Published online 7 November 2007
Published in Crop Sci 47:2340-2345 (2007)
© 2007 Crop Science Society of America
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CROP BREEDING & GENETICS
Cytogenetic Studies on Diploid and Autotetraploid Common Buckwheat and Their Autotriploid and Trisomics
Qing-Fu Chena,b,*,
Sai L. K. Hsamb and
Friedrich J. Zellerb
a Institute of Plant Genetics and Breeding, School of Biological Technology and Engineering, Guizhou Normal Univ., Guiyang 550001, Guizhou, P.R. China
b Technical Univ. of Munich, Institute of Plant Breeding, D-85354 Freising-Weihenstephan, Germany
* Corresponding author (cqf1966{at}163.com).
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ABSTRACT
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Crossing autotetraploid plants with diploid parent is an efficient way to produce autotriploid and trisomics. Comparative studies on cytogenetics and pollen fertility in diploid and autotetraploid common buckwheat (Fagopyrum esculentum Moench) and their autotriploid hybrids and trisomics were conducted. Observation of meiosis metaphase I of pollen mother cells showed that the average configurations of chromosome pairing in the autotetraploid cultivar Emka, its inbreeding line Emka-1, and autotriploid hybrids were 0.67I + 11.60II + 0.32 III + 1.79IV, 0.71I + 7.42II + 1.19 III + 3.22IV, and 4.71I + 4.71II + 3.29III, respectively. The average numbers of micronuclei in telophase II cells in the diploid cultivar Siva, autotetraploids Emka and Emka-1, and the autotriploid hybrids were 0.03, 0.09, 0.20, and 0.86, respectively. Siva had the highest pollen fertility (98.89%), followed by the autotetraploids Emka and Emka-1 (95.33 and 93.33%); the autotriploid hybrids had the lowest (31.28%). When autotriploid hybrids were backcrossed with diploid cultivar Sobano, 27.72% of the flowers pollinated yielded shriveled dead seeds, and only 3.08% yielded normal seeds. Of the 34 achenes of the autotriploid hybrids, 6 were double trisomics (2n + 2), 23 trisomics (2n + 1) and 5 diploids (2n). The segregation of short-style (Ss) and long-style (ss) plants in backcrossing progenies of trisomics showed that the S gene of common buckwheat was located on chromosome 4E. The common buckwheat trisomic lines produced in this study will become an important tool for common buckwheat genetics and breeding.
Abbreviations: AFLP, amplified fragment length polymorphism • AI, anaphase I • BC, backcross • FISH, fluorescence in situ hybridization • m, median kinetochore • MI, metaphase I • PMC, pollen mother cell • SAT, satellite chromosome • sm, submedian kinetochore • TI, telophase I • TII, telophase II • Tr, trisomics
Cytogenetic Studies on Diploid and Autotetraploid Common Buckwheat and Their Autotriploid and Trisomics
Qing-Fu Chena,b,*,
Sai L. K. Hsamb and
Friedrich J. Zellerb
a Institute of Plant Genetics and Breeding, School of Biological Technology and Engineering, Guizhou Normal Univ., Guiyang 550001, Guizhou, P.R. China
b Technical Univ. of Munich, Institute of Plant Breeding, D-85354 Freising-Weihenstephan, Germany
* Corresponding author (cqf1966{at}163.com).
