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CROP BREEDING & GENETICS

Genotype Dependent Interspecific Hybridization of Sorghum bicolor

^a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77840-2474
^b USDA-ARS, Crop Germplasm Research Unit, 430 Heep Center, Texas A&M Univ., College Station, TX 77843-2474
^c Australian Tropical Crops and Forages Collection, Queensland Dep. of Primary Industries and Fisheries, Biloela, QLD, Australia

^* Corresponding author (g-hodnett{at}tamu.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Wild Australian Sorghum species are a tertiary gene pool to grain sorghum [Sorghum bicolor (L.) Moench], and they are of interest to breeders because of their resistance to important insects and pathogens. However, strong reproductive barriers have prevented hybridization between S. bicolor and these wild species. The purpose of this study was to determine if the recessive iap allele (dominant allele Iap = inhibition of alien pollen) would reduce or eliminate the pollen–pistil incompatibilities that prevent hybridization between S. bicolor and divergent Sorghum species. Cytoplasmic male-sterile S. bicolor plants, homozygous for the iap allele, were pollinated with three divergent Sorghum species, S. angustum Blake, S. nitidum (Vahl) Pers., and S. macrospermum Garber. The pollen of these three wild species readily germinated and the pollen tubes grew to the base of the S. bicolor ovary within 2 h after pollination. Hybrid embryos were detected in the S. bicolor florets 13 to 20 d post-pollination. Sorghum bicolor x S. angustum and S. bicolor x S. nitidum hybrids were obtained using embryo rescue followed by in vitro culture techniques and hybrids between S. bicolor and S. macrospermum were obtained by simply germinating the hybrid seed. These hybrids were confirmed by their morphological and cytological traits. These findings clearly demonstrate that the recessive iap allele circumvents pollen–pistil incompatibilities in the genus Sorghum and permits hybrids to be made between S. bicolor and species of the tertiary gene pool.

Abbreviations: Iap, inhibition of alien pollen

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
THE GENUS SORGHUM consists of 25 species that are classified into five taxonomic subgenera or sections, Eu-sorghum, Chaetosorghum, Heterosorghum, Para-sorghum, and Stiposorghum (Garber, 1950; de Wet, 1978; Lazarides et al., 1991). Sorghum belongs to the Eu-sorghum section. Sorghum breeders have utilized divergent races of S. bicolor as the primary gene pool for genetic improvement (Duncan et al., 1991; Rosenow and Dahlberg, 2000). The secondary gene pool is composed of the other Sorghum species within the section Eu-sorghum and breeders have accessed genes from these species by introgression. Nineteen Sorghum species belonging to sections other than Eu-sorghum, distributed primarily in Australia, southern Asia, and Africa, comprise an untapped tertiary gene pool. The wild Australian Sorghum species are of particular interest to plant breeders because some of them are resistant to important insects and pathogens including midge [Stenodiplosis (Contarinia) sorghicola (Coquillett)] and sorghum downy mildew [caused by Peronosclerospora sorghi (Weston and Uppal) Shaw] (Hacker et al., 1992; Franzmann and Hardy, 1996; Sharma and Franzmann, 2001; Kamala et al., 2002). However, sorghum breeders have not been able to use this tertiary gene pool because S. bicolor would not hybridize with these species (Garber, 1950; Schertz and Dalton, 1980; Doggett, 1988).

Hodnett et al. (2005) reported that pollen–pistil interactions are the primary reasons why S. bicolor will not hybridize with divergent Sorghum species. They determined that the pollen tubes of 14 alien Sorghum species behaved abnormally in the pistils of S. bicolor. For most of the alien species, pollen tubes did not grow beyond the stigma. However, a very small number of tubes of three species, S. ecarinatum Lazarides, S. macrospermum, and S. matarankense Garber & Snyder, grew into the S. bicolor ovary, but fertilization and subsequent embryo development were not common. When fertilization and embryo development did occur, the endosperm deteriorated which resulted in embryo death (Hodnett et al., 2005). Only one embryo was observed in 1237 ovaries dissected from florets of cytoplasmic male-sterile S. bicolor that had been pollinated by S. macrospermum (Hodnett et al., 2005). This embryo was rescued and cultured in vitro. The resultant seedling was verified as a hybrid based on its cytological and morphological characteristics (Price et al., 2005a).

