The Journal of Heredity 2002:93(3)
© 2002 The American Genetic Association 93:185-192
Occurrence and Cytological Mechanism of 2n Pollen Formation in a Tetraploid Accession of Ipomoea batatas (Sweet Potato)
From the International Potato Center (CIP), P.O. Box 1558, Lima 100, Peru. We are grateful to the International Potato Center (CIP) for financially supporting this research. L. A. Becerra Lopez-Lavalle is currently at the Centre for Plant Biodiversity Research, CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, Australia. G. Orjeda is currently at Genoscope, 2 rue Gaston Cremieux, CP 5706, 91057 EVRY CEDEX, France.
Address correspondence to L. A. Becerra Lopez-Lavalle at the address above or e-mail: Augusto.BecerraLopez-Lavalle@csiro.au.
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Abstract |
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We evaluated a 4x accession of Ipomoea batatas (L.) Lam. and four 2x accessions of Ipomoea triloba for 2n pollen production. Approximately 90% of the genotypes of accession 81.2 (I. batatas, 4x) produced 2n pollen with different frequencies. In contrast, none of the genotypes of I. triloba produced 2n pollen. The diameter of the 2n pollen was approximately 30% (
) larger than that of the n pollen, making it easy to identify, measure, and quantify. The correlation (r = 0.93**) between the frequency of giant pollen and the frequency of dyads and triads was highly significant, strongly suggesting that the giant pollen grains were 2n pollen. The 2n pollen producers presented either a parallel or tripolar spindle arrangement (Y shaped) at anaphase II instead of the normal 60° crossed spindle orientation. These two abnormal spindle configurations produced dyads and triads, with different frequencies (13–67%), instead of tetrads. Occasionally a metaphase II spindle variation was found with a single fused spindle, which also forms a dyad. The correlation (r = 0.89**) between the frequency of 2n pollen and the frequency of parallel, fused, and tripolar spindle arrangements was also highly significant, suggesting that these abnormal spindle configurations are involved in the production of 2n pollen in I. batatas. When we evaluated the efficiency of 2n pollen in polyploidization using 4x x 4x (2n) crosses, all progenies were 4x, suggesting the existence of barriers to crossability between 4x genotypes and their 2n pollen-producer counterparts. |
Introduction |
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The sweet potato, Ipomoea batatas (L.) Lam., is the ninth most important crop in the world in terms of production. In 2000 it yielded 141 x 106 metric tons (http://apps.fao.org/page/collections, 2000). It belongs to the family Convolvulaceae, genus Ipomoea, section Batatas (Austin 1987; McDonald and Austin 1990). The sweet potato is the only cultivated species of the 15 that compose this section (Austin 1987).
For the origin of the cultivated species, it is not yet clear whether it is an autopolyploid (Nishiyama 1971, 1975; Oracion et al. 1990; Shiotani and Kawase 1987, 1989) or an allopolyploid (Jones and Dioner 1965; Magoon et al. 1970). Yen (1974) suggested that the cultivated species can be autoallohexaploid, in which the formation of multivalents during meiosis as well as some secondary associations of chromosomes are remnants of the original species. Yen's interpretation is strongly supported by the cytological observations made by Jones and Dioner (1965). This hypothesis is also supported by the tetradisomic model of inheritance found for ß-amylase in sweet potato storage roots (Kumagai et al. 1990).
The species in section Batatas have a basic number of chromosomes (x) equal to 15. The cultivated species I. batatas is hexaploid (2n = 6x = 90), although a tetraploid cytotype (2n = 4x = 60) has been collected in Ecuador (Bohac et al. 1993). The other species in this section are diploid (2x) or tetraploid (4x). Some species have cytotypes with more than one ploidy (Austin 1987), such as I. cordato-triloba, which has 2x and 4x cytotypes (Ozias-Akins and Jarret 1994). The presence of a polyploid series in section Batatas is one indication of the occurrence of 2n gametes.
