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Published online 27 May 2005
Published in Crop Sci 45:1403-1409 (2005)
© 2005 Crop Science Society of America
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CROP BREEDING, GENETICS & CYTOLOGY

Pollen–Pistil Interactions Result in Reproductive Isolation between Sorghum bicolor and Divergent Sorghum Species

George L. Hodnetta, Byron L. Bursonb, William L. Rooneya, Sally L. Dillonc and H. James Pricea,*

a Dep. of Soil & Crop Sciences, Texas A&M Univ., College Station, TX
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 (hj-price{at}tamu.edu)


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Sorghum [Sorghum bicolor (L.) Moench] breeders have long recognized the importance of exotic germplasm and noncultivated sorghum races as sources of valuable genes for genetic improvement. The genus Sorghum consists of 25 species classified as five sections: Eu-sorghum, Chaetosorghum, Heterosorghum, Para-sorghum, and Stiposorghum. Species outside the Eu-sorghum section are sources of important genes for sorghum improvement, including those for insect and disease resistance, but these have not been used because of the failure of these species to cross with sorghum. An understanding of the biological nature of the incompatibility system(s) that prevent hybridization and/or seed development is necessary for the successful hybridization and introgression between sorghum and divergent Sorghum species. The objectives of this study were to determine the reason(s) for reproductive isolation between Sorghum species. The current study utilized 14 alien Sorghum species and established that pollen–pistil incompatibilities are the primary reasons that hybrids with sorghum are not obtained. The alien pollen tubes showed major inhibition of growth in sorghum pistils and seldom grew beyond the stigma. Pollen tubes of only three species grew into the ovary of sorghum. Fertilization and subsequent embryo development were not common. Seeds with developing embryos aborted before maturation, apparently because of breakdown of the endosperm.

Abbreviations: EBN, endosperm balance number


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
SORGHUM is a major cereal crop of marginal rainfall areas of the tropics, with selected varieties widely grown in temperate regions. Recent annual grain sorghum yields have exceeded 61000000 Mg worldwide, with 13207000 Mg produced in the USA (Smith, 2000).

Twenty-five species form the genus Sorghum (Lazarides et al., 1991) which consists of five subgenera or sections, Eu-sorghum, Chaetosorghum, Heterosorghum, Parasorghum, and Stiposorghum (Garber, 1950; deWet, 1978). Members of the Eu-sorghum section have a natural range through Africa and southern Asia. The cultivated sorghum (S. bicolor) and its subspecies drummondii and arundinaceum, as well as the wild species S. almum Parodi, S. propinquum (Kunth) Hitchc., and Johnsongrass [S. halepense (L.) Pers.] are in the Eu-sorghum section (deWet, 1978). Chaetosorghum and Heterosorghum are monotypic sections that are native to the Australo-Pacific region; whereas, the Para-sorghum section consists of seven Asian, Australian, and central American species (Lazarides et al., 1991). Ten species that occur in northern Australia comprise the Stiposorghum section (Lazarides et al., 1991).

Sorghum breeders have long recognized the importance of exotic germplasm (Duncan et al., 1991). Noncultivated sorghum races have been extensively used as sources of genes for sorghum improvement (Rosenow and Dahlberg, 2000). However, no species outside the eu-sorghum section have been utilized because of strong reproductive barriers (Garber, 1950; Schertz and Dalton, 1980; Doggett, 1988). Resistance to major insects and diseases, for example, midge [Stenodiplosis (Contarinia) sorghicola (Coquillett)] and downy mildew [caused by Peronosclerospora sorghi (Weston and Uppal) Shaw], that attack sorghum has been found in species of the Chaetosorghum, Heterosorghum, Para-sorghum, and Stiposorghum sections (Franzmann and Hardy, 1996; Sharma and Franzmann, 2001; Kamala et al., 2002). These species are potential sources of resistance genes (Hacker et al., 1992).

A prerequisite for using wild species as germplasm is successful hybridization and backcrossing. It is apparent that the production of hybrids between sorghum and diverse Sorghum species will require an understanding of the biological nature of the incompatibility system(s) that prevent hybridization and/or seed development. Reproductive barriers occur at both the prezygotic or postzygotic levels. Prezygotic mechanisms may involve the failure of pollen germination, pollen tube growth, and/or fertilization. Postzygotic mechanisms include embryo lethality due to genotypic interactions or embryo death following abortion of the endosperm. There is little reported in the literature characterizing reproductive barriers between sorghum and wild Sorghum species. Sun et al. (1991) studied pollen tube growth in reciprocal pollinations between sorghum and S. versicolor Anderss. They determined that the primary reason for reproductive isolation in reciprocal crosses between the two species was the inhibition of pollen tube growth.

