Tetraploid Germplasm Research Articles

Regeneration and molecular characterization of interspecific somatic hybrids between Satsuma mandarin and two seedy sweet oranges for scion improvement

With 2 figures Abstract In an effort to produce novel and elite tetraploid germplasm for citrus seedless scion cultivar improvement, symmetric fusions were conducted between embryogenic suspension protoplasts of Citrus unshiu Marc. cv. ‘Guoqing No. 1’ (G1) and mesophyll protoplasts isolated from the leaves of ‘Red Anliu’ (Citrus sinensis Osbeck cv. ‘Red Anliu’) or ‘Taoye’ sweet oranges (C. sinensis Osb. cv. ‘Taoye’). Eight plants from G1+ ‘Red Anliu’ and 20 plants from G1+ ‘Taoye’ were regenerated, respectively. Flow cytometry analysis showed that all five randomly selected plants from G1+ ‘Taoye’ were tetraploid, and for four randomly selected plants recovered from G1+ ‘Red Anliu’, three were tetraploid and the remaining one was diploid. Genetic compositions of regenerated plants were determined by nuclear simple sequence repeat (SSR), chloroplast-SSR and mitochondrial cleaved amplified polymorphic sequence analysis. It was confirmed that all tetraploid plants from both fusions were true somatic hybrids, with chloroplast DNA and mitochondrial DNA from embryogenic callus parent. The diploid from G1+ ‘Red Anliu’ was identified to be originated from G1 protoplast, the callus parent. The potential of these novel allotetraploid somatic hybrids in citrus scion breeding was discussed.

POLYPLOIDY AND VARIETY IMPROVEMENT OF PHALAENOPSIS ORCHIDS

Polyploidy played an important role in the development of Phalaenopsis varieties. Many popular commercial hybrids are tetraploids. However, analysis of the pedigree of these hybrids indicated that their genetic base was quite narrow. On the other hand, it is difficult to transfer favorable genes of the diploid wild species to the commercial hybrids because of the difference in the ploidy levels. In addition, the chromosome sizes of Phalaenopsis species varied tremendously. Hybrids developed from the crosses between these species were usually sterile due to the problem of genome homology. Chromosome doubling is one of the methods to restore the fertility of these hybrids. Due to the above reasons, a research project was initiated in 2004 to develop simple and effective methods to detect the nuclear DNA content by flow cytometry and to double the chromosome of Phalaenopsis species and varieties by the use of tissue culture without application of anti-microtubule agents. The nuclear DNA contents of the native species and some of the related hybrids of Phalaenopsis in several germplasm banks and orchid nurseries in Taiwan were determined. Based on this information, Phalaenopsis species were grouped into low, medium and high nuclear DNA content which reflects the chromosome sizes of the species. This will provide useful information to orchid breeders for the selection of parental varieties for hybridization. Through chromosome doubling techniques, a conversion program to develop a series of tetraploid native species of Phalaenopsis is in progress. This project will provide a wide range of tetraploid germplasm matching the ploidy level of the commercial hybrids to accelerate the breeding of novel varieties of Phalaenopsis for the market and to create a significant impact on the orchid industry in the coming years.

Wild Tetraploid Wheat (Triticum turgidum L.) Response to Heat Stress

Identifying reliable screening tools and characterizing tolerant germplasm sources are essential for developing wheat (Triticum aestivum L.) varieties suited for the hot areas of the world. Our objective was to evaluate heat tolerance of promising wild tetraploid wheat (Triticum turgidum L.) accessions that could be used as sources of heat tolerance in common- and durum-wheat (Triticum durum) breeding programs. We screened 16 wild tetraploid wheat accessions and two common wheat checks for their response to heat stress by measuring damage to the thylakoid membranes, flag leaf temperature depression (FLTD), and spike temperature depression (STD) during exposure to heat stress for 16 days post-anthesis (DPA). Measurements were taken on the day of anthesis then 4, 8, 12, 16 DPA under controlled optimum and heat-stress conditions. Individual kernel weight (IKW) and heat susceptibility index (HSI) measurements were also obtained. Prolonged exposure to heat stress was associated with increased damage to thylakoid membranes, as indicated by the high ratio of constant fluorescence (O) to peak variable fluorescence (P). Some wild tetraploid wheat accessions exhibited a better HSI than the common heat-tolerant wheat cultivar ‘Kauz.’ A positive and significant correlation was found between O/P ratio and each of FLTD and STD under heat-stress conditions. A negative and significant correlation was found between FLTD and HSI and between STD and HSI based on the second and third measurements (4 and 8 DPA). Correlations obtained after the third measurement were not significant because of heat-induced, accelerated maturity and a lack of green leaf tissue. This study identified potential heat-tolerant wild tetraploid wheat germplasm that can be incorporated into wheat breeding programs to improve heat tolerance in cultivated common and durum wheat.