The rescue and culture of rare hybrid embryos are possible for only a few crosses between S. bicolor and species in sections other than Eu-sorghum. Therefore, approaches to increase the frequency of hybridization are needed. Since pollen–pistil incompatibilities in grasses may be genetic in nature (Riley and Chapman, 1967; Lange and Wojciechowska, 1976; Sitch and Snape, 1986; Snape et al., 1979), one method to overcome barriers to hybridization is to screen S. bicolor germplasm for accessions that will hybridize with the wild species. Using this approach, Laurie and Bennett (1989) screened S. bicolor accessions using maize (Zea mays L.) pollen. They discovered a S. bicolor accession (Nr481) from China in which the maize pollen germinated and pollen tubes grew into the styles of some of the plants. When the dominant allele of the gene (Iap) was present, growth of maize pollen tubes into S. bicolor styles was inhibited. However, when the recessive allele, iap was present in the homozygous state, the maize pollen tubes grew through the S. bicolor pistils. The objective of this study was to determine if the recessive iap allele will reduce or eliminate the pollen–pistil incompatibilities that prevent hybridization between S. bicolor and three widely divergent Sorghum species, S. angustum, S. macrospermum, and S. nitidum, which represent different taxonomic sections.

	MATERIALS AND METHODS

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Plant Materials
All plants were grown in greenhouses at College Station, TX. Male-sterile S. bicolor plants (2n = 2x = 20) with the iap allele were derived from progeny of the backcross [DL1325 (cytoplasmic male-sterile) x S. bicolor Nr481] x S. bicolor Nr481 (Laurie and Bennett, 1989). These plants were used as the female parent in crosses with S. angustum (2n = 2x = 10), S. nitidum (2n = 4x = 20), and S. macrospermum (2n = 4x = 40) (Table 1).

The length of time required for S. bicolor pollen grains to germinate and the pollen tubes to grow through a S. bicolor ovary is less than 1 h (Hodnett et al., 2005). To account for possible slower growth of alien pollen tubes through S. bicolor pistils, alien pollen tubes in S. bicolor pistils (with and without the iap allele) were observed at 24 h post-pollination. Data regarding pollen germination and tube growth in the stigma branches, stigma axes, styles, and ovaries were statistically analyzed in a completely randomized design using PC SAS (SAS Institute, 2002). Data from individual pistils were used for replications. Since different numbers of pistils were evaluated for each sample, a general linear model was used and differences among means were detected using a Fisher protected LSD (Steel and Torrie, 1980).

Recovery of Hybrid Plants
Embryos from crosses between cytoplasmic male sterile S. bicolor, iap iap x S. angustum and S. nitidum, were excised from the ovaries 13 to 20 d after pollination. These rescued embryos were placed in petri dishes on a culture medium that consisted of Murashige–Skoog (Murashige and Skoog, 1962) basal salts and vitamins supplemented with 10 mg L^–1 glycine, 10 mg L^–1 L-arginine-HCl, 10 mg L^–1 L-tyrosine, 100 mg L^–1 inositol, and 50 g L^–1 sucrose, solidified with 0.7% agar (plant tissue culture grade, Phytotechnology Laboratories, Shawnee Mission, KS) (Sharma, 1999). The dishes were placed into an environmental chamber set to 16 h light/8 h dark at 24°C. After the embryos germinated and several leaves had developed, seedlings were transplanted into soil in pots and grown in a greenhouse. Hybrids between S. bicolor, iap iap, and S. macrospermum were obtained by simply germinating the seeds.

Chromosome Analysis
Chromosome spreads were prepared using a modification of the protocol reported by Jewell and Islam-Faridi (1994). Actively growing roots were treated with a saturated aqueous solution of -bromonaphthalene for 1.75 h at room temperature, fixed in 95% ethanol/glacial acetic acid (4:1 v/v), rinsed several times with distilled water, hydrolyzed for 5 min in 0.2 M HCl, and rinsed 10 min with distilled water. Cell walls were digested with an aqueous solution of 5% cellulase (Onozuka R-10, Yakult Honsha Co. Ltd., Tokyo) and 1.0% pectolyase Y-23 (Seishin Corporation, Tokyo) at pH 4.5 for 15 to 25 min at 37°C and rinsed three times with distilled water. Meristems were placed on a clean glass slide in an ethanol/glacial acetic acid (3:1) solution, macerated, and spread with fine-tipped forceps, air-dried at room temperature for 2 d, and stained with Azure Blue.