The 2n gametes (2n pollen or egg) possess a sporophytic chromosome number instead of the gametophytic number and they are the most conspicuous mechanism of sexual polyploidization in angiosperms (De Wet 1980; Harlan and De Wet 1975; Peloquin et al. 1999; Ramsey and Schemske 1998). It is likely that 2n gametes may have played a fundamental role in the origin and evolution of the 6x sweet potato (Bohac et al. 1992; Clement and Stanford 1961; McCoy and Smith 1983; Pfeiffer and Bingham 1983). Most 2n gametes are products of abnormalities occurring during meiosis. Male 2n gametes are known as 2n pollen or diplandroids and female 2n gametes as 2n eggs or diplogynoids. The term "2n gamete" is preferred to the term "unreduced gamete," which implies failure of the reductional or first meiotic division (Bingham and Saunders 1974). The 2n gametes are produced by different mechanisms, each with its own genetic consequence (Barone et al. 1999; Peloquin et al. 1999; Veilleux 1985). The cytological mechanism involved in 2n gamete production in Ipomoea is unknown. In potato, Solanum tuberosum, the cytological mechanisms involved in 2n pollen production and their genetic control have been reported by Mok and Peloquin (1975a, b). The mechanisms correspond to two types of nuclear restitution: first division restitution (FDR) and second division restitution (SDR).
Parallel spindles in the second meiotic division result in 2n pollen grains of FDR type and premature cytokinesis I or II after telophase I produces 2n pollen grains of SDR type. It was reported that different recessive single genes controlled each of these three mechanisms in potato. Mok and Peloquin (1975b) emphasized the genetic value of parallel spindles (ps) for 2n pollen formation. They indicated that in FDR 2n gametes, all parental heterozygous loci from the centromere to the first crossover, one-half of those between the first and second crossover, and 75% of those beyond the second crossover will be heterozygous in the 2n gametes produced. In SDR 2n gametes, only half of the heterozygous loci between the first and the second crossover and 25% of those beyond the second crossover in the parental chromosomes will be maintained in the 2n gametes (Mok and Peloquin 1975b). Peloquin et al. (1999) estimated that the percentage heterozygosity transmitted by gametes is approximately 80% with FDR and less than 40% with SDR.
The wild species of section Batatas represent the main source of wild germplasm for sweet potato breeding. Their wide distribution and adaptation to many different environments make the use of these wild species desirable. Nevertheless, their use is constrained by the lack of storage-root production and the presence of sexual barriers to crossability with the cultivated species, mainly due to differences in ploidy.
In addition to their value for cytological and taxonomical studies, 2n gametes have an important role in sweet potato breeding programs as a genetic tool for the introgression of valuable traits from 2x and 4x wild species into the cultivated 6x species (Freyre 1989; Iwanaga et al. 1991; Jones 1990; Orjeda et al. 1991). This value has been demonstrated in breeding in many other polyploid crops (Veilleux 1985).
In Ipomoea, 2n gametes have been shown to function in polyploidization from the 2x to the 3x and 4x levels (Eckenwalder and Brown 1986; Orjeda et al. 1991) and also from the 3x to the 6x level (Freyre et al. 1991), as well as in several other ploidy combinations (Freyre 1989; Ling 1984; Oracion et al. 1990; Orjeda and Iwanaga 1989). Although several investigators have proposed that the 6x crop arose from 4x genotypes that produced either 2n eggs or 2n pollen, no successful attempts have been made to generate 6x genotypes from 4x ones (Jones A, personal communication).
The objectives of this study were to determine the occurrence and frequency of 2n pollen in two species of Ipomoea, I. batatas (2n = 4x = 60) and I. triloba (2n = 2x = 30), which are considered ancestral species of the sweet potato (Austin 1987), to investigate whether 2n pollen functions in polyploidization from the 4x to the 6x level, and to determine the cytological mechanism(s) of 2n pollen production in 4x I. batatas.
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Materials and Methods |
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Plant Material
We used 64 open-pollinated seeds of accession 81.2 (I. batatas, 4x) that came from a polycross where all the genotypes produced more than 10% 2n pollen, and one stem cutting of the same accession (I. batatas, 4x), which was designated Ch 7.3. The seeds and stem cutting were kindly provided by Dr. J. Bohac of the USDA/ARS, Charleston, South Carolina. We also used four accessions—DLP 1679, DLP 1822, DLP 1895, and DLP 2429 of I. triloba (2x)—from the International Potato Center (CIP) collection (Table 1).
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To study the cytological mechanism, we used 44 2n pollen producer genotypes of accession 81.2. As a control we used eight genotypes of the same accession without 2n pollen production. The plants were kept in the greenhouses and facilities of the International Potato Center (CIP) in Lima, Peru.
Occurrence and Frequency
Pollen grains and young flowers buds were collected from 52 4x I. batatas and 45 2x I. triloba 2-month-old plants. These plants were generated from seeds and stem cuttings, in the summer of 1993 and 1994, at CIP (La Molina, Lima, Peru), respectively. The pollen grains (five flowers per plant) were collected immediately after anthesis using acetic-carmine glycerol jelly (Marks 1954). The frequency of 2n pollen grains was recorded by counting all the pollen grains of each sample with a microscope and separating them into two classes, normal and giant, according to their diameter. The diameter of 2n pollen grains is approximately times that of n pollen grains, thus there was no difficulty in separating them into two classes (Orjeda et al. 1990).
The pollen diameter was recorded in units with the aid of a micrometer placed in the eyepiece of the microscope and later transformed into millimeters. The percentages of giant pollen production were calculated by dividing the number of giant grains by the total number of grains in the sample, giant and normal, and the quotient was multiplied by 100. Only 52 genotypes out of 64 of accession 81.2 were evaluated; the rest did not flower.
At least 80 pollen grains per flower were measured in each of these genotypes using a 40x objective lens. Forty-four genotypes of I. batatas (4x) but none of I. triloba (2x) produced giant pollen. The genotypes that did not produce giant pollen were used to plot the distribution frequency of the normal pollen diameter. The 44 genotypes of I. batatas (4x) with giant pollen grains were also evaluated for pollen diameter distribution.
To confirm that the giant pollen effectively represents 2n pollen production, microspores at the tetrad stage were examined for the presence of dyads and triads in the sample. Young flower buds from 52 genotypes of I. batatas (4x) with and without giant pollen production were fixed in Farmer's fixative (3:1 absolute ethanol:glacial acetic acid) for 24–36 h and then stored in 70% ethanol until use. Other samples were prepared using 2% acetic-carmine. Expected giant pollen frequencies were calculated using the dyad and triad frequencies and then correlated with the observed giant pollen frequencies. The correlation was performed using plot options from MSTAT-C statistical software version 1.41 (Department of Crop and Soil Sciences, Michigan State University).
Polyploidization
Genotypes with different 2n pollen production frequencies were crossed and their progenies analyzed for ploidy level. Chromosomes were counted in root tips stained with acetic orcein.
Small cuttings were placed in vials with water until roots were produced. The roots were collected and pretreated with 8-hydroxyquinoline for 4 h at 15°C and then fixed in Farmer's fixative for 24 h. The roots were then hydrolyzed with 1 N HCl at 60°C for 20 min. Hydrolysis was stopped by filling the vials with cold tap water. The water with HCl was discarded and the roots were left immersed in 2% acetic orcein for at least 24 h. To count the chromosomes, the root tips were squashed. Chromosomes were counted at 100x magnification.