Acquiring homozygous tetraploid germplasm by PEG-mediated protoplast fusion of Rhodiola sachalinensis

To acquire homozygous tetraploid germplasm of Rhodiola sachalinensis. PEG-mediated protoplast fusions were conducted using callus of Rh. sachalinensis as materials. Protoplast fusion products were embedded and cultured in low-density, low-melting-point agar and marked according to the protoplast size, and single-celled sister lines were established to acquire genetically homozygous tetraploid germplasm. R(D) and R(M) of newborn daughter cells or protoplasm, metaphase cells or protoplasm were approximately in line with the formula R(D) = 0.793 7R(M). The change range in diameter of the diploid cells without fusion, two protoplasts fusion product were: 16.7 microm < or = R < 21.3 microm, 21.0 microm < or = R' < 26.8 microm respectively. There is an overlap between the two diameter ranges. The protoplast inoculation density of 1 x 10(4) cells x mL(-1) was appropriate when protoplasts were anchored by low-intensity, low-melting-point agar. Under the conditions of this density, plating efficiency was high and single cell origin of the sister lines microclones grew rapidly, and it was easy to mark the single cell microclones, and separate from each other to subculture. The chromosome counts results showed that chromosome numbers of diploid and tetraploid of single cell lines were 26 and 52, respectively. The result from flow cytometry assay showed that there is no presence of chimerism in single-cell regeneration plantlets. The results of this study provide a scientific basis for polyploid breeding of Rh. sachalinensis.

Potential root depth development and nitrogen uptake by tetraploid bahiagrass hybrids

The objectives were to develop a screening technique to evaluate the tetraploid germplasm of Paspalum notatum Flugge (bahiagrass) for potential rate of root depth development (RRDD), estimate the variability for RRDD among hybrids, and analyze the association between RRDD and nitrogen (N) uptake. First, a screening technique was developed based on the evaluation of two clones grown in clear acrylic columns of different sizes, filled with mineral or organic substrates. The RRDD response was determined to be constant across substrates, and tube sizes. Second, differences in RRDD among thirteen clones were compared by growing plants in acrylic columns filled with mineral substrate. Genetic variability was identified among the bahiagrass germplasm for RRDD. Greater RRDD values resulted in greater root and shoot mass, indicating that RRDD was related to early vigor. Finally, to evaluate the relation between RRDD and N uptake, labeled N (15N) was injected either at the depth of the deepest root of each clone or at the maximum depth of all clones. Rapid root depth development was a determining factor for rapid access and uptake of nitrogen present in deep soil layers. This relationship might explain why clones exhibiting rapid root penetration also showed greater early vigor.

Bahiagrass Tetraploid Germplasm: Reproductive and Agronomic Characterization of Segregating Progeny

Bahiagrass Tetraploid Germplasm: Reproductive and Agronomic Characterization of Segregating Progeny

Bahiagrass Tetraploid Germplasm: Reproductive and Agronomic Characterization of Segregating Progeny