	RESULTS

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Interspecific Pollinations of S. bicolor (Iap Iap)
The mean germination of S. angustum, S. nitidum, and S. macrospermum pollen when placed on S. bicolor, Iap Iap, stigmas was significantly lower than that of the control, S. bicolor pollen on S. bicolor, Iap Iap, stigmas (Table 2). The mean number of alien pollen tubes that grew through the stigma, style, and ovary of S. bicolor, Iap Iap, was significantly less than for the control (Table 2). Among the alien species, significant differences were observed in the growth of their pollen tubes in S. bicolor pistils (Table 2). For S. nitidum, tubes were not observed beyond the stigma axes of S. bicolor, Iap Iap. Sorghum angustum tubes grew well into the stigma branches but few grew through the stigma axes and into the style of S. bicolor, Iap Iap. Only 0.04% of the tubes were observed in the style and none reached the ovary. Pollen tubes of S. macrospermum readily grew into the stigma axes of S. bicolor, Iap Iap, but less than 2.5% of them grew into the style and rarely were they observed in the ovary (Table 2).

View larger version (62K): [in this window] [in a new window]   Fig. 1. Germination of S. nitidum pollen on S. bicolor stigmas and tube growth 24 h after pollination in pistils of S. bicolor genotypes with and without the iap allele. (A) Sorghum nitidum pollen on a stigma of S. bicolor, Iap Iap. Pollen germinated but the pollen tubes seldom grew beyond the stigma branches. (B and C) Sorghum nitidum pollen tubes have grown through the stigma, style, and into the ovary of S. bicolor genotype, iap iap. Scale bars = 1 µm.

Recovery and Identification of Interspecific Hybrids
Following interspecific pollination, the frequency of S. bicolor, iap iap, ovules with an embryo was usually between 10 and 19% (Table 4). Because the seed aborted in these interspecific crosses with S. bicolor (with exception of S. macrospermum) following breakdown of the endosperm, the embryos were rescued and cultured in vitro. The excised hybrid embryos grown under these culture conditions frequently produced various amounts of a reddish-brown non-anthocyanin precipitate. This precipitate often encircled the embryo and killed it. The frequency of hybrids recovered via embryo culture was 20.7 and 4.5% for the S. bicolor x S. angustum and S. bicolor x S. nitidum crosses, respectively (Table 4). Embryo rescue and culture were not required to recover plants from the S. bicolor x S. macrospermum crosses, where 60% of the sampled seeds germinated (Table 4).

View larger version (75K): [in this window] [in a new window]   Fig. 2. Somatic chromosomes of hybrids between S. bicolor (2n = 2x = 20) and S. angustum (2n = 2x = 10), S. bicolor and S. nitidum (2n = 4x = 20), and S. bicolor and S. macrospermum (2n = 4x = 40). (A) Chromosomes of a hybrid between S. bicolor and S. angustum consisting of five large chromosomes from S. angustum and 10 small chromosomes from S. bicolor. (B) Chromosomes of a S. bicolor x S. nitidum hybrid with 10 large chromosomes from S. nitidum and 10 small chromosomes from S. bicolor. (C) Chromosomes of a S. bicolor x S. macrospermum hybrid with 20 from S. macrospermum and 10 from S. bicolor. The S. bicolor chromosomes are smaller than most of the S. macrospermum chromosomes. Upper arrow shows two chromosomes. Lower arrow shows a chromosome in which the centromere is not fully condensed and appears as a strand. Scale bars = 5 µm.

	DISCUSSION

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Laurie and Bennett (1989) discovered a S. bicolor accession (Nr481) in which maize pollen tubes were able to grow through its pistils. A genetic analysis revealed that maize pollen tubes grew through the S. bicolor pistils when the S. bicolor plant was homozygous for a recessive allele iap (Laurie and Bennett, 1989). It remains to be determined if the iap allele is evolutionarily related to the Kr₁ and Kr₂ alleles of wheat.