If hybrids can be produced between sorghum and wild species outside the Eu-sorghum section, additional barriers to introgression may occur. The base chromosome number of species in the Para-sorghum and Stiposorghum sections is x = 5; whereas, in the Eu-sorghum section, including sorghum, it is x = 10. Species belonging to the Para-sorghum and Stiposorghum sections have larger chromosomes than those of sorghum, and their genome size is up to 2.5 times larger than sorghum (Price et al., 2005). Hybrids between sorghum and species of the Para-sorghum and Stiposorghum sections would likely be sterile because of reduced or lack of chromosome pairing and irregular chromosome segregation during meiosis involving members of the x = 5 and x = 10 genomes. The monotypic sections Heterosorghum (S. laxiflorum Bailey) and Chaetosorghum (S. macrospermum Garber) are more closely related to Eu-sorghum, based on morphology (Garber, 1950), phylogenetic affinity (Dillon et al., 2004), karyotype (Wu, 1990, 1993), and genome size (Price et al., 2005). Although they are apparently polyploid (2n = 40), the karyotypic similarities suggest that chromosome pairing would be more likely during meiosis in hybrids between sorghum and both S. laxiflorum and S. macrospermum than in hybrids between sorghum and species with a base number of x = 5. Therefore, S. laxiflorum and S. macrospermum may be the most promising for introgression into sorghum.

The objectives of this research were to observe pollen germination and tube growth of divergent Sorghum species in sorghum pistils to determine if pistil–pollen interactions are reproductive barriers to producing interspecific hybrids.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Plant Materials
Accession, herbarium voucher, and chromosome numbers of the 14 wild Sorghum species are listed in Table 1. The cultivated sorghum used was a paired inbred line (BTx623 and ATx623) with subtropical adaptation that is widely used as the female parent in the production of commercial hybrids. The ATx623 plants are male sterile; whereas, the BTx623 plants are male fertile. All plants were grown in greenhouses at College Station, TX, and were maintained at a temperature from 20 to 33°C. The plants flowered from January through May without any supplemental light.


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  		Table 1. Fifteen Sorghum species used in this study.

 
Pollination Techniques
To determine if pollen–pistil interactions contribute to the reproductive isolation between sorghum and Sorghum species classified in sections other than Eu-sorghum, pollen from one plant of each accession of 14 different Sorghum species (Table 1) was transferred to receptive sorghum stigmas. Pollen was collected at anthesis and brushed onto stigmas of the cytoplasmic male-sterile sorghum line, ATx623. To prevent contamination, the male-sterile sorghum plants were grown in a different greenhouse from the male-fertile plants.

Fixation and Storage of Inflorescences
Inflorescences were collected at intervals after pollination and fixed in either FAA (18:1:1, 70% ethanol/glacial acetic acid/formaldehyde) or 3:1, 95% ethanol/glacial acetic acid for 12 to 18 h. Initially FAA was used but slight discoloration of the pistils occurred. Discoloration did not occur in tissues fixed in 3:1, 95% ethanol/glacial acetic acid and, therefore, it was the preferred fixative. After fixation, the pistils were excised from the inflorescences and stored in 70% ethanol at –20°C until examined.

Quantification of Pollen Germination and Pollen Tube Growth
Pistils were processed using a slightly modified version of the protocol described by Kho and Baer (1968). Pistils were cleared and softened in 0.8 M NaOH overnight, stained with 0.025% (w/v) aniline blue in 0.1 M K2PO4 for approximately 30 min, and mounted on microscope slides in 50% 0.1 M K2PO4 and 50% glycerol. The slides were kept in the dark until observed with a Zeiss Universal II microscope (Carl Zeiss Inc., Gottingen, Germany). Callose, a ß–1, 3-polyglucan, occurs in pollen tubes, and when exposed to aniline blue stain, it fluoresces under 350- to 400-nm light (Martin, 1959; Dumas and Knox, 1983). Fluorescence was induced using 390- to 420-nm light filtered from a mercury lamp with a 450-nm emission filter. Images were captured with an Optronics VI-470 system (Optronics Inc., Goleta, CA) and digitally stored and processed with Optimas (v. 6.1) image analysis software (Optimas Corp., Bothell, WA).