The tetraploid germplasm of bahiagrass, Paspalum notatum Flüggé, is an unexploited source of variability that can be used for the genetic improvement of this species as forage or turf. The objectives of this research were to develop a segregating population by hybridizing induced sexual and apomictic clones and to characterize the resulting population for mode of reproduction, seed fertility, growth habit, seasonal growth pattern, and frost resistance. A population of 591 hybrids had a 4:1 ratio between sexual and apomictic (facultative + obligate apomictic) plants and an 8:1 ratio between others (facultative apomictic + sexual) and obligate apomictic plants. The general fertility of the progeny was higher than that of the induced sexual parents. Significant differences were observed for self‐ and cross‐fertility among apomictic progeny and for self‐fertility among sexual progeny. Most sexual progeny set the same amount of seed when self‐ or cross‐ pollinated, indicating an absence of self‐incompatibility. Marked variation was observed for growth habit, seasonal growth patterns, and frost resistance among the progeny. Heritability estimates obtained for these characteristics indicate that selection can be successfully used for the genetic improvement of this species.

Heritability of Chip Color and Specific Gravity in a Long-day Adapted Solanum phureja–S. stenotomum Population

Acceptable chip color and high specific gravity are important characteristics for chipping potatoes. High specific gravity in US chipping varieties traces back to B5141-6 (Lenape). In an effort to expand the germplasm base for high specific gravity, a long-day adapted diploid hybrid Solanum phureja–S. stenotomum population with high specific gravity was developed. The purposes of this study were to evaluate this population for its chipping potential and estimate heritability for chip color and specific gravity. The population consisted of four clones from each of 72 maternal half-sib families and represents the third cycle of selection for high specific gravity in this population. Clones were grown in a randomized complete block design at Presque Isle, Maine in 2004 and 2005 along with the check variety ‘Atlantic’. Five tubers from each clone per replicate were processed into chips in early December following 10°C storage both years. Individual chips were rated on a 1–10 scale, with ≤7 considered an acceptable color. The average chip scores of the diploid clones and Atlantic were 7.3 and 6.8, respectively. The average specific gravities of the diploid clones and Atlantic were 1.099 and 1.091, respectively. Four diploid clones had significantly higher specific gravity and lighter chip color than Atlantic; 93 diploid clones had significantly higher specific gravity and chip color equal to Atlantic. Broad-sense heritabilities and their 95% confidence interval for chip color and specific gravity were 0.68 (0.59–0.74) and 0.78 (0.70–0.81), respectively. Narrow-sense heritabilities for chip color and specific gravity were 0.24 ± 0.26 and 0.32 ± 0.26, respectively. About one-third of the population possesses both high specific gravity and acceptable chip color and could prove useful in expanding the tetraploid germplasm base for processing traits.

Reproductive Characterization of Bahiagrass Germplasm

Bahiagrass (Paspalum notatum Flüggé) is one of the major forage grasses used for beef cattle production in the southern USA. Description of the reproductive characteristics of bahiagrass germplasm is needed for genetic improvement of this forage crop. The objective was to determine and compare the mode of reproduction of diploid and tetraploid germplasm of bahiagrass. Reproductive behavior was determined by embryo sac observations and a series of controlled pollination studies in the greenhouse and field in 2004 and 2005. Two sexual diploid populations (‘Pensacola’ and ‘Tifton 9’) were not different in self‐ or cross‐fertility, indicating that the general fertility of the crop was not affected by phenotypic mass selection. Sexual diploids, as a group, were determined to be primarily cross‐pollinated, with low but variable levels of self‐fertility. A group of 20 artificially induced tetraploids was determined to be primarily sexual and cross‐pollinated. Fertility of these autotetraploids was lower than the original diploids. Two selected sexual tetraploids produced similar amounts of seed when self‐ or cross‐pollinated. In contrast, six other tetraploids, including the cultivars Argentine and Wilmington, the experimental hybrid Tifton 7, and three plant introductions 315732, 315733, and 315734 reproduced primarily by apomixis. The sexual or apomictic expression, the general fertility, and other agronomic characteristics of these groups of tetraploids define their usefulness for genetic improvement research.

Segregation of total carotenoid in high level potato germplasm and its relationship to beta-carotene hydroxylase polymorphism

High carotenoid potato may have particular value for human health due to the antioxidant properties and the therapeutic value for eye health in patients at risk for macular degeneration. Carotenoid concentrations were determined among the progeny of a cross between two high carotenoid lines derived from diploidPapa Amarilla germplasm from South America. The total carotenoid content ranged from 82 to 2686 µg / 100 g fresh weight (FW). The higher values greatly exceeded the mid-parent value of the cross. An index for yellow tuber flesh color was determined for a subset of the progeny. A cleaved amplified polymorphic sequence (CAPS) assay was developed to distinguish the alleles of beta-carotene hydroxylase (bch) in the two high-carotenoid parents. Abch allele (denoted B) common to the high carotenoid parents co-segregated with yellow flesh in the progeny of a white-flesh x yellow-flesh cross, makingbch an excellent candidate for the classicalY locus, required for yellow tuber flesh. The same allele was also present in all other yellow-fleshed potato clones tested. Genotype atbch explained a portion of the variation of total carotenoid (R2=0.42). Clones homozygous for the B allele (BB) contained, on average, slightly more carotenoid than heterozygous Bb clones, which in turn had much more carotenoid than homozygous bb clones, suggesting a partially dominant gene model. Similarly, bb flesh was significantly less yellow than Bb and BB, the latter two being quite close. Total carotenoid varied considerably between progeny in the Bb and BB genotype categories, suggesting that variation at one or more additional loci have a significant effect on total carotenoid levels. Since the total carotenoid levels in manyPapa Amarilla cultivars and progeny are much higher than those in white- and yellow-fleshed tetraploid cultivars, it may be possible to breed for high carotenoid values in tetraploid germplasm by introducing one or more genes derived fromPapa Amarilla germplasm.

Usefulness of SSR Derived from Tetraploid Gossypium spp. for Analyses of Diploid Gossypium spp.

This investigation is the first attempt to utilize tetraploid species-derived primers to generate markers for mapping in diploid cotton species. About 113 individual F2 plants, developed from the cross between Gossypium nelsoni and G. australe, were used to construct a linkage map. One allotetraploid (ADj) G. hirsutum (TM-1) plant and its putative diploid ancestors, G. arboreum (A2) and G. raimondii (D5), were also included for comparative analysis. We developed a high-throughput multi-mix PCR assay for simple sequence repeat (SSR) marker separation using fluorescent-labeled capillary electrophoresis. Of a total of 205 tetraploid-derived SSR primers, 83% amplified in all four diploid species, indicating that flanking primer sequences are conserved in the diploid and tetraploid ADj genome of cotton. DNA sequence information of 18 randomly selected clones revealed that all of the clones contained SSR repeat motifs in the diploid species, suggesting that SSR markers including the flanking primer sequences have been conserved at the inter-specific level in genomic DNA across years of evolution. About 85% of the amplified primers showed polymorphism among the diploid genome species. Forty percent of SSRs in the diploid G. nelsoni and 35% in G. australe amplified the same size DNA fragment as in the tetraploid, (TM-1). This is further evidence that many of the tetraploid SSRs are conserved across diploid and tetraploid species. G. nelsoni compared with G. australe had more conserved genomic regions in common with G. hirsutum. From 73 markers scored in the F2 population, 50 exhibited codominance and 23 dominance. There were 47 markers linked across 10 linkage groups covering a total map distance of 754 cM with an average of 16 cM per marker. One linkage group (lg-3) with six loci has been assigned to chromosome 20. Comparative analysis of DNA sequences between G. hirsutum and its two putative parental diploid species indicated that changes within a few selected clones have occurred during evolution as follows: (1) by increasing or decreasing the number of the repeat motif, (2) by single nucleotide mutations occurring within or outside of the repeat motif, (3) by inter conversion of a simple to a compound SSR motif, and (4) by inversion of the repeat regions. Tetraploid-derived microsatellites should be useful in comparative genetic mapping of both tetraploid and diploid Gossypium species for evolutionary studies and introgression of agriculturally important traits from exotic diploid and tetraploid germplasm sources.

Mapping Late Blight Resistance inSolanum microdontumBitter

A diploid Solanum population was developed with the objective of mapping quantitative trait loci (QTL) associated with resistance to Phytophthora infestans (Mont.) de Bary, the causal organism of late blight. The mapping population was a cross between a late blight resistant selection of Solanum microdontum Bitter and a susceptible diploid breeding clone. The progeny of 109 clones and the parents were field tested for foliar late blight reaction in 1999 and 2000 and for vine maturity in 2000 and 2001. Parents and progeny were genotyped with isozymes and simple sequence repeats (SSR) markers. A total of 161 pairs of SSR primers were screened, from which 74 amplified polymorphic bands in Metaphor agarose or polyacrylamide gels yielding a total of 109 SSR loci. The isozyme and SSR combined analyses resulted in a total of 118 marker loci in the population, of which 54 were heterozygous (band present) in S. microdontum and 64 in MSA133-57. High phenotypic correlation (r = 0.89, P < 0.0001) was found for late blight reaction between years and no correlation was found between late blight and vine maturity. Solanum microdontum transmitted high levels of resistance to more than 50% of the offspring. There was a major QTL linked with the SSR marker STM0020b associated with foliar late blight resistance located at the same position in both years of field testing explaining over 60% of the phenotypic variance. This major QTL is suitable for marker-assisted selection to introgress a new source of resistance to P. infestans to the cultivated tetraploid germplasm of potato.

Genetic Diversity in Diploid and Tetraploid Late Blight Resistant Potato Germplasm

An understanding of the genetic relationship within potato germplasm is important to establish a broad genetic base for breeding purposes. The objective of this study was to assess the genetic diversity of potato (Solanum tuberosum subsp. tuberosum Hawkes) germplasm that can be used in the development of cultivars with resistance to late blight caused by Phytophthora infestans (Mont.) de Bary. Thirty-three diploid and 27 tetraploid late blight resistant potato clones were evaluated for their genetic diversity based on 11 isozyme loci and nine microsatellites. A total of 35 allozymes and 42 polymorphic microsatellite fragments was scored for presence or absence. The germplasm was clustered based on the matrix of genetic similarities and the unweighted pair group means analysis of the isozyme and microsatellite data, which were used to construct a dendrogram using NTSYS-pc version 1.7. Twenty-three allozymes and DNA fragments were unique to the wild species. The diploid Solanum species S. berthaultii Hawkes and S. microdontum Bitter formed two distinct phenetic groups. Within S. microdontum, three subgroups were observed. The tetraploid germplasm formed another group, with S. sucrense Hawkes in one subgroup and the cultivated potato and Russian hybrids in another subgroup. Based upon the genetic diversity and the level of late blight resistance, S. microdontum and S. sucrense offer the best choice for strong late blight resistance from genetically diverse sources. This potato germplasm with reported late blight resistance should be introgressed into the potato gene pool to broaden the genetic base to achieve stronger and more durable resistance.

Evaluation of genetic variation in the daylily (Hemerocallis spp.) using AFLP markers

The daylily (Hemerocallis spp.) is one of the most economically important ornamental plant species in commerce. Interestingly, it is also one of the most heavily bred crops during the past 60 years. Since the American Hemerocallis Society began acting as the official registry of daylily cultivars in 1947, more than 40 000 registrations have been processed. In order to determine the effects of intensive breeding on cultivar development, and to study relationships among different species, genetic variation in the daylily was estimated using AFLP markers. Nineteen primary genotypes (species and early cultivars) and 100 modern cultivars from different time periods were evaluated using 152 unambiguous bands (average 79% polymorphism rate) derived from three AFLP primer combinations. Overall, pairwise similarity estimates between entries ranged between 0.618 and 0.926 (average=0.800). When comparing cultivar groups from different time periods (1940–1998), genetic similarity was initially increased, compared to the primary diploid genotypes, remained constant from 1940 to 1980, and then steadily increased as breeding efforts intensified and hybridizers began focusing on a limited tetraploid germplasm pool derived by colchicine conversion. Among modern (1991–1998) daylily cultivars, genetic similarity has increased by approximately 10% compared to the primary genotypes. These data were also used to evaluate recent taxonomic classifications among daylily species which, with a few minor exceptions, were generally supported by the AFLP data.

Gas Exchange and Water Relations in Diploid and Tetraploid Russian Wildrye

Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski] is a drought tolerant, cool‐season forage grass used in seeded pastures in the Northern Great Plains where water often limits production. Seedling vigor is generally poor in diploid cultivars, but tetraploid germplasm has improved seedling vigor. Objectives of this study were to determine the relationships between water‐use efficiency (WUE), carbon isotope discrimination (CID), and gas exchange rates for a diploid cultivar (Vinall) and a tetraploid entry over 3 yr, two water treatments (50 and 150% of mean monthly precipitation), and two fertilizer rates (10 and 134 kg N ha−1) in a rain shelter. The tetraploid entry exhibited higher carbon exchange rate (CER), stomatal conductance (gs), and transpiration (T) than the diploid entry at the 50% but not the 150% water treatment. Leaf water potentials (LWP) were 0.6 and 0.3 MPa less negative for the tetraploid entry than diploid entry at the 50 and 150% water treatment, respectively. Values of CER averaged 12.4 and 14.2 μmol m−2 s−1, T averaged 5.2 and 6.0 mmol m−2 s−1, and gs averaged 0.21 and 0.28 mol m−2 s−1 for the diploid and tetraploid entry, respectively. The diploid entry had a significantly greater CID, averaging 20.37‰ compared with 19.65‰ for the tetraploid entry and CID was negatively associated with WUE. The more favorable plant water relations, gas exchange, and dry matter production under limited soil water suggests tetraploid populations of Russian wildrye should be emphasized in future breeding and management programs.

Characterization of tetraploid wheat germplasm for resistance to Pseudocercosporella herpotrichoides, cause of eyespot disease

Eyespot disease, caused by Pseudocercosporella herpotrichoides, can be devastating to winter wheat grown in northern Europe and the northwest USA. Accessions from eight different tetraploid wheat species randomly extracted from core collections were scored for resistance to eyespot disease using a β-glucuronidase (GUS)-transformed strain of P. herpotrichoides. The GUS values for the combined population followed a quasi-Gaussian distribution. Three species, Triticum dicoccoides, T. durum and T. turanicum, showed significant variation (P < 0.001) in disease response with T. dicoccoides having the lowest disease scores, i.e. highest levels of resistance. All tetraploid accessions were less resistant than resistant diploid T. tauschii accessions. Thirteen percent of tetraploid accessions had disease scores that ranged between the average of the resistant accessions of T. tauschii and the moderately resistant hexaploid germplasm line Cerco. Eight accessions (three accessions of T. dicoccoides, two of T. turgidum and three of T. durum) with low disease scores (resistance) to infection were selected for further genetic analysis.

Genetic Variances of Agronomic and Morphological Traits of Diploid Crested Wheatgrass

Crested wheatgrass breeding efforts in the USA have focused on tetraploid germplasm, resulting in little genetic information available for diploid germplasm. This study was conducted to estimate progenymean heritability (h2) of eight traits of diploid crested wheatgrass (Agropyron cristatum L.) grown under seeded competitive conditions in the northern Great Plains. Thirty half‐sib families were randomly selected from a genetically broad‐based population, and evaluated at one location over 2 yr, in single‐row 1.5‐m plots, spaced 38 cm apart, and seeded at 90 seed m−1. Heritabilities based on the progeny meansof 30 families were determined for forage dry matter yield (h2 = 52%), filler height (h2 = 63%), first‐cut vigor (h2 = 15%), regrowth vigor (h2 = 68%), proline content (h2 = 49%), spikelets per spike (h2 = 59%), anthocyanin pigmentation of stem nodes (h2 = 36%), and flag leaf pubescence (h2 = 70%). Heritability of forage yield, tiller height, regrowth vigor, number of spikelets per spike, and flag leaf pubescence were estimated with the greatest precision. Firstcut vigor, proline content, and node color were estimated with less precision. Proline accumulation was not associated with either dry matter yield or regrowth vigor under water stress. Regrowth vigor was not associated with first‐cut yield (P ≥ 0.24), indicating that improvement in first‐cut yield would have little effect on forage regrowth potential. A moderately high heritability estimate for regrowth vigor indicated that selection for this trait could extend the grazing season of crested wheatgrass. Genetic variance components for all traits except first‐cut vigor exceeded their standard errors. Significant family × year effects were detected only for node color. The results indicate that evaluation over multiple years would not be needed for most traits studied.

Identification of a Gene Conferring High Levels of Resistance to Verticillium Wilt in Solanum chacoense

Verticillium wilt (Verticillium albo-atrum) is an important disease affecting potato tuber yield and quality. In North America the major commercial cultivars are susceptible and management strategies for control of the pathogen rely mainly on soil fumigation and crop rotation. In this study 398 genotypes from accessions of Solanum berthaultii, S. chacoense, and S. tarijense were screened for resistance to Verticillium albo-atrum. Resistant genotypes were identified in all but two accessions; however, results indicate that tolerance is more common than resistance. We identified two genotypes in S. chacoense (PI 472819) that had low stem-colonization levels and also did not develop wilt symptoms when inoculated with V. albo-atrum. These genotypes and a susceptible genotype from PI 472810 (S. chacoense) were studied to determine genetic inheritance. Segregation ratios in F1, F2, and backcross populations indicated that resistance in one of the resistant genotypes (18-21R) was controlled by a single dominant gene. Transfer of the Vc gene to tetraploid germ plasm could provide effective and economical control of Verticillium wilt.

The Reproductive Versatility of Eastern Gamagrass

Eastern gamagrass [Tripsacum dactyloides (L.) L.], a native, warmseason species, is a nutritious and palatable forage for grazing livestock. The grass has potential for rangeland and pasture improvement. This field study was conducted over 10 yr and one location to determine the reproductive characteristics of the various cytotypes of Tripsacum and identify methods for breeding agronomically superior cultivars. Intra‐ and interspecific crosses were made and evaluated between several Tripsacum cytotypes. Cytological and polymerase chain reaction‐random amplified polymorphic DNA (PCR‐RAPD) techniques were used to identify methods of reproduction and types of individuals generated. Results of the study identified two methods for facilitating horizontal gene flow among sexual and apomictic cytotypes. Materials with triploid and hexaploid constitutions were essential in the movement of sexual germplasm into apomictic tetraploids and of tetraploid germplasm into sexual diploids, respectively. Triploids and hexaploids were useful materials to introgress and transfer desirable alleles across species, cytotypes, and reproductive barriers. In addition, the generation of triploid and hexaploid genotypes will continue to exert major influences on Tripsacum evolution and speciation. This study identified additional methods and systems for improving Tripsacum germplasm by utilizing its distant relative maize (Zea mays L.) in a breeding or genetic evaluation program.

Potential of New Tetraploid Germplasm in Russian Wildrye

Induced and natural tetraploids have been proposed as promising sources of germplasm in breeding programs to improve Russian wildrye [Psathyrostachys juncea (Fisch.) Nevski]. Studies were conducted under semiarid conditions to evaluate the potential of tetraploid (2n=4x=28) Russian wildrye germplasm recently obtained from Kazakhstan. The teraploids had significantly heavier seeds, greater seedling vigor, and they were significantly taller, and had longer and wider leaves than standard diploid (2n=2x=14) cultivars. Carbon isotope discrimination, which has been negatively correlated with water-use efficiency in cool-season grasses, was significantly lower in the tetraploid accessions than the diploid cultivars. Dry matter and seed yield of these unselected tetraploid accessions were superior to the diploid cultivar Vinall and equivalent to more recently developed diploid cultivars, Bozoisky-Select and Syn-A. In general, relative phenological development and forage quality of the tetraploid populations did not differ significantly from the diploid cultivars; however, water content, which has been associated with greater succulence, was significantly higher in the tetraploid accessions. Significant variation was found among entries within ploidy levels for most characters indicating that genetic variability is available for additional improvement through selection. Results indicate that these tetraploid accessions can be used in the development of promising breeding populations and support earlier conclusions that tetraploid germplasm should receive emphasis in future Russian wildrye breeding programs.