The iap allele dramatically increases our ability to hybridize divergent Sorghum species with S. bicolor. This is demonstrated from our efforts to cross S. bicolor with S. macrospermum, S. angustum, and S. nitidum. When S. angustum and S. nitidum were crossed onto S. bicolor, Iap Iap, hybrid embryos were never formed because their pollen tubes failed to grow into the S. bicolor ovaries, and the S. nitidum tubes never grew beyond the stigma axes (Table 3; Hodnett et al., 2005). However, when the S. bicolor parent was homozygous for the iap allele, pollen tubes of these two alien species grew rapidly through the stigmas, styles, and ovaries (Table 3) and into the ovules, where fertilization occurred and embryos were produced and hybrids were eventually recovered (Table 4). In the case of S. macrospermum, a limited number of its pollen tubes grew into the ovaries of S. bicolor, Iap Iap, plants not homozygous for the iap allele (Tables 2 and 3), but embryo formation was rare. Hodnett et al. (2005) reported only one embryo from 1237 (0.08%) S. bicolor, Iap Iap, pistils pollinated with S. macrospermum pollen. In the present study, when a S. bicolor plant, homozygous for the iap allele, was pollinated with S. macrospermum pollen, about 10% of the pistils formed seeds and 60% of these germinated (Table 4). Vigorous S. bicolor x S. macrospermum hybrids were readily obtained without embryo rescue and were identified by their intermediate morphology.

The lack of vigor in the S. bicolor x S. angustum and S. bicolor x S. nitidum hybrid seedlings may be related to culture conditions. The presence of the reddish-brown non-anthocyanin precipitate surrounding the cultured embryos and the formation of extensive anthocyanin in the hybrid seedlings' leaves may be an indication of stress, because the presence of anthocyanin in S. bicolor is often stress related. Improvements in the composition of the embryo culture medium and in light and temperature regimes may result in more vigorous hybrid plants. However, it is more likely that weakness of the hybrids is due to genetic incompatibility between the S. bicolor genome and those of S. angustum and S. nitidum.

The range of species that will hybridize with S. bicolor, iap iap, is not known. In the current study, hybrids were recovered from crosses between S.bicolor, iap iap, and S. angustum, S. nitidum, and S. macrospermum (Table 1). These three species were chosen because they represent taxonomic and phylogenetic divergence in the genus based on a combined ITS/ndhF sequence comparison (Dillon et al., 2004) and their chromosome size and numbers (Price et al., 2005b). Sorghum macrospermum (2n = 4x = 40), a member of the Chaetosorghum section, is considered to be one of two species outside the Eu-sorghum section that is most closely related to S. bicolor (Dillon et al., 2004). It has relatively small chromosomes with a base chromosome number of x = 10. Sorghum nitidum (2n = 4x = 20), a member of the Para-sorghum section, is basal in a lineage with relatively large chromosomes and a base chromosome number of x = 5. Sorghum angustum (2n = 10), a member of the Stiposorghum section, is a derived species within the same lineage as S. nitidum (Dillon et al., 2004). It also has large chromosomes and a base chromosome number of x = 5. In addition to the three species mentioned above, we have recovered hybrid embryos from crosses between S. bicolor, iap iap, and 11 other diverse Sorghum species. Detailed studies of these crosses and the resultant hybrids have not been completed. However, it appears that hybrids can be recovered from crosses between S. bicolor and most, if not all, of the Sorghum species of the Para-sorghum, Stiposorghum, Chaetosorghum, and Heterosorghum sections, if the S. bicolor female parent is homozygous for the iap allele. It has not been determined if hybrids can be produced between S. bicolor, iap iap, and species of other grass genera and, if so, how divergent a species can be and still produce viable hybrids.

The recovery of interspecific Sorghum hybrids is important to the success of long-term sorghum improvement programs. Collectively, species classified in sections other than Eu-sorghum possess many useful traits that have not been available for sorghum improvement (Hacker et al., 1992). The production of these interspecific hybrids represents the first step in transferring genes from the tertiary gene pool to the primary gene pool. While considerably more research is needed to determine optimal methods for inducing fertility and achieving introgression between the genomes, there is real potential to extract new genes for resistance to several biotic stresses as well as abiotic stresses such as drought resistance and heat tolerance which would not be possible without the iap allele.

	ACKNOWLEDGMENTS

 
We thank Dr. David Laurie for providing seed of S. bicolor Nr481 and seed from the backcross [S. bicolor CMS DL132/Nr481] x Nr481.

	NOTES

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The first and second authors contributed equally to this research. Research supported by the Texas Agricultural Experiment Station, the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant no. 2004-35300-14686, and the USDA Sorghum Crop Germplasm Committee.

Received for publication September 1, 2005.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Price, H. J. Articles by Rooney, W. L. Search for Related Content PubMed Articles by Price, H. J. Articles by Rooney, W. L. Agricola Articles by Price, H. J. Articles by Rooney, W. L.