For a control, pollen from the male-fertile sorghum line, BTx623, was transferred onto stigmas of its cytoplasmic male-sterile counterpart, ATx623. Pollen germination and pollen tube growth were observed at 45 min and 1 h following pollination. Germination of alien pollen and the extent of tube growth on sorghum pistils were initially recorded for pollinations involving all species at 24 h after pollination. Statistical analysis of pollen germination and pollen tube growth into the stigma branches, stigma axis, style, and ovary was completed in a completely randomized design using data from individual pistils for replication. Because different numbers of pistils were evaluated for each species, a general linear model was used and differences among means were detected using a Fisher protected LSD (Steel and Torrie, 1980). A subset consisting of S. angustum S.T. Blake, S. ecarinatum Lazarides, S. macrospermum, S. matarankense Garber & Snyder, and S. purpureo-sericeum (A. Rich.) Aschers & Schweinf., was used to further characterize the progress of pollen tube growth at selected earlier intervals between 2 and 12 h. These were chosen because pollen tubes of these five species had grown into the style or ovary of S. bicolor by 24 h.

Detection of Fertilization and Embryo Development
Pollen from S. ecarinatum, S. macrospermum, and S. matarankense were individually dusted on stigmas of male-sterile sorghum ATx623. On Day 15 post-pollination, florets were dissected and examined for embryo formation using a dissecting microscope.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Pollen Germination and Pollen Tube Growth in S. bicolor
More than 98% of the S. bicolor BTx623 pollen germinated when placed on stigmas of ATx623. The pollen tubes rapidly entered the stigma branches and within 45 min post-pollination they had grown into the stigma branches, axis of the stigma, and the style, respectively, but not into the ovary. Pollen tubes had grown through the ovary within 1 h after pollination (Table 2). Figures 1A and 1B show pollen tubes of BTx623 in the stigmas and ovary, respectively, of ATx623 at 1 h after pollination.


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  		Table 2. Pollen germination and tube growth in Sorghum bicolor (ATx623) pistils following pollination with alien Sorghum species.

 


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  		Fig. 1. Growth of sorghum and alien species' pollen tubes in sorghum pistils stained with aniline blue and observed with fluorescent microscopy. (A) Sorghum pollen tubes in stigmas at 1 h post-pollination; (B) Sorghum pollen tubes that have grown into the ovary at 1 h post-pollination; (C) Germinated pollen grain of S. interjectum with a pollen tube that has not penetrated the stigma branch at 24 h post-pollination; (D) A germinated pollen grain of S. nitidum that grew in a convoluted fashion without entering the stigma branch at 24 h post-pollination; (E) Pollen tubes of S. ecarinatum in the stigma and style at 6 h post-pollination; (F) Stigma branches with an ungerminated pollen grain and a pollen tube of S. interjectum in the stigma branch at 24 h post-pollination; (G) Sorghum ecarinatum pollen tubes in the style and ovary at 24 h post-pollination; (H) Pollen tube of S. plumosum growing outside and parallel to the stigma branch; (I) Pollen tube of S. intrans that has grown out of and back into a stigma branch; (J) A pollen tube of S. interjectum in the stigma axis that reversed its direction and grew toward the apex of the stigma; (K) Pollen tube of S. purpureo-sericeum with a swollen tip; (L) Pollen tube of S. timorense terminating at accumulated callose in the stigma axis. Scale bars are 100 µm, except when designated otherwise.

 
Alien Pollen Germination and Tube Growth in Sorghum Pistils
Pollen germination and tube growth of 14 different Sorghum species in sorghum ATx623 pistils were first examined at 24 h post-pollination (Table 2). Pollen of all species showed a numerical reduction in germination on sorghum stigmas relative to the control, but germination was significantly lower only for those pollinations involving S. bulbosum Lazarides and S. leiocladum (Hack.) C.E. Hubb. Pollen of S. leiocladum displayed an exceptionally low germination (5.4%). Because S. leiocladum produced a limited number of inflorescences and a small amount of pollen under the greenhouse conditions at College Station, lack of available pollen prevented further studies using this species.

There was a significant inhibition in the growth of pollen tubes from alien Sorghum pollen on sorghum stigmas, relative to the controls (Table 2). Figure 1F shows an ungerminated pollen grain of S. interjectum adhering to a stigma branch of sorghum. Following germination, inhibition of pollen growth can occur at any time before entering the micropyle at the base of the ovary and the subsequent fertilization of the egg cell and central cell. In this study, the pollen tubes ceased growing: (i) before entering the stigma branch (Fig. 1C); (ii) in the stigma branch (Fig. 1E, 1F); (iii) in the stigma axis (Fig. 1E); (iv) in the style (Fig. 1G); or (v) in the ovary (Fig. 1G). Pollen tubes of most Sorghum species grew no further than the axis of the stigma (Table 2). Pollen tubes of six species [S. angustum, S. ecarinatum, S. macrospermum, S. matarankense, S. plumosum (R. Br.) P. Beauv., and S. purpureo-sericeum] grew into the style of sorghum. Pollen tubes of only three species (S. ecarinatum, S. macrospermum, and S. matarankense) were observed in the ovary of sorghum at 24 h (Table 2).

