell) support the conclusion that there are close homologies between the St genomes in bluebunch and thickspike.


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  		Table 3. Chromosome associations at metaphase I in hybrids between bluebunch and thickspike wheatgrasses.

 
Chromosome numbers in the amphiploid breeding population ranged from 2n = 39 to 42. Hyperploid plants accounted for 14% of the population and hypoploids made up 30%. Chromosome pairing in the amphiploid population was not consistent with what was expected based on the pairing associations in the triploid hybrid between bluebunch and thickspike. On the basis of the proposed genomic composition in the C0 (StStStStHH) and the close association between the St genomes in the triploid hybrid (Table 3), chromosome associations with three or more chromosomes should be observed in most cells. The two most frequently observed chromosome pairing associations in the amphiploid breeding population were 21 bivalents and 19 bivalents and one quadrivalent (Fig. 1A ; Table 3), which occurred in 28 and 23% of the cells, respectively. The occurrence of trivalents and quadrivalents in 55% of the metaphase I cells suggests the possible presence of a heterozygous interchange between the St genomes of bluebunch and thickspike. Unpaired chromosomes (Fig. 1B) were observed in 41% of the metaphase I cells and up to three quadrivalents cell–1 were observed in 47% of the cells (Fig. 1C). Chromosome paring in the amphiploid was more typical of a segmental autoallohexaploid with a genomic makeup of St1St1St2St2HH than one with a genomic makeup of StStStStHH. Chromosome pairing in the amphiploid hybrid suggests that when the exact homologous chromosome is present, preferential pairing will occur. However, in the absence of an exact homologous chromosome (i.e., triploid hybrid), homeologous chromosomes are capable of pairing. Lagging chromosomes averaged 0.54 laggards cell–1, which resulted in 0.70 micronuclei quartet–1. There was a negative correlation between number of univalents and seeds spike–1 (r = –0.29, P < 0.05) and micronuclei and percentage stainable pollen (r = –0.29, P < 0.05) (data not shown). Univalent frequency was correlated with the frequency of lagging chromosomes (r = 0.78, P < 0.01) and the frequency of micronuclei quartet–1 (r = 0.52, P < 0.01). As the incidence of univalents cell–1 increased, percentage stainable pollen and seed set declined (r = –0.29, P < 0.05). Univalents were negatively correlated (r = –0.52, P < 0.01) with bivalent frequency.


Figure 1
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  		Fig. 1. Meiotic chromosome association for the bluebunch x thickspike amphiploid. (A) 19 II + 1 IV, (B) 1 I + 17 II + 1 III + IV, and (C) 15 II + 3 IV.

 
Percentage Stainable Pollen and Seed Set
Percentage stainable pollen in the amphiploid breeding population ranged from 56 to 99% and averaged 85 ± 0.1% stainable pollen. This is similar to the 90% stainable pollen reported for bluebunch and thickspike (Dewey, 1965). Consistent with crested wheatgrass (Jensen et al., 2006), level of aneuploidy had no effect on percentage stainable pollen (data not shown). However, associated with aneuploidy in the amphiploid was a significant (P < 0.05) decrease in seeds spikelet–1 (data not shown). This similar effect was observed in the amphiploid hybrid between E. lanceolatus and E. caninus (Jensen, 2005) and in crested wheatgrass (Jensen et al., 2006).

Despite the parents being self-sterile, amphiploid plants were self-fertile (data not shown). Euploid plants produced 23% (P < 0.05) more seed under self-pollination than did aneuploid plants (data not shown). This may not be so surprising since most of the H genome Hordeum species are highly self-fertile (Jensen et al., 1990). In addition, autotetraploid forms of bluebunch have increased levels of self-fertility (Jensen et al., 1990). With the exception of the thickspike complex, all species comprised of the St and H genomes are capable of producing seed under self-pollination (Jensen et al., 1990).

Morphology
Principal component analysis was used to explore the multivariate patterns of variation among populations of bluebunch, thickspike, and the amphiploid breeding population. On the basis of 18 morphological characters (Table 1), principal components accounted for 71% of the variation in the first three axes and separated bluebunch, thickspike, and the amphiploid (Table 4, Fig. 2 ). Within the first component, which accounted for 45% of the variation, spikelets spike–1, first and second glume length and width, lemma length and width all had factor loadings > 0.82. Component 2 was much less diagnostic, with spike length, leaf height on the culm, and leaf blade width having the highest weightings. Leaf height on the culm and glume and lemma awn lengths characterized principal component 3.


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  		Table 4. Principle component analysis of morphological characters for 42 plants each of bluebunch, thickspike, and the amphiploid breeding population.

 

Figure 2
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  		Fig. 2. Cluster analysis of 18 morphological traits for bluebunch, thickspike, and the amphiploid hybrid.

 
Bluebunch wheatgrass was characterized by having longer spikes (P < 0.05) and fewer spikelets spike–1 (P < 0.05) than thickspike and the amphiploid (Table 1). Characteristic of bluebunch are more distantly spaced spikelets on the rachis. Thickspike wheatgrass had more spikelets and shorter spikes than either the amphiploid or bluebunch populations. The amphiploid was intermediate to its parental species in both respects (Table 1). Culm length (cm) and leaf length (cm) and width (mm) in the amphiploid hybrid were significantly (P < 0.05) longer and wider than in bluebunch and thickspike. Leaves were oriented significantly (P < 0.05) higher on the culm with more leaf nodes per culm in the amphiploid hybrids than either parent.

Examination of the position of individual plants with respect to the first two principal component axes showed that the species tended to be located in different but contiguous portions of the cluster (Fig. 2). This was expected, since it has been hypothesized that bluebunch and thickspike have introgressed with each other over time. These observations were supported by Dewey (1965, 1970), who obtained progeny and backcross derivatives from thickspike x bluebunch hybrids that closely resembled Montana and Wyoming wheatgrasses.

Effect of Chromosome Doubling
All character contrasts between the F1 hybrid and its genetically identical C0 derivative were significant (Table 2). Compared with F1 plants, C0 plants had significantly (P < 0.05) longer and wider leaf blades, thicker culms, longer spikes and spikelets, longer and wider stomatal apparatus (Fig. 3 ), longer and wider first and second glumes, longer first and second glume awns, longer and wider lemmas and lemma awns (Table 2). The C0 plants had fewer leaf nodes per culm and spikelets spike–1 (Table 2). The F1 plants produced >2.5 times as many flowering culms as the C0.


Figure 3
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  		Fig. 3. Guard cells of the F1 and C0 hybrids showing the effect of chromosome doubling. (A) F1 triploid hybrid and (B) C0 amphiploid hybrid.

 

   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
The amphiploid breeding population derived from the hybrid bluebunch x thickspike was 56% euploid and 44% aneuploid. Meiosis in the euploids was essentially regular, 50% of the cells formed either 21 bivalents, or 19 bivalents and one quadrivalent. Aneuploid plants were significantly less fertile than their euploid counter parts. The frequency of aneuploidy can likely be reduced in subsequent generations by increasing the selection pressure for seed set (Jensen et al., 2005). The St genomes of bluebunch and thickspike are similar but display preferential pairing in the amphiploid. Self-pollination in the amphiploid breeding population ranged from highly self sterile to completely self fertile so that opportunities were evident for selection of either mode of pollination.

On the basis of the 18 floral and vegetative characteristics measured, the amphiploid hybrid was more similar to bluebunch than thickspike, but morphologically can be effectively separated from both parents. Studies of F1 and C0 counterparts indicated that induced amphiploidy in this population resulted in an initial reduction in vegetative vigor, larger guard cells and floral parts, and thicker stems. Through the use of chromosome doubling, this amphiploid successfully combined genes from of bluebunch and thickspike into a partially fertile hexaploid population that is vegetatively more vigorous than bluebunch wheatgrass. Reduced fertility and increased occurrence of aneuploid plants continue to be a problem in the amphiploid after four cycles of selection. However, there appears to be enough variation for pollen stainability and seed set to indicate that selection for increased fertility is possible. In addition, further selection emphasis on plants with a chromosome number of 2n = 6x = 42 will likely increase fertility.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Cooperative investigations of the USDA-ARS and the Utah Agric. Exp. Stn., Logan, UT 84322. Approved Journal Paper No. 7702.

Received for publication June 22, 2005.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 





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