Published in Crop Sci 39:1722-1727 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Crop Science 39:1722-1727 (1999)
© 1999 Crop Science Society of America
CELL BIOLOGY & MOLECULAR GENETICS
Maize Genotypes Show Striking Differences for Induction and Regeneration of Haploid Wheat Embryos in the Wheat x Maize System
Vinesh Vermaa,
N.S. Bainsa,
G.S. Mangata,
G.S. Nandaa,
S.S. Gosala and
Kuldeep Singha
a Dep. Plant Breeding, Punjab Agric. Univ., Ludhiana 141 004, India
kuldeep{at}pau.chd.nic.in
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ABSTRACT
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A number of maize (Zea mays L.) genotypes were tested for their influence on induction and regeneration of haploid wheat (Triticum aestivum L.) embryos to improve further the efficiency of the wheat x maize system of haploid production. Fifteen maize genotypes were crossed to five diverse wheat F1 hybrids in a line x tester fashion in three replications. Two efficiency parameters, caryopses formation frequency (CFF—number of caryopses/100 pollinated florets) and embryo formation frequency (EFF—number of embryo-carrying caryopses/100 pollinated florets), were estimated. Analysis of variance revealed that lines (wheat genotypes), testers (maize genotypes), and their interaction showed significant effects on both efficiency parameters, but the effect of maize genotypes was greater than that of wheat genotypes. The EFF of individual maize genotypes ranged from 1.1 to 23.4% and the EFF of individual wheat genotypes ranged from 8.4 to 10.2%. Maize genotypes also showed significant differences for general combining ability estimates. In addition to EFF, maize genotypes had a striking effect on haploid embryo regeneration as analyzed using one of the wheat genotypes. The values ranged from 0.0 to 87.8%. For maize genotypes producing the highest EFFs, the regeneration frequency was not necessarily higher. Hence, we suggest a new index, haploid formation efficiency (HFE—number of haploid plants formed /100 pollinated florets), be used for identification of efficient pollinators. The HFE ranged from 0.0 to 9.9%. In this study, a pop corn cultivar, Pearl Pop Corn, was identified as the best pollinator with an average EFF of 15.1% across five wheat genotypes and an HFE of 9.9% with the one wheat genotype tested.
Abbreviations: CFF, caryopses formation frequency • EFF, embryo formation frequency • HRF, haploid regeneration frequency • HFE, haploid formation efficiency • GCA, general combining ability
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INTRODUCTION
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THE WHEAT X MAIZE SYSTEM has emerged as the system of choice for inducing haploids in wheat (Snape, 1998). It represents a simple and relatively inexpensive technology which augurs well for its adoption. It can mesh perfectly with conventional plant breeding programs and help speed up the breeding cycle for at least the most promising material. The first wheat haploid from the wheat x maize system was reported in 1988 (Laurie and Bennett, 1988), 4 yr after Zenkteler and Nitzsche (1984) had reported that embryos could be obtained by crossing wheat with maize. Many studies aimed at exploring the basic biology of the system (Laurie and Bennett, 1986, 1987; Laurie, 1989; Suenaga and Nakajima, 1989) have been conducted. In the last 5 yr, however, the focus has shifted to application oriented studies (Suenaga, 1994; Lefebvre and Devaux, 1996; Zhang et al., 1996; Snape, 1996, 1998).
The components of the system which were initially taken up for refinement involved the crossing technique (Laurie and Bennett, 1988; Laurie, 1989), the hormonal treatments (Suenaga and Nakajima, 1989; Riera-Lizarazu and Mujeeb-Kazi, 1990; Suenaga, 1994; Matzk and Mahn, 1994; O'Donoughie and Bennett, 1994), and tissue culture medium manipulations (Comeau et al., 1992; Suenaga, 1994; Kammholz et al., 1996). The genotypic specificity became evident gradually, more as a result of a consensus emerging from several studies with varied objectives than from a direct investigation of this phenomenon. As a consequence of this situation, very few studies address this problem systematically, e.g., the genotypic specificity of only the maize or only the wheat genotypes was studied, the data was either based on a single replication (Suenaga and Nakajima, 1989; Laurie and Reymondie, 1991; Ushiyama et al., 1991; Amrani et al., 1993; Sarrafi et al., 1994; Suenaga, 1994; Bains et al., 1995), or the study was based on only a few parents (Li et al., 1996; Zhang et al., 1996). This gap in the information served as the rationale for conducting this study. Second, the large variations in the efficacy of pollinator genotypes might be used for the identification of an efficient pollinator for improving the efficacy of the wheat x maize system many fold without incurring any extra effort or expense. This study was conducted under field conditions, unlike other wheat x maize studies, which in general were conducted in greenhouses.
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Materials and methods
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Plant Material
The plant material consisted of five wheat F1s and 15 maize genotypes. The five wheat F1 hybrids used were DL 983-1 x WH 542 (W1), DL983-1 x HI 601 (W2), PBW 343 x WH 542 (W3), PBW 343 x Chilero (W4), and PBW 343 x HD 2329 (W5). Parentage of the wheat genotypes used for generating F1 hybrids is presented in Table 1
. Among the genotypes used for making F1s, PBW 343, WH 542 and HD 2329 are commercial cultivars, whereas Chilero, DL 983-1, and HI 601 are genetic stocks. All these are spring wheats. Wheat varieties PBW 343 (=Attila`s') and WH 542 (=Kauz`s') have a IBL.IRS wheat-rye chromosomal translocation and have winter wheat pedigrees, whereas HD2329 is a typical spring wheat variety. The maize genotypes used for crossing are presented in Table 2
. These maize genotypes included inbred lines, composites, and F1 hybrids. Most of these lines belonged to tropical and sub-tropical maize germplasm and only a few of these, which were developed by introgressing temperate maize germplasm into tropical and sub-tropical germplasm, were known to have cold tolerance.
Experimental Design
The experiment was conducted at Punjab Agricultural University, Ludhiana, India, situated 30.55°N latitude and 75.54°E longitude. The five wheat genotypes were planted in the field in a randomized block design (RBD) with three replications and plot size of four rows, 1 m long. The plant-to-plant spacing was 20 cm and distance between rows was 23 cm. The maize genotypes were planted under a temporary greenhouse covered with transparent polyethylene sheet to protect against frost and to promote normal (non-stressed), timely flowering.
Crossing Technique
Emasculation
Wheat spikes were emasculated 2 to 3 d before anthesis. The procedure involved removing central florets and keeping intact the primary and secondary florets. Anthers were removed manually with the help of fine pointed forceps without cutting lemma and palea, unlike conventional practice which involves cutting of glumes to expose the androecium. This practice promotes caryopsis development in wheat x maize crosses. After emasculation, the spikes were covered with glassine bags.
Pollination
In each of the three replications, three spikes of every wheat genotype were pollinated with each of the 15 maize genotypes in a line x tester fashion. The emasculated spikes were pollinated 3 to 5 d after emasculation with freshly collected pollen from 5 to 8 plants of each maize genotype. Pollination was done by opening the lemma and palea with forceps and applying pollen onto individual stigmas with a number zero camel hair brush to avoid pollen wastage. Pollinated spikes were individually covered again with glassine bags. The pollinations were completed within a span of 20 d (26 Feb.–17 March) and maize pollinators were used in the order presented in Table 2 (i.e., M1 at the beginning and M15 at the end; these genotypes flowered in that order).
Hormonal Application
The pollinated tillers were administered 100 mg L-1 2,4-D (2,4-dichlorophenoxy acetic acid; Sigma Chemical Company, St. Louis) 24 h after pollination once a day, for three successive days. The 2,4-D solution was injected with the help of a syringe having a fine needle (no. 24) at the base of the uppermost internode. A hole was pricked at the top of the uppermost internode before injecting 2,4-D. This allows air from hollow space to escape and prevents splitting of the tiller because of pressure exerted by the injection. Also, individual florets were given a drop of 50 mg L-1 2,4-D in between the lemma and palea 24 h after pollination.
Identification of Embryo-Carrying Caryopses
Caryopses were harvested 15 to 17 d after pollination. The embryo-carrying caryopses were identified as per the method given by Bains et al. (1998). In this procedure, caryopses were placed in a glass petri dish with the crease on the upper side. A light source was placed above the petri dish. The embryo-carrying caryopses can be identified by viewing from below because the suspended embryos settle to the lower surface within the caryopses.
Embryo Rescue
Embryos of only one wheat genotype (PBW 343/WH 542) pollinated with all the 15 maize lines were plated for regeneration. The embryo-carrying caryopses were surface sterilized with absolute ethanol for 2 min followed by 1 g L-1 mercuric chloride treatment for 10 min. The caryopses were subsequently rinsed 3 to 4 times with sterile distilled water and surface dried. The embryos were then dissected from sterilized caryopses. All these operations were carried out in a sterile air flow chamber. The embryos were transferred to test tubes containing half-strength Murashige and Skoog (MS) medium (Murashige and Skoog, 1962) supplemented with 0.5 mg L-1 benzylaminopurine, 20 mg L-1 L-alanine (Sigma), 30 g L-1 sucrose and 8 g L-1 agar agar. The embryos were incubated at 20 ± 2°C in dark for regeneration. The regenerated embryos were kept at 20 ± 2°C with 8 to 10 h light in culture room until 1-to-2 leaf stage when the data were recorded.
Data Recording
The following observations were recorded for each cross combination in all the three replications: (i) total number of florets pollinated, (ii) total number of caryopses formed, and (iii) total number of embryo-carrying caryopses. On the basis of the above recorded information, the following efficiency parameters were calculated.
Data were also recorded on total number of embryos regenerated into plants for each of the 15 maize pollinators crossed to one wheat genotype, w3. Haploid regeneration frequency (HRF) and haploid formation efficiency (HFE) were calculated as follows.
Statistical Analysis
Three spikes of each wheat genotype were pollinated with each of the 15 maize genotypes in all the three replications and the parameters, CFF and EFF, were recorded in percentages for each replication and two-way tables obtained for each of the above parameters. These parameters were subjected to analysis of variance for RBD after arc sine transformation. Similarly for combining ability estimates of wheat and maize genotypes for the above three parameters, line x tester analysis (Kempthorne, 1957) was done.
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Results and discussion
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Genotypic Influence: Wheat Versus Maize
Pollination with maize pollen coupled with post-pollination treatments yielded caryopses and haploid embryos in all the 75 wheat x maize combinations. However, the frequency of both caryopses and haploid embryo development varied considerably in different cross combinations. All the wheat F1 hybrids were crossable with maize as shown by caryopses and embryos forming in all five wheat genotypes. None of the parental lines of these hybrids are known to carry recessive crossability alleles kr1 and kr2. Thus, the wheat x maize system, as demonstrated earlier (Laurie and Bennett, 1987; Suenaga, 1994), is independent of crossability alleles kr1 and kr2.
Caryopsis formation frequencies in the 75 cross combinations ranged from 34.3 to 93.7% and for embryo formation the extremes were 0.3 and 31.3% (data not shown). Analysis of variance for CFF and EFF showed significant effects of wheat parents, maize pollinators, and their interaction at P < 0.01 (Table 3)
. The magnitude of mean squares indicates that an overwhelming proportion of the variation observed for caryopsis and embryo formation was due to the maize parents. The means of the efficiency parameters for the five wheat parents (averaged over maize pollinators) show a narrow range of 60.1 to 68.2% for CFF and 8.4 to 10.2% for EFF (Table 4)
. This is in sharp contrast with the ranges obtained when the same parameters are averaged for the maize parents (Table 5)
. The caryopsis formation with different maize parents ranged from 36.7 (for M15) to 84.6% (for M9) and embryo formation varied from 1.1 (for M15) to 23.4% (for M9). This comparison of ranges may be biased by the fact that the number of maize and wheat parents was not equal. However, larger number of maize genotypes served the overall objective of this study.
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Table 4 Mean values for caryopsis formation frequency (CFF) and embryo formation frequency (EFF) in percentages (averaged over replications and maize genotypes)
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Table 5 Mean values for caryopsis formation frequency (CFF) and embryo formation frequency (EFF) in percentages (averaged over replications and wheat genotypes)
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Several studies have analyzed the influence of wheat and maize genotypes on embryo formation in wheat x maize crosses (Inagaki and Tahir, 1990; Suenaga et al., 1991; Amrani et al., 1993; Sarrafi et al., 1994; Suenaga, 1994; Bains et al., 1995; Kammholz et al., 1996; Lefebvre and Devaux, 1996; Li et al., 1996; Zhang et al., 1996). The present study shows the influence of maize genotypes to be striking in comparison to the milder influence of wheat genotypes. This has important implications for the wheat x maize system of haploid induction. Low genotypic specificity from the wheat side enhances the wheat breeding applicability of the system. High genotypic specificity of maize parents provides an excellent opportunity for identification of efficient pollinators. Superior pollinators have been selected in Hordeum bulbosum L. (Pickering and Rennie, 1990) and in Solanum phureja Juz. & Buk. (Kotch and Peloquin, 1987) for improving haploid induction efficiencies in these systems. Apart from the within-system comparison of genotypic specificity made here, anther culture and the H. bulbosum system have been shown to have much stronger genotypic responses of wheat as compared to the wheat x maize system (Inagaki and Tahir, 1990; Kisana et al., 1993).
General Combining Abilities
The term general combining ability (GCA) in the context of wheat x maize crosses denotes the average ability of a parent to induce caryopsis formation or embryo formation in a series of crosses. The present study used the full set of 15 maize and 5 wheat genotypes crossed in a line x tester fashion. This allowed estimations of GCA effects, which identify the value of each parent clearly by depicting it as deviation from the overall mean (Fig. 1)
. In wheat, one (W3) of the five genotypes had significant positive effects for caryopses formation while for embryo formation genotype W5 had negative GCA effects. The majority of genotypes did not differ significantly from the set average in their ability.
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Fig. 1 General combining ability effects of wheat and maize genotypes on caryopsis formation and embryo formation frequencies. ** Indicates significance at P < 0.01
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The GCA effects have greater relevance in the case of maize pollinators for identifying the efficient pollinator. The majority of genotypes differed significantly (Fig. 1). M1, M2, M7, M8, M9, M12, and M13 showed positive and significant GCA effects with M9 having the highest GCA for embryo formation. A divergence is observed in maize GCA effects for caryopsis formation and embryo formation. Several of the parents having non significant GCA effects for caryopsis formation (M1, M2, M7, M8, M10, M13, and M14) showed significant GCA for embryo formation. M5, however, with a significant effect for caryopsis formation was non significant for embryo formation. Overall there seems to be a much higher specificity for embryo formation as compared to caryopsis formation. In fact caryopsis formation is greatly influenced by 2,4-D treatment (Suenaga, 1994) thus overriding genotypic differences. In terms of GCA effects for embryo formation, M9, M13, M12, and M2 (in that order) can be identified as the desirable genotypes. Maize lines M15, M12, M11, M3, and M4 were poor general combiners.
Influence of Maize Genotypes on Haploid Embryo Regeneration
For plant regeneration studies, embryos of only one wheat genotype, W3 (PBW 343 / WH 542), crossed with all 15 maize pollinators were cultured in vitro. After culturing, not all the embryos developed into complete plants—some did not germinate, some produced only shoot and no root, and others produced both root and shoot but subsequently died. Data on regeneration were recorded when plants reached a 2-to-3 leaf stage. Embryos which did not regenerate even after one month of culture were discarded. Maize genotypes showed a significant influence on plant regeneration. Regeneration of haploid embryos into plants varied from 0.0 to 87.9% (Table 6)
, the highest being for the maize genotype M2 followed by M5 and M8 (50.0% both). M2, the maize genotype which induced exceptionally high plant regeneration, is a commercial pop corn composite known as Pearl Pop Corn. Ushiyama et al. (1991) observed regeneration from 0.0 to 81.8%, the highest being for a teosinte [Zea mays subsp. mexicana (Schrader) Iltis] genotype. Zhang et al. (1996) reported haploid regeneration of wheat genotype Bb 540/322 to be 46.2 and 78.2%, when pollinated with maize lines Rp1-D-Mutator and Rp1-A-Mutator, respectively. Lefebvre and Devaux (1996) also observed a significant effect of maize genotypes on plant regeneration. Almouslem et al. (1998), however, reported that maize pollinators did not have any significant influence on germination of haploid embryo in durum wheat. Their data were based on three maize genotypes only and plant regeneration frequency was very low (maximum being 0.6%). It is intriguing that the pollen parent genotype can affect embryo development and consequently plant regeneration. It is difficult to discern whether these differences are related to general vigor and quality of the pollen or to specific factors directly interacting at the biochemical level. We speculate that there are some differences in dynamics of maize chromosome elimination and expression of some maize genes before chromosome elimination which result in differences in embryo development. This is an area for future study.
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Table 6 Embryo formation frequency (EFF), haploid regeneration frequency (HRF) and haploid formation efficiency (HFE) of wheat genotype, W3 (PBW 343/WH 542) pollinated with different maize genotypes
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Identification of Efficient Maize Pollinators
Genotypic differences in maize pollinators for induction of haploid embryos seem to be independent of differences observed for embryo regeneration (Table 6). The correlation between EFF and HRF across maize genotypes was non significant
. Thus, maize genotypes inducing high embryo formation did not induce high embryo regeneration (Fig. 2)
. Combining these two parameters to obtain the proportion of florets giving haploid plants, i.e., haploid formation efficiency (HFE), could serve as an appropriate index for the efficiency of maize pollinator. Based on this index, maize genotype M2 (Pearl Pop Corn) ranks highest with 9.9% of the pollinated florets resulting in haploid plants (Table 6). M6, an inbred line, is ranked next with 7.6%. Hence a new index, HFE is suggested to be used for identification of efficient maize pollinators. Apart from the rankings obtained in this manner, the choice of pollinator would also be dependent upon other considerations such as pollen availability and cold tolerance. This is particularly relevant for situations where the wheat x maize program is conducted in field or with minimal environmental control. Pearl Pop Corn (M2) possessed abundant pollen and a high level of cold tolerance. The tassel is large with pollen availability lasting for about 14 d on an average.
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Fig. 2 Influence of maize genotypes on embryo formation frequency and haploid formation efficiency of wheat genotype, W3 (PBW 343/ WH 542)
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In conclusion, pronounced differences exist in maize genotypes for their ability to induce and regenerate haploid embryos. In contrast, the influence of wheat genotypes was mild. Together, these observations imply a lack of genetic recalcitrance in wheat to the wheat x maize system on one hand and an excellent opportunity to enhance the efficiency of the system with a superior pollinator on the other hand. In the present study, the efficiency obtained by the best pollinator in terms of number of haploid plants produced (HFE) is more than five times the average of the 15 pollinators used.
Genetic specificity for induction of haploid embryos was independent of specificity for embryo regeneration and the traits are likely to be under the control of different maize loci. Probability dictates fewer chances of identifying genotypes superior for both induction and regeneration. Crossing genotypes excelling in individual traits may be a viable option for breeding super pollinators. Another future option for improving maize pollinators further in the context of the present study includes selection within the Pearl Pop Corn population. Useful variability for these traits is likely to exist in the absence of any previous selection for this purpose. It may also be rewarding to explore a larger set of pop corn genotypes as only one was used in this study.
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ACKNOWLEDGMENTS
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The authors gratefully acknowledge the Maize Section of the Department of Plant Breeding for providing maize lines used in this study and the Biotechnology Centre of the University for providing tissue culture facilities.
Received for publication November 23, 1998.
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ABSTRACT
INTRODUCTION