Sorghum (BTx623) pollen tubes grew relatively straight and parallel to the axis in sorghum (ATx623) styles (Fig. 1A, 1B), whereas alien pollen tubes typically grew down the style in a crooked manner (Fig. 1E, 1G). In addition to growth inhibition and crooked growth paths, several other aberrant forms of alien pollen tube growth were observed in sorghum pistils that were not observed in the control. These included: (i) pollen tube growth in a convoluted fashion without entering a stigma branch (Fig. 1D); (ii) tubes growing parallel to the stigma branch without entering the stigma (Fig. 1H); (iii) tubes growing out of and then re-entering the stigma branch (Fig. 1I); (iv) tubes that grew toward the apex of the stigma (Fig. 1J); (v) swelling of the tip of the tube (Fig. 1K); and (vi) tube growth terminating at callose in the stigma axis (Fig. 1L). For S. amplum, S. angustum, S. brachypodum Lazarides, S. intrans F. Muell. ex Benth., S. nitidum (Vahl.) Pers., S. plumosum, S. purpureo-sericeum, and S. timorense (Kunth) Buse, the frequency of aberrant tube growth in sorghum pistils exceeded 15%. The most common aberrations were swollen pollen tube tips and tube growth terminating at callose deposited in the stigma axis.

Five of the six alien Sorghum species in which the pollen tubes had grown into sorghum styles at 24 h after pollination were selected to analyze pollen tube growth at intervals less than 24 h post-pollination (Table 3). These were S. angustum, S. ecarinatum, S. macrospermum, S. matarankense, and S. purpureo-sericeum. The sixth species, S. plumosum, was not flowering at the time this experiment was conducted. For these species, pollen tubes had penetrated the style 2 h after pollination, but only between 0.1% and 3.5% of the tubes were observed in the style after 24 h. Less than 0.6% of the tubes of S. ecarinatum, S. macrospermum, and S. matarankense were observed in the sorghum ovary. The data in Table 3 indicate that most pollen tubes of S. ecarinatum and S. macrospermum that reach the sorghum ovary have done so within 2 to 6 h after pollination. However, for S. matarankense, a higher percentage of pollen tubes were observed in the sorghum ovary at 24 h than at 2, 4, and 6 h, thus suggesting that some slower growing pollen tubes of this species continue to reach the ovary over the 24 h period.


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  		Table 3. Pollen germination and tube growth of five alien Sorghum species in sorghum ATx623 pistils at progressive intervals post-pollination.

 
Detection of Fertilization and Embryo Development
The frequency of fertilization, as measured by embryo formation, was determined for the three species in Table 3 that had pollen tubes penetrating the sorghum ovary. Sorghum florets that had been pollinated with S. ecarinatum, S. macrospermum, and S. matarankense pollen were dissected from inflorescences 15 d after pollination. From these pollinations, the frequency of seed with immature embryos was 10/1119, 1/1237, and 13/533 for S. ecarinatum, S. macrospermum, and S. matarankense, respectively. However, the endosperm aborted in all hybrid seed produced, which resulted in seed failure.


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
Interspecific hybridization and introgression is a means to tap genes of agronomic importance for crop improvement programs. However, reproductive isolation barriers often exist between divergent relatives and crop species that render introgression difficult. These barriers may affect any part of the plant's reproductive cycle including lack of fertilization, endosperm failure, embryo abortion, seedling lethality, hybrid sterility, and hybrid breakdown.

A very common prezygotic reproductive barrier results from pollen–pistil incompatibility, where growth of pollen tubes from one species is inhibited in the stigma of another species. There is currently considerable interest regarding the physiology and molecular biology of pollination and fertilization in plants (Franklin-Tong, 2002; Lord, 2003). The poorly understood events starting at pollination and terminating at fertilization involve complex and harmonious interactions between the microgametophyte and the pistil. Signaling occurs between the microgametophyte and the cells and the extracellular matrix of the pistil. Pollen tubes are guided to the micropyle by signals originating in the style and embryo sac (Lord and Russell, 2002). Adverse pistil–pollen interactions that include inhibition of tube growth following interspecific pollinations involving sorghum conceptually may be viewed as the consequence of inharmonious genetic interactions due to genetic divergence among the species.