Cytological Evaluation
Young flower buds were collected between 8:00 and 10:00 A.M. According to previous reports this is the best time for finding meiotic stages (Jones 1990). Once collected, the buds were fixed in Farmer's fixative for 24 h and then transferred to 70% ethanol for at least another 24 h. Before the buds were cytologically examined, they were selected by size to roughly estimate the size of the buds with the meiotic stage of their pollen mother cells (PMCs).
The anthers were dissected from the buds under a stereoscope (Olympus VMZ 1x–4x). All five anthers per flower were placed on a microscope slide with a drop of acetic carmine (2%). The anthers were cut with a scalpel and then pressed gently to detach the PMCs from the anther wall. A cover slip was applied and samples were then observed using a microscope (Olympus BH2) with a 40x objective lens to record the number of dyads, triads, and tetrads, as well as any other type of sporads. The same was done with slightly less mature buds, looking for any abnormalities during meiosis such as parallel, fused, or Y-shaped spindles.
We obtained the expected 2n pollen frequencies using the observed frequencies of normal, parallel, fused, and tripolar spindles with the following formula:
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Results |
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Occurrence and Frequency
Because the genotypes of accession 81.2 come from a polycross in which the plants produced at least 10% 2n pollen, some were expected to be 6x, but according to the chromosome counts in root tips, the 65 genotypes of accession 81.2 were 4x (Figure 1A), and accessions DLP 1679, DLP 1822, DLP 1895, and DLP 2429 were 2x (Figure 1B). Table 1 shows the ploidy of the species studied, the number of individuals per accession evaluated, and their frequency of giant pollen.
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From 52 genotypes of accession 81.2 that flowered, 44 (86%) produced giant pollen grains (Figure 1C), from which 38 produced more than 1% giant pollen (Table 1). None of the accessions of I. triloba showed giant pollen production (Figure 1D).
The pollen diameter distribution of all 45 genotypes of 2x I. triloba that were selected for this study was similar to a normal distribution and had only one mode (Figure 2A) (Tondini et al. 1993). In contrast, the pollen diameter distribution of 4x I. batatas was bimodal, with a clear difference between the two classes of pollen grains (Figure 2B). The diameter of I. triloba pollen grains ranged between 5.6 x 10-2 mm and 7.2 x 10-2 mm, with a modal class of 6.6 x 10-2 mm. On the other hand, the 52 genotypes of 4x I. batatas showed a clear bimodal distribution of the pollen diameter: the first group had diameters that ranged between 8.8 x 10-2 mm and 10.8 x 10-2 mm and the second group had diameters that ranged between 11.8 x 10-2 mm and 14 x 10-2 mm (Figure 2B). Low frequency of pollen grain diameters fell between the two groups (Figure 2B). This bimodal distribution indicated the presence of grains from different pollen populations in the same sample.
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The presence of dyads and/or triads in the genotypes with giant pollen production was the second indicator of 2n pollen production. Monads, dyads, triads, and polyads (Figure 3) were found in anther samples at the end of telophase II. Based on the abnormal sporad frequencies, the frequency of expected giant pollen was estimated (Table 2). The correlation analysis showed a highly significant association (0.93**) between the observed and expected giant pollen frequencies (Table 2), strongly suggesting that the giant pollen grains were, in fact, 2n pollen.
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Polyploidization
To evaluate the efficiency of 2n pollen for polyploidization, the ploidy of progenies from selected 4x x 4x (2n) crosses was evaluated by counting chromosomes in root tips with the standard acetic-orcein squash technique. Twelve families were obtained from crosses between three non- 2n pollen producers (81.2-8, -50, and -53) and six 2n pollen producers (81.2-13, -18, -25, -52, and -63, and Ch 7.3-1), with their 2n pollen production percentages ranging from 2.7 to 84.2%. All crosses (six families) with the high 2n pollen producers (81.2-13 and Ch 7.3-1) did not set seeds. The root tips of five individuals per family were analyzed. All 60 genotypes evaluated were tetraploid (4x).
Cytological Evaluation
In section Batatas, normal microsporogenesis results in a tetrad-shaped sporad, which will form four haploid (n) pollen grains. The tetrad is the product of the particular spatial orientation of the spindles with respect to each other during anaphase II (Figure 4A,B). At the end of telophase II, a cleavage furrow is formed between the four meiotic products (Figure 4C) and a tetrad is formed (Figure 4D).
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One variation from normal microsporogenesis occurs with parallel or fused second-division spindles, so that the four groups of chromosomes are aligned in pairs side by side in the same plane (Figures 5A,B and 6A,B) and only one cleavage furrow along a common metaphase plate is formed (Figures 5C and 6C) producing a dyad (Figures 5D and 6D). Parallel, fused, tripolar, and normal spindle frequencies for each genotype, as well as the number of dyads, triads, and tetrads are shown in Table 2. Only 13 of 44 2n pollen-producer genotypes and 2 of 8 without 2n pollen production could be cytologically evaluated during anaphase II, as well as after telophase II; the rest were only evaluated after telophase II and the sporads counted. All 13 2n pollen-producer genotypes evaluated had the same type of spindle disorientation (parallel spindles).
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No tripolar spindle configurations were observed for genotypes 81.2-6, 81.2-7, and 81.2-13; thus no triads were observed in these genotypes. One exception was genotype 81.2-56, which did not have bipolar spindles, but produced one triad. There were several genotypes without 2n pollen production. In these genotypes we only found normal crossed spindles and tetrads, as expected.
A significant positive correlation was found (0.89**) between the expected 2n pollen frequencies using the parallel, fused, and tripolar spindle frequencies and the observed 2n pollen frequencies. This correlation strongly indicates that 2n pollen production is caused by parallel, fused, and tripolar spindles at metaphase II of microsporogenesis. Parallel and fused spindles result in dyads and tripolar spindles result in triads instead of tetrads.
Another variation is tripolar or Y-shaped spindles (Figure 7A,B) in which the PMC chromosomes end up split into three groups (Figure 7C) forming a triad. Two groups would form two n pollen grains and the remaining group makes the 2n pollen grain (Figure 7D).
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Discussion |
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Veilleux (1985) indicated the following symptoms of 2n pollen occurrence: bimodal distribution of the pollen diameter, the presence of dyads and/or triads in the microspores in the tetrad stage, and unexpected ploidy levels in the progeny of 4x x 2x or 2x x 4x crosses. Two of these three symptoms were found in the population of 4x I. batatas that we studied. First, a bimodal distribution was obtained when the pollen diameter frequency of giant pollen-producer genotypes was plotted (Figure 2B). This type of distribution suggested the presence of pollen grains from two different populations in 85% of the genotypes evaluated: a population of pollen grains of normal or standard size and a population of giant grains, presumably 2n pollen (Figure 1C).
The second indication of 2n pollen production was the presence of dyads and/or triads in genotypes with giant pollen production. Monads, dyads, triads, and polyads (Figure 3) were found in anther samples at the tetrad stage of meiosis. A correlation was made between the frequency of giant pollen in 52 genotypes of accession 81.2 and the frequencies of dyads and triads. A highly significant correlation (r = 0.93**) was found. This result strongly suggested that the giant pollen grains were 2n pollen grains. This result also agreed with that reported by Jones (1990), who studied 2n pollen production in similar 4x accessions of Ipomoea and found a high correlation (0.86 ± 0.17) between the production of giant pollen grains and the frequency of dyads.
The presence of dyads, triads, or both is strong evidence that the cytological mechanism involved in the formation of 2n pollen is "parallel spindles" (Mok et al. 1975). Orjeda et al. (1990) mention that Shiotani found that the 2n pollen was produced by a mechanism equivalent to the first division restitution (FDR) in 4x hybrids between 6x I. batatas and 2x I. trifida. These results suggest that a primary cytological mechanism involved in 2n pollen production in section Batatas is parallel spindles at metaