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Published online 27 May 2005
Published in Crop Sci 45:1305-1311 (2005)
© 2005 Crop Science Society of America
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CROP BREEDING, GENETICS & CYTOLOGY

Characterization of Hybrids from Induced x Natural Tetraploids of Russian Wildrye

Kevin B. Jensen*, Steven R. Larson, Blair L. Waldron and Douglas A. Johnson

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
 
Because of restrictions imposed by hybridization barriers, Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski] breeders have usually limited themselves to selection and hybridization within ploidy levels. Plants of diploid (2n = 2x = 14) Russian wildrye tend to be shorter, finer stemmed, leafier, and generally higher in forage production than tetraploids (2n = 4x = 28), which have larger seeds and superior seedling vigor. The principle objectives of the present study were to combine the genes from diploid and tetraploid Russian wildrye into a stable population, and evaluate variability in chromosome behavior, molecular genetic diversity, forage production, seed characteristics, and seedling vigor of the hybrids. Cytologically, the hybrid population behaved as an autotetraploid averaging 0.50 univalents + 9.37 bivalents + 0.24 trivalents + 1.98 quadrivalents per cell. Narrow-sense heritabilities were 31, 63, 17, and 23% for dry matter yield (DMY), total seed yield, 100-seed weight, and rate of seedling emergence, respectively. Coefficients of determination (r2) among parent and half-sib family means were 0.12, 0.29, 0.09, and 0.72 for the above traits, respectively. Essentially all variation among half-sib families for rate of seedling emergence was associated with environmental effects. Positive responses to selection pressure for improved seed yield, rate of seedling emergence, and dry matter production were achieved and additional gains are expected. Any major improvement in seed mass would be limited without broadening the genetic base of the hybrid.

Abbreviations: DMSO, dimethylsulfoxide • DMY, dry matter yield


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
RUSSIAN WILDRYE is a cool-season, dryland bunchgrass that provides excellent forage for livestock and wildlife on rangelands of the Northern Great Plains and the Intermountain Region of North America. This species is native to steppe and semidesert regions of Eurasia and, although introduced into the USA in 1927, its potential in range seeding programs was not recognized until the 1950s (Hanson, 1972). Russian wildrye is valued as a source of forage during the early spring, and its dense basal leaves retain their nutritive value better than those of most other temperate range grasses during the late summer and fall (Knipfel and Heinrichs, 1978).

Relatively slow seedling growth and development are the most serious limitations with Russian wildrye. Poor seedling vigor has reduced Russian wildrye's usage as a range grass (Asay and Jensen, 1996). Seedling vigor has been positively correlated with individual seed weight, and screening procedures based on individual seed weight and emergence from deep seedings were effectively used in breeding programs to improve seedling vigor in Russian wildrye (Lawrence, 1979; Asay and Johnson, 1980; Berdahl and Barker, 1984).

Bingham et al. (1994) proposed that increased vigor of tetraploid alfalfa compared with diploid forms may be associated with greater complementary gene interaction (nonallelic gene interaction or epistasis) at the tetraploid level. Dewey (1980) concluded that induced polyploidy usually results in increased cell size and is often associated with reduced fertility. Induced tetraploids had greater seedling vigor than their diploid counterparts in Italian (Lolium multiflorum Lam.) and perennial ryegrasses (L. perenne L.) (Sjödin and Ellerström, 1986).

As a perennial forage grass with a chromosome number of 2n = 2x = 14, Russian wildrye appears to be an excellent candidate for polyploid breeding. 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).

The parental germplasm of ‘Tetracan’, a tetraploid cultivar (Lawrence et al., 1990), was derived from colchicine-induced tetraploids from several diploid strains of Russian wildrye (Dhindsa and Slinkard, 1963). Tetracan plants have larger spikes and seeds, wider leaves, larger tillers, and longer roots than plants from diploid cultivars, particularly at the seedling stage (Jefferson, 1993).

Sources of naturally occurring tetraploid Russian wildrye were collected by K.H. Asay, D.A. Johnson, and M.D. Casler during a collection expedition to Kazakhstan in 1988. In 1998, 26 half-sib families were combined to generate the broad-based ‘RWR-Tetra-1’ germplasm of naturally occurring tetraploids (Jensen et al., 1998). Dry matter yields of the tetraploids were equal to or less than those reported for typical diploids, particularly in harsh environments. However, 100-seed weight and rate of emergence from a 7.6-cm planting depth in the tetraploids were significantly greater than that for the diploids (Jensen et al., 1998).

Merging diploid and tetraploid populations in a breeding program could increase hybrid vigor and lead to strains with desirable characteristics. Such a breeding scheme in crested wheatgrass [Agropyron cristatum (L.) Gaertn., A. desertorum (Fisch. Ex Link) Schultes] was used to develop the cultivars Hycrest and CDII (Asay et al., 1985a, 1997). The principle objectives of the present study were to combine the gene(s) from diploid and tetraploid Russian wildrye into a stable population and evaluate variability in mitotic and meiotic chromosome behavior, molecular genetic diversity, forage productivity, seed characteristics, and seedling vigor of the hybrid population.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Plant Materials
Tillers from 35 plants of ‘Bozoisky-Select’ (2n = 14) (Asay et al., 1985b) were exposed to 0.1% colchicine {7-acetamido-6,7-dihydro-1,2,3,10-tetramethoxybenzo[a]heptalen-9(5H)-one} in 1% dimethylsulfoxide (DMSO) in 1 L of H2O for 4 h followed by a wash in tap water for 1 h. Doubled plant sectors (2n = 28) were identified in one of the original 35 treated plants. These doubled sectors were tillered and established at the Utah Agricultural Experiment Station Evans Experimental Farm, located about 2 km south of Logan, UT (41°41' N, 111°50' W, 1350 m above sea level). Soil at the site is a Nibley silty clay loam (fine, mixed, active, mesic Aquic Argixerolls). Ten-year (1990–1999) annual precipitation at the site averaged 475 mm, with about 50% occurring during May through October. Monthly total precipitation from October 1998 to September 1999 was 2.9, 2.8, 0.9, 1.0, 7.1, 0.9, 6.6, 5.8, 6.6, 1.8, and 2.2 cm, respectively. From October 1999 to September 2000, the annual precipitation was 0.3, 0.5, 0.5, 3.7, 7.4, 2.5, 1.3, 0.0, 1.0, 3.0, and 0.0 cm, respectively. During the same time period in 2000 and 2001, annual precipitation was 6.0, 2.8, 2.5, 1.5, 1.8, 2.0, 4.2, 1.7, 0.9, 0.0, 0.4, and 0.8 cm, respectively.

Three natural Russian wildrye tetraploids (PI 565063, PI 565044, and PI 565065) were obtained from the N.I. Vavilov All-Union Institute of Plant Industry, Leningrad, USSR, in October 1988 (Jensen et al., 1998). Spaced plant nurseries of these entries were established in 1989 at the Evans Experimental Farm.

Because Russian wildrye is highly self-sterile (Jensen et al., 1990), all crosses (Table 1) were made without emasculation. Five spikes of the doubled Bozoisky-Select clone were enclosed in a white-parchment bag several days before anthesis. Three to five pollen-bearing spikes of each of the natural tetraploid Russian wildrye accessions were introduced into separate isolation bags of the induced tetraploid at the initiation of anthesis. Controlled pollinations were repeated for three consecutive days.


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  		Table 1. Hybrid parentage, crossing data, and source origin data.

 
Fifty-nine F1 hybrids resulted from the paired crosses, and two subsequent open-pollinated generations were subjected to phenotypic recurrent selection for plant vigor, seed yield, and individual seed weight. On the basis of these selection criteria, 25 generation-three clones were selected and polycrossed to produce half sib families. Although the 25 clones were not randomly selected from generation three, they represent the genetic base for future breeding work within this hybrid population.

Field Plot Design
The study was conducted on a semiarid range site receiving an average of 365 mm precipitation annually and located at the Utah State University Blue Creek Experimental Farm (41°56' N, 112°26' W), about 80 km northwest of Logan, UT. During the course of the study, the site received 386, 226, and 244 mm of precipitation in 1999, 2000, and 2001, respectively. The soil type at this site is a Parleys silt loam (fine-silty, mixed, superactive, mesic, Calcic Argixerolls). Field plots were established April 1998, and 1 yr was allowed for plants to become fully established before data were collected.

The 25 parental clones were established as spaced plants on 1-m centers arranged as a randomized complete block design with 20 replications. Half-sib families were established on 1-m centers arranged as a randomized complete block design with four replications. Plots consisted of 10 plants (40 total plants per each half-sib family). Half-sib families were established from seed in pots in the greenhouse before transplanting in the field. The parents and half-sib progeny were planted in different areas of the Blue Creek Experimental Farm.

Cytological Analysis
Root tips were treated in an aqueous solution containing 0.05% colchicine plus 0.025% 8-hydroxyquinoline and 25 drops 100 mL–1 of DMSO for 2 to 3 h at room temperature in darkness. They were then fixed and stained in 2% aceto-orcein at 4°C for a minimum of 3 d. The meristematic portion of the root tip was squashed in 45% acetic acid.

Pollen mother cells from the hybrid population were preserved in Carnoy's fixative (six parts absolute alcohol–three parts chloroform–1 part 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, and chromosome pairing was analyzed at metaphase I.

DNA Analysis
A total of 84 plants from eight cultivars, experimental populations, and hybrids of Russian wildrye were screened with six AFLP primer pairs (E.ACAC//M.CTAC, E.ACAG//M.CTTG, E.ACCA//M.CTAG, E.ACCT//M.CTCT, E.ACCT//M.CTGA, and E.ACGT//M.CTGT). The AFLP technique was performed as described by Vos et al. (1995), except that the selective EcoRI primers were fluorescent labled with 6-FAM and fractionated by capillary electrophoresis on an ABI3100 instrument (PE Applied Biosystems, Foster City, CA) with GS-400 size standards (PE Applied Biosystems). Moreover, the EcoRI and MseI preamplification primers (Vos et al., 1995) included two selective nucleotides, AC and CT, respectively. Likewise, selective amplification primers (Vos et al., 1995) included four selective nucleotides as described above. Additional selective nucleotides were used to reduce the complexity of DNA profiles and simplify genotype interpretation. The relative migration (molecular size) of amplified DNA fragments was determined using GeneScan (PE Applied Biosystems) and classified into allelic categories using the Peakmatcher program (DeHaan et al., 2002). Selective amplifications were replicated as suggested by DeHaan et al. (2002). An unrooted neighbor-joining cluster analysis tree was developed using PAUP* version 4.0b8 (Sinauer Associates, Inc., Publishers, Sunderland, MA) and TREEVIEW (Page, 1996).

Forage and Seed Characteristics
Fifteen months following establishment in 1998, determinations were made of DMY. The first five half-sib plants in the 10-plant plot were harvested to an 8-cm stubble height with a Swift Current sickle bar harvester (Swift Machining & Welding LTD, Swift Current, SK, Canada) for determination of DMY on 15 July 1999 and 20 July 2001. Dry matter yields of the parental clones were obtained when the entire plant was harvested at seed maturity in July. All harvests were made at the mature seed stage, and dry weights were expressed as megagrams per hectare. Forage samples were taken from each plot and dried to a constant weight in a forced-air oven at 60°C to determine dry matter percentage.

Seed yield (g) and 100-seed weight (g) were measured in 1999 and 2001 from the 25 parental clones at the Blue Creek Experimental Farm. Parental plants were harvested in their entirety at seed maturity. For the 25 half-sib families, the remaining five plants in each plot were harvested for seed yield. Seed heads were threshed, and total seed weight and 100-seed weight was determined. Seedling vigor was determined as the ability of seedlings to emerge from a 7.6-cm seeding depth in 1999 and 2001 from seed harvested that year. An index that measures emergence as seedlings day–1 (Maguire, 1962; Asay and Johnson, 1980) was used.

Data Analysis
Variance components for DMY and seed characteristics were estimated considering replications, parent clones, and half-sib families, and years as random variables using the REML option of PROC VARCOMP (SAS Institute, 1999). Narrow-sense heritabilities were computed based on the variation among half-sib families as {sigma}g2/{sigma}2ph, where {sigma}g2 is the total genetic variance arising from differences among half-sib families, and {sigma}2ph is the phenotypic variance among entries (Hallouer and Miranda, 1981; Fehr, 1987). The phenotypic variance is defined as {sigma}2ph = {sigma}g2 + {sigma}2rg/4 + {sigma}2gy/2 + {sigma}2rgy/8, where {sigma}2rg, {sigma}2gy, and {sigma}2rgy are the variance components for replications x entries, entries x years, and replications x entries x years, respectively. The relative contribution (%) of the genotype x year ({sigma}gy2/{sigma}2ph) to the phenotype variance was estimated and compared with variance-derived narrow-sense heritability. Parent–progeny correlations were computed on entry x year means.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY
 REFERENCES
 
Cytology
Root tips of the induced-tetraploid P. juncea parent had 28 chromosomes. Meiosis in the induced tetraploid was mixed with 44% of the cells having 14 bivalents and 38% having two or three univalents. The most frequently observed metaphase I association in the induced tetraploid parent was 0.81 univalents + 12.87 bivalents + 0.06 trivalents + 0.32 quadrivalents (Table 2). What was not anticipated was the lack of observed quadrivalents, which occurred in only 25% of the cells. Dewey and Pendse (1968) reported a mean of 2.25 quadrivalents in an induced autotetraploid of A. cristatum. In artificially induced P. juncea, Wang and Berdahl (1990) reported a mean chromosome association of 0.60 univalents + 9.10 bivalents + 0.47 trivalents + 1.96 quadrivalents. The lack of quadrivalents in the induced tetraploid of Bozoisky-Select would suggest the possible presence of a pairing regulator gene.


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  		Table 2. Chromosome pairing in doubled Bozoisky-Select x natural tetraploid Russian wildrye hybrid.

 
All 25 hybrids studied had a chromosome number of 2n = 28. At metaphase I, univalents were less common in the hybrids than in the parents. Zero to six trivalents or quadrivalents were found per cell with an average of 0.24 and 1.98, respectively. The mean quadrivalent frequency in the hybrid cells exceeded that of the parents by more than one configuration (Table 2). The hybrids averaged about three less bivalents than the induced tetraploid parent. Frequency of bivalents ranged from 2 to 14 and averaged 9.37 per cell (Table 2). The limited number of chromosome associations higher than four, no lagging chromosomes at anaphase I, and no micronuclei suggest that there is no structural heterozygosity between the induced and natural tetraploids. The level of multivalent formation was similar to that reported in the genus Agropyron, which has the P genome in an autoploid series of diploid, tetraploid, and hexaploid species (Dewey, 1984; Wang, 1989).

Molecular Characterization
The neighbor-joining tree (Fig. 1) demonstrated that all of the hybrid population genotypes formed a group, relative to other Russian wildrye cultivars and experimental populations. Hybrid population genotypes displayed the same average number of fragments as other populations except ‘Mankota’, which displayed significantly fewer fragments per plant (Table 3). The average similarity coefficient among genotypes of the hybrid population was notably greater than all other populations except RWR-Tetra-1, which is a composite of naturally occurring tetraploids from Kazakhstan (Tables 3 and 4).



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  		Fig. 1. Neighbor-joining cluster analysis based on genotypic similarity determined by amplified fragment length polymorphism analysis. Diploid entries include ‘Bozoisky-Select’, ‘Mankota’, ‘Vinall’, and ‘Swift’. Tetraploid entries include ‘Tetracan’, ‘RWR-Tetra-1’, and doubled Bozoisky-Select x tetraploids (B x T).

 

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  		Table 3. Average number of AFLP fragments (SE) and average similarity coefficients within cultivars corrected for covariance as described by Leonard et al. (1999).

 

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  		Table 4. Probability values from t test among pairwise comparisons of the average similarity coefficients within cultivars corrected for covariance as described by Leonard et al. (1999).

 
Analysis of molecular variance (Excoffier et al., 1992) indicated that the average number of polymorphisms between the hybrid population and the other seven Russian wildrye populations was significantly greater (11.7 to 26.6%) than the average number of polymorphisms within respective cultivars and experimental populations (Table 5). Genotypes of the two parents, RWR-Tetra-1 and Bozoisky-Select, generally grouped together in the same major branch of the hybrid (Fig. 1); however, a few genotypes were dispersed throughout the tree. Genotypes of ‘Vinall’ and ‘Swift’ were mixed. No relationships based on chromosome number were apparent.


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  		Table 5. Pairwise comparisons of the average total number of polymorphisms between varieties (PXY, above diagonal), average number of polymorphisms within varieties (PX, diagonal), and the average number of polymorphisms among varieties, corrected by polymorphism within varieties [PXY – (PX + PY)/2, below diagonal]. The relative apportionment of variation among varieties, {Phi}ST[2PXY/(PX + PY)], is also shown in parentheses (below diagonal). All {Phi}ST values were significant (P < 0.01).

 
Forage Yield
Combined across years, the narrow-sense heritability estimate based on the variation among half-sib families in the hybrid population was 0.31 for DMY. The parent–progeny correlation was 0.35. Nguyen and Sleper (1983) pointed out that the parent–progeny coefficient of determination (r2) is a useful statistic that describes the variation present in the progeny accounted for by corresponding covariation in the parents. The coefficient of determination (r2) was 0.12 for DMY. Moderate heritability estimates for DMY indicate that increases in DMY should be possible in this hybrid population. Mean DMY in the half-sib families ranged from 0.7 to 3.3 Mg ha–1 (Table 6).


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  		Table 6. Variance components and means for half-sib families of Bozoisky-Select x Tetraploid Russian wildrye for dry matter production (DMY) and seed characteristics.

 
On the basis of the magnitude of {sigma}2gy compared with that of {sigma}2ph, 37% of the variation for DMY was attributed to possible environmental affects, suggesting that the need to evaluate and select among half-sib families in multiple environments. On the basis of the Spearman-rank coefficient of determination (r2), 14% of the variation in DMY rank in 1999 was explained by corresponding covariation in DMY rank in 2001. Only eight half-sib families were common to the top 50% in both years.

Seed Yield
The narrow-sense heritability estimate for total seed yield (g) was 0.63 (Table 6). Selection within all three cycles of the hybrid population for seed yield may have biased estimates of narrow-sense heritability upward. Parent–progeny correlation was 0.54 and the coefficient of determination (r2) was 0.29 for total seed yield.

Heritability estimates were higher than the {sigma}2gy/{sigma}2ph ratio for total seed yield (Table 6), indicating that the environment affected seed production less than DMY. Despite the high heritability estimate for seed yield, only 12% of the variation in total seed yield rank in 1999 was explained by corresponding covariation in total seed yield rank in 2001. Only seven half-sib families were common to the top 50% in both years, suggesting the need to evaluate and select for seed yield under multiple environments.

Individual Seed Weight
Selection for heavier seeds is often used as an indirect method to screen for increased seedling vigor (Asay and Johnson, 1980). Consequently, an understanding of genetic variation for seedling vigor in this hybrid population is critical for further improvement. After three cycles of selection for 100-seed weight, genetic variances were essentially zero (Table 6) in the half-sib families. Narrow-sense heritability estimates of zero, parent–progeny correlations of 0.13, and a low coefficient of determination (r2 = 0.09) confirm the lack of genetic variation for individual seed weight in the hybrid population (Table 6). As a result, prospects for increased seed weight in this population are low without introducing additional genetic variation. The lack of genetic variation for seed weight may have resulted because only four genotypes were used to generate this hybrid population.

Rate of Emergence
Breeding lines of Russian wildrye have responded to selection pressure for increased seedling rate of emergence across repeated cycles of selection (Berdahl and Barker, 1984). A narrow-sense heritability estimate of 0.23 (Table 6) and a parent–progeny correlation of 0.35 suggest that large increases in the rate of emergence from a deep planting depth may be limited unless the genetic base can be broadened through additional hybridization of doubled diploids with natural tetraploid Russian wildrye populations.

On the basis of {sigma}2gy/{sigma}2ph, essentially all the variation observed in the half-sib families for rate of seedling emergence was associated with environmental effects (Table 6). Variance components for rate of emergence were 7.8 times larger in seed harvested in 2001 than 1999 (data not shown). Rate of emergence from a deep planting depth was higher in 1999 than 2001 (Table 6). Other than a decrease in annual precipitation from 386 mm in 1999 to 244 mm in 2001, environmental factors were very similar. On the basis of these data, seedlings from seed produced under drier conditions are less likely to exhibit the same level of seedling vigor as those produced under wetter conditions. The results suggest that when selecting for rate of emergence from a deep planting depth, the seed used in planting the nursery should be produced under uniform environmental conditions.

Intercharacter Correlations
On the basis of plants grown on 1-m centers, intercharacter correlations indicated that only total seed yield (g plant–1) was significantly associated with DMY (r = 0.48, P = 0.05), but not 100-seed weight (g) (r = 0.04) and seedling vigor (r = 0.06), as measured by seedling emergence from a deep planting depth (7.6 cm). The strongest positive correlation was between 100-seed weight and seedling vigor (r = 0.82, P = 0.01). These correlations suggest that selecting for traits associated with increased DMY and seed yield will not affect seedling vigor or individual seed weight. However, selection for increased individual seed weight would likely increase seedling vigor.


   SUMMARY
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
REFERENCES
 
Because of restrictions imposed by crossing barriers, Russian wildrye breeders have usually limited themselves to selection and hybridization within ploidy levels. The prospects of combining diploid and tetraploid P. juncea into a breeding program appear promising. The apparent seedling vigor advantages of tetraploids are encouraging (Asay et al., 1996). An increase in vigor is a common consequence of merging genetically diverse germplasm, which can occur between diploid and tetraploid Russian wildrye after the diploids are brought to the tetraploid level.

Cytologically, the hybrid population behaved as an autotetraploid averaging 0.50 univalents + 9.37 bivalents + 0.24 trivalents + 1.98 quadrivalents. Narrow-sense heritabilities were 31, 62, 17, and 23% for DMY, total seed yield, 100-seed weight, and rate of seedling emergence, respectively. Coefficients of determination (r2) among parental and progeny means were 0.12, 0.29, 0.09, and 0.72 for the above traits, respectively. Positive responses to selection pressure for improved seed yield, rate of seedling emergence, and DMY were achieved and additional gains are expected, particularly in seed yield and DMY. A concentrated breeding effort to broaden the genetic base is critical to achieve measurable increases in seed weight and seedling vigor. Crosses between the hybrid population and artificially derived tetraploids, which originated from different diploid parentage, would broaden the genetic base in the hybrid population.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY
 REFERENCES
 
Cooperative investigations of the USDA-ARS and the Utah Agric. Exp. St., Logan Utah 84322. Approved Journal Paper No. 7618.

Received for publication July 26, 2004.


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


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K. B. Jensen, K. W. Maughan, and K. H. Asay
Characterization of Amphiploid Hybrids between Bluebunch and Thickspike Wheatgrasses
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