Crossing autotetraploid plants with diploid parent is an efficient way to produce autotriploid and trisomics. Comparative studies on cytogenetics and pollen fertility in diploid and autotetraploid common buckwheat (Fagopyrum esculentum Moench) and their autotriploid hybrids and trisomics were conducted. Observation of meiosis metaphase I of pollen mother cells showed that the average configurations of chromosome pairing in the autotetraploid cultivar Emka, its inbreeding line Emka-1, and autotriploid hybrids were 0.67I + 11.60II + 0.32 III + 1.79IV, 0.71I + 7.42II + 1.19 III + 3.22IV, and 4.71I + 4.71II + 3.29III, respectively. The average numbers of micronuclei in telophase II cells in the diploid cultivar Siva, autotetraploids Emka and Emka-1, and the autotriploid hybrids were 0.03, 0.09, 0.20, and 0.86, respectively. Siva had the highest pollen fertility (98.89%), followed by the autotetraploids Emka and Emka-1 (95.33 and 93.33%); the autotriploid hybrids had the lowest (31.28%). When autotriploid hybrids were backcrossed with diploid cultivar Sobano, 27.72% of the flowers pollinated yielded shriveled dead seeds, and only 3.08% yielded normal seeds. Of the 34 achenes of the autotriploid hybrids, 6 were double trisomics (2n + 2), 23 trisomics (2n + 1) and 5 diploids (2n). The segregation of short-style (Ss) and long-style (ss) plants in backcrossing progenies of trisomics showed that the S gene of common buckwheat was located on chromosome 4E. The common buckwheat trisomic lines produced in this study will become an important tool for common buckwheat genetics and breeding.
Abbreviations: AFLP, amplified fragment length polymorphism • AI, anaphase I • BC, backcross • FISH, fluorescence in situ hybridization • m, median kinetochore • MI, metaphase I • PMC, pollen mother cell • SAT, satellite chromosome • sm, submedian kinetochore • TI, telophase I • TII, telophase II • Tr, trisomics
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INTRODUCTION
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COMMON BUCKWHEAT (Fagopyrum esculentum Moench) is widely cultivated in many countries of the world. It is regarded as highly nutritious and beneficial to health, and it possesses great potential in human nutrition, medicine, and animal forage (Li and Zhang, 2001; Zeller, 2001; Zeller and Hsam, 2004; Zemnukhova et al., 2005; Wang and Chen, 2006). Common buckwheat is diploid (2n = 2x = 16) (Lin 1994; Morris, 1951). Zu et al. (1984) reported that the chromosomes of common buckwheat are similar to each other in size, and each has a median kinetochore (m). Chen (1999a,b) reported the chromosome number of new species F. megaspartanium, F. pilus, F. zuogongense, and F. pleioramosum. Karyotype analysis (Chen, 2001; Chen et al., 2004) showed that F. esculentum, F. tataricum, F. megaspartanium, and F. pilus are all diploid species with two pairs of satellite chromosomes (SAT), and their karyotype formulas are 12 m + 4 m (SAT), 12 m + 4 submedian kinetochore (sm) (SAT), 8 m + 4 sm + 4 m (SAT), and 12 m +2 m (SAT) + 2 sm (SAT), respectively. Within each species, the different chromosomes are of similar size. Cytogenetical studies on buckwheat have been limited to general observation, and basic genetic research has been scarce. Ohnishi and Ohta (1987) have established some mutant lines and assigned some morphological and isozyme mutants to seven linkage groups, constructing a tentative linkage map. Quite recently, a linkage map based on microsatellite and amplified fragment length polymorphism (AFLP) markers was published by Konishi and Ohnishi (2006).
The assignment of genes to chromosomes in diploid species has been determined mainly by cytogenetical analyses using trisomics. The trisomics may be produced spontaneously or artificially, a common method being selection from backcrossed derivatives of autotriploid hybrids with a diploid accession. Since trisomics were discovered in Datura stramonium by Belling (1920, see Li and Song 1993), complete or incomplete trisomic sets have been produced and used in many plants such as corn (Zea mays L.), barley (Hordeum spontaneum), rice (Oryza sativa L.), broomcorn (Sorghum vulgare Pers.), beet (Beta vulgaris L.), rye (Secale cereale L.), soybean [Glycine max (L.) Merr.], tomato (Lycopersicon peruvianum), tobacco (Nicotiana tabacum) and pea (Pisum sativum L.) (Ahmad and Hymowitz, 1994; Auger and Birchler, 2002; Dong et al., 2001; Imanywoha et al., 1994; Li and Song, 1993; Leang et al., 2006; Schondelmaier and Jung, 1997; Zeller et al., 1977). The trisomic lines play an important role in genetic analyses of these crops. No autotriploid or trisomic lines have been available in common buckwheat because of strong reproductive isolation and difficulties of cytological observation, the chromosomes being small and difficult to distinguish by normal staining methods. The development of trisomic lines in common buckwheat is essential for assigning linkage groups to individual chromosomes.
Homology is necessary for the success of meiosis and is also important in evaluating the structural similarity between chromosomes or chromosome segments in eukaryotic organisms because it provides the basis on which chromosomes recognize one another, synapse, and cross over with a high degree of fidelity. Pairing of homologous chromosomes and formation of bivalents during the meiosis of diploid species are the prerequisite of orderly chromosomal segregation and production of normal spores and seeds. Autotriploids and autotetraploids are invaluable for investigating the synapse of chromosomes and their relationships with the production of spores and seeds.
To improve the low grain set and low productivity of common buckwheat, many hybridizations among buckwheat species have been attempted. Despite considerable difficulties in obtaining interspecific hybrids, some interspecific crosses among F. esculentum, F. tataricum, F. cymosum, and F. zuogongense have been achieved (Chen, 1999a; Krotov and Dranenko, 1973; Lee et al., 1994; Samimy, 1991; Ujihara et al., 1990; Woo et al., 1999). Among these studies, only Chen (1999a) and Ujihara et al. (1990) gave some information regarding chromosome pairing.
Some cytological studies of common buckwheat have been attempted (Lin, 1994; Zu et al., 1984), but chromosome pairing in autotriploid buckwheat has not yet been described.
The main objective of this study was to develop autotriploid and trisomic common buckwheat and to report on cytogenetical findings that provide new information for buckwheat genetics and breeding.
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MATERIALS AND METHODS
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Four heterostylous accessions of common buckwheat were used: the diploid cultivars Siva and Sobano and the autotetraploid cultivar Emka and its inbreeding line Emka-1. The first three were kindly supplied by Instituto Sperimentale per la Cerealicoltura, Sant'Angelo Lodigiano, Italy, Suedwestdeutsche Saatzucht, Rastatt, Germany, and Institut Uprawy Nawozenia I Gleboznawstwa, Pulawy, Poland, respectively. Emka-1 was developed by the authors through self-pollination of Emka. Common buckwheat is sporophytically heteromorphic, consisting of the long-style type and the short-style type of flowers and self-incompatible. The genotype of diploid common buckwheat for the short style is heterozygous (Ss) and that for the long-style recessive homozygous (ss). Emka plants with the long-style type of flowers as female parent were crossed to Siva plants with the short-style type of flowers as male parent by means of rubbing newly dehisced anthers on the styles.
The resultant seeds and their parental seeds were sprouted by means of floating on 0.2% KNO3 solution in culture dishes at room temperature; they were then transplanted to pots of compost in a growth chamber at the Technical University of Munich. Morphology, mitosis, meiosis, pollen fertility, and grain set of these plants were observed. For meiotic investigation, young inflorescences of parents and hybrid plants were fixed in Carnoy's I solution and stained in 2% aceto-carmine solution. Some parameters of pollen mother cell (PMC) meiosis, such as the average numbers of univalents and micronuclei at telophase II (TII), and the rate of TII cells with micronuclei (%) were scored in Siva, Emka, their autotriploid hybrids, and Emka-1; the rate of TII cells with micronuclei was expressed as a percentage of the total number of TII cells observed. The pollen fertility was determined from the percentage of pollen grains that stained in 1% I2–KI solution.
The autotriploid hybrids obtained in the crosses between autotetraploid Emka and diploid Siva were backcrossed with diploid Sobano (with good agronomic characteristics) to produce trisomic plants and to study their inheritance. At the beginning of the flowering stage, autotriploid hybrids with long-style and short-stamen flowers were used as female parents in crosses to Sobano plants with short styles and long stamens. Autotriploid hybrids with short styles and long stamens were used as female parents in crosses with Sobano plants having long styles and short stamens. As flowers opened (before anther dehiscence), the short-style flowers chosen as female parents were emasculated. Some minutes later, these emasculated flowers were pollinated by rubbing on them the newly dehisced anthers of the male parent. For long-style flowers used as female parents, the emasculation is unnecessary, and they were pollinated directly. The procedure was repeated every day until the end of the flowering stage, when the seed set of all crosses was evaluated. The rate of normal seeds set and the rate of shriveled dead seeds set were recorded as percentages of the total number of flowers pollinated.
At the same time, the segregations of short-style and long-style plants were scored in backcross (BC)1 populations of diploid Sobano (long style) as female parent with trisomic plants (short style) from the cross diploid Sobano x (tetraploid Emka x diploid Siva). The negative crosses [(Emka x Siva) x Sobano]x Sobano were used for propagation of trisomics.
The chromosomes of the parents, the hybrids, and their progenies were also observed in mitosis of root tip cells following the method of Chen (1999a,b).
The Pearson correlation coefficient was used to evaluate the relationships among the rate of stained (fertile) pollen grains (x1), the average number of univalents in metaphase I (MI) cells (x2), and the average number of micronuclei in TII cells (x3). Chi-square (
2) analysis was used to compare observed and expected genotypic frequencies.
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RESULTS
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Seed Set and Pollen Grain Fertility of Autotriploid and Autotetraploid Common Buckwheat
More than 1000 flowers of Emka were crossed with Siva, and about 200 seeds were obtained. Among them, however, only three were hybrids. The autotriploid hybrids were weaker than Emka plants. Two were of the short-style type, and the remainder were of the long-style type. The proportion of fertile pollen grains in Emka (95.33%) and Emka-1 (93.33%) was close to that in diploid Siva (98.89%), indicating that male fertility in autotetraploid Emka and Emka-1 is only slightly decreased (Table 1
). In contrast, autotriploid hybrids had a much lower rate of fertile pollen grains (see Fig. 1
), only 31.28% (Table 1), indicating that the autotriploid hybrids have greatly decreased male fertility. Since male fertility was slightly lower in the inbreeding line Emka-1 than in Emka (Table 1), inbreeding may have caused an increase in abnormal meiosis in autotetraploid common buckwheat.
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Figure 1. Chromosomes and pollen grains of diploid and autotetraploid common buckwheat and progenies of their hybrids. 1. Diploid buckwheat ‘Sobano’: 2x = 16. 2. Autotetraploid buckwheat ‘Emka’: 4x = 32. 3. Autotriploid hybrid of the cross between Emka and ‘Siva’, 3x = 24. 4. Metaphase I (MI) of pollen mother cell (PMC) meiosis of diploid buckwheat Sobano: 2x = 8II. 5. MI of PMC meiosis of Emka-1: 4x = 8IV. 6: MI of PMC meiosis of the autotriploid hybrid between Emka and Siva: 3x = 6III + 2II + 2I. 7. Telophase II (TII) of PMC meiosis of the autotriploid hybrid, with two micronuclei. 8. Pollen grains of the autotriploid hybrid. 9. Chromosome number of trisomic Tr15, 2n + 1 = 17. 10. MI of PMC meiosis of trisomic Tr15: 2n + 1 = 7II +1III (arrow). 11. Anaphase I of PMC meiosis of trisomic Tr15 with segregation model of 8/9. bar = 2 µm, except in No. 8, where bar = 30 µm.
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In the autotriploid hybrids backcrossed with Sobano, shriveled dead seeds were produced by 27.72% of the flowers pollinated, and normal seeds by only 3.08% of the flowers, indicating that the fertility of pistils is 31.00% (= 27.72% + 3.08%). This is similar to the rate (31.28%) of fertile pollen grains of the autotriploid hybrids (Table 2
), but because of serious imbalance in the genetics and physiology, most of the backcrossed embryos in the autotriploid hybrids aborted, resulting in a much lower rate of normal seeds set.
Meiosis in Autotriploid Hybrids and Autotetraploid Common Buckwheat
The results of observation of mitosis and meiosis in parents, their hybrids, and progenies are given in Fig. 1 and Tables 3
and 4
. In MI of PMCs (Table 3), the average configurations of chromosome pairing in Emka and its inbreeding line Emka-1 are 4x = 0.67I + 11.60II + 0.32III + 1.79IV = 32 and 4x = 0.71I + 7.42II + 1.19III + 3.22IV = 32, respectively, and that in autotriploid hybrids 3x = 4.71I + 4.71II + 3.29III = 24. Average tetravalency was 1.79 in Emka and 3.22 in Emka-1, and the average multivalency was 2.11 in Emka and 4.41 in Emka-1. Hence, inbreeding results in an increase in the proportion of multivalents at meiosis MI of autotetraploid common buckwheat.
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Table 3. Average configuration of metaphase I (MI) cells of meiotic pollen mother cells (PMCs) in ‘Emka’, Emka-1, and autotriploid hybrids.
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Table 4. Frequency of tetrad micronuclei of autotriploid and autotetraploid common buckwheat at telophase II (TII) of pollen mother cell meiosis.
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The rates of TII tetrads without micronuclei were 98.02% in diploid Siva, 93.40% in autotetraploid Emka, 85.51% in Emka-1, and 42.37% in the autotriploid hybrids (Table 4), indicating that autotriploid hybrids have a much higher incidence of abnormal meiosis than the parent and that the inbreeding line Emka-1 has a higher rate of abnormal meiosis than autotetraploid Emka.
Relationship between Pollen Grain Fertility and Abnormal Meiosis
The average number of univalents in MI cells of PMC meiosis was 0.04 in diploid Siva, 0.67 in autotetraploid Emka, 0.71 in autotetraploid Emka-1, and 4.42 in autotriploid hybrids (Table 1), and the average number of micronuclei in TII cells of PMC meiosis was 0.03 in Siva, 0.09 in Emka, 0.20 in Emka-1, and 0.86 in autotriploid hybrids (Table 1). There was a significantly positive correlation (r23 = 0.994, p 
0.01) between the average number of univalents (x2) and the average number of micronuclei (x3), indicating that the number of univalents is an important factor in the formation of micronuclei at TII of meiosis. Significant negative correlation of rate of fertile pollen grains (x1) with the average number of univalents (x2) and with the average number of micronuclei (x3), with r12 = –0.996 and r13 = –0.993 (p 0.01), respectively, indicates that univalents and micronuclei can cause genetic and physiological imbalances that result in an increase in abortion of pollen grains.
Chromosome Number of Progenies of Autotriploid Hybrids
Cytological observations on the 34 progenies generated by crossing the autotriploid hybrids with diploid common buckwheat Sobano showed six double trisomics (2n + 2), 23 trisomics (2n + 1) and five diploid plants (2n). The trisomics were temporarily designated Tr1–23. The ratio of trisomics (2n + 1) to diploids, 4.6:1, agreed with the theoretical probability; according to binomial distribution, if p = 1/3 and q = 2/3, the rate of trisomics/diploid plants = (C87 p7q1 + C81 p1q7)/(C80 p0q8 + C88 p8q0) = 4.06:1. Figure 1 shows that trisomics of common buckwheat can form a configuration 7II + 1III or 8II + 1I and segregate as a 8/9 model in anaphase I (AI) of meiosis, indicating that the transmission of trisomics on common buckwheat is similar to that in other species (Ahmad and Hymowitz, 1994; Dong et al., 2001; Imanywoha et al., 1994; Li and Song, 1993; Schondelmaier and Jung, 1997; Zeller et al., 1977).
Segregation of Short-Style and Long-Style Plants in Progeny Populations of Trisomics
The segregations of short-style and long-style plants in BC1 populations of diploid Sobano as female parent with trisomic plants from the cross diploid Sobano x (tetraploid Emka x diploid Siva) (Table 5
) all fit a ratio of 1:1, except the cross Sobano (long style, ss) x Tr15 short style. This indicates that the gene for short-style and long-style characters is on the Tr15 trisomic chromosome.
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Table 5. Segregation of short-style and long-style plants in populations of the crosses ‘Sobano’ (long style, ss) x trisomics (short style, Ss, Sss, or SSs).
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DISCUSSION
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Genetic Stability of Autotetraploid Common Buckwheat
The results of this study support previous findings that the univalents in MI can increase the number of micronuclei at telophase I (TI) and TII of meiosis and decrease the rate of fertile pollen grains (Chen and Zhang, 1991; Loidl, 1995; Mendes-Bonato et al., 2002). Zhang and Chen (2000) reported that the average numbers of micronuclei in TII cells in diploid and autotetraploid common buckwheat are 0.04 and 0.08. Wu and Chen (2001) showed that the average numbers of micronuclei in TII cells in diploid and autotetraploid tartary buckwheat were 0.019 and 0.586. These reports indicate that artificial autotetraploid plants from pure lines of diploid tartary buckwheat derived by means of doubling chromosomes have more abnormality in meiosis than those from heterozygous diploid common buckwheat. The autotetraploid variety Emka in the present study had less abnormal meiosis than its inbreeding line Emka-1. The correlation analyses indicated that univalents are important in the formation of micronuclei at TII of meiosis and that univalents and micronuclei can cause genetic and physiological imbalances that lead to an increase in abortion of pollen grains. These results indicate that heterozygosity can decrease the abnormality of meiosis in autotetraploid buckwheat and thereby increase the chances of producing genetically stable and highly fertile autotetraploid lines by means of chromosome doubling of diploid plants.
Autotriploid and Trisomic Common Buckwheat
This is the first report of the successful development of autotriploid and trisomic common buckwheat plants and their cytological behavior. This may be an important breakthrough in studies of buckwheat genetics, since production of autotriploids and trisomics has hitherto proved very difficult in this species. The low grain set in the autotriploid common buckwheat is similar to those in autotriploid watermelon and autotriploid Musa spp. (Ortiz and Vuylsteke, 1995). It is clear that autotriploid plants can produce many pollen grains and female sex cells with chromosome number varying from n to 2n or more. Most are sterile because of serious imbalances in their genetic and physiological constituents. Imanywoha et al. (1994) reported that a population of 157 plants generated by crossing autotriploid wheatgrass (Agropyron cristatum) with the diploid comprised many types: 2n (58 plants), 2n + 1 (76 plants), 2n + 2 (17 plants), 2n + 3 (4 plants), 2n + 4 (1 plant) and 2n +1t (1 plant). In soybean also, there was a range of ploidy (2n, 2n + 1, 2n + 2, etc.) in progenies generated by crossing an autotriploid with the diploid (Hymowitz et al., 1991; Ahmad and Hymowitz, 1994). The results with wheatgrass and soybean differ from those of the present study, in which the number of trisomics was 4.6 times higher than the number of diploid plants in progenies of autotriploid common buckwheat backcrossed to the diploid. Because of the small chromosomes, their similarity in size, and the heterozygous genetic background, identification of the trisomics of common buckwheat is difficult.
Chromosomal Location of Style Character of Common Buckwheat
This study located the S locus for length of style on the trisomic chromosome of Tr15. A recent report of Fesenko et al. (2006) inferred that the S locus in common buckwheat (F. esculentum) in the present paper may be at the same position of the S4 homostyly of the wild species F. homotropicum. A preliminary study (T. Wang and Q.-F. Chen, unpublished data) of chromosomes of common buckwheat by fluorescence in situ hybridization (FISH) has shown that Tr15 is trisomic for chromosome 4E. This is the first gene in common buckwheat to have been associated to its chromosome by means of trisomic lines.
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ACKNOWLEDGMENTS
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The authors are grateful to the Alexander von Humboldt Foundation for a research fellowship and to the Natural Science Foundation of China (30270852, 30471116), Program for New Century Excellent Talents in University (NCET-2004-0913), Guizhou Key International Cooperation Project (Qianke Hewai G Zi 2005#400108), and Mega-projects of Science Research for the 11th Five-Year Plan of China (2006BAD02B06) for providing funds.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication March 7, 2007.
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ABSTRACT
INTRODUCTION