When fertilization and embryo development do occur in sorghum interspecific crosses, the seed abort due to early breakdown of the endosperm. Endosperm breakdown in interspecific crosses is a common form of post-zygotic reproductive isolation. Whether or not endosperm develops normally in seed from interploidy intraspecific and interspecific crosses has been proposed to be due to the endosperm balance number (EBN) which determines the effective ploidy in the endosperm of each species (Johnston et al., 1980). This hypothesis is that either the maternal/paternal genome ratio or the EBN must be in a 2:1 maternal/paternal ratio for successful endosperm development.

In addition to inhibition of pollen tube growth, several types of aberrations were observed in alien tubes growing in sorghum pistils that were not observed in the control. Alien pollen tubes typically displayed a crooked growth path through sorghum pistils. Irregular pollen tube growth was observed in ovules of blue panicgrass (Panicum antidotale Retz.) when pollinated with Kleingrass (Panicum coloratum L.) pollen (Burson and Young, 1983). Kleingrass pollen tubes became disoriented in the ovaries and grew in a random manner that prevented the tubes from entering the micropyle. A common growth-form aberration of alien pollen tubes in sorghum pistils was the enlargement or swelling of tips. This phenomenon has been observed for other species. In crosses between wheat (Triticum aestivum L.) and rye (Secale cereale L.), swollen rye pollen tubes have been reported in wheat pistils (Lange and Wojciechowska, 1976; Jalani and Moss, 1980). Sorghum pollen tubes with swollen tips and a twisted growth pattern were observed in pearl millet [Pennisetum glaucum (L.) R. Br.] styles (Heslop-Harrison, 2000). In the sorghum pistils of the current study, a common observation was the presence of callose in the stigma axis. During normal growth of a grass pollen tube, callose may form near the pollen tube and block plasmodesmata connections between cells (Heslop-Harrison and Heslop-Harrison, 1981). Callose accumulation in the stigma, in response to pollen from related species, is a common phenomenon (Dumas and Knox, 1983). The accumulation of callose in sorghum stigmas, in response to alien pollen tubes, is apparently a manifestation of inharmonious gene interaction between pollen and the pistil.

One approach to increase the frequency of interspecific hybridization is to discover genes that eliminate or reduce the factor(s) that cause reproductive isolation. In wheat, duplicate crossability genes Kr1 and Kr2 influenced interspecific crossability (Riley and Chapman, 1967). In the crosses, wheat x rye and wheat x bulbous barley (Hordeum bulbosum L.), the dominant alleles retarded and inhibited pollen tube growth at the base of the wheat style and in the ovary wall (Lange and Wojciechowska, 1976; Snape et al., 1979; Jalani and Moss, 1980). Genes exist in bulbous barley that override the action of the Kr1 allele, and allow the barley pollen tubes to grow into the wheat pistils (Sitch and Snape, 1986). In sorghum, variation exists among genotypes that influences pollen–pistil incompatibilities for at least one interspecific cross. Sun et al. (1991) reported that the growth of S. versicolor pollen tubes into sorghum pistils was influenced by the genotype of the sorghum line used. Of the three genotypes used, KS36A, KS5A, and ATx623, the S. versicolor pollen tubes grew further into ATx623 pistils than into those of the other two genotypes. However, successful hybridization was not achieved. Nonetheless, screening divergent sorghum lines may result in the discovery of genes that allow interspecific hybridization in sorghum.

In conclusion, the primary reason why interspecific hybridization does not occur in sorghum is that growth of alien pollen tubes is inhibited in sorghum pistils. For three species where limited fertilization and embryo formation occurred, the endosperm in immature seed aborted and no viable seed were produced. Thus pollen–pistil interactions and post-fertilization events are the reasons why hybrids have not been produced. Potential utilization of interspecific hybrids in sorghum breeding programs will require overcoming the inhibition of alien pollen tube growth and the rescue of hybrid embryos before the seed aborts. In vitro culture, a commonly used method to grow rescued embryos from aborted seeds following interspecific hybridization (Sharma, 1999), may be an approach to obtain interspecific sorghum hybrids. Additional obstacles to introgression may include hybrid sterility due to differences in chromosome number or the lack of homology between the genomes in the hybrid.


   ACKNOWLEDGMENTS
 
We acknowledge the contributions of Ms. Adrienne Broussard and Mr. Andrew Sitorus in scoring pollen germination and pollen tube growth. We thank Dr. J. Spencer Johnston for comments on the manuscript and Dr. David Stelly for access to fluorescence and imaging facilities.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Research supported in part by the USDA Sorghum Crop Germplasm Committee.

Received for publication July 8, 2004.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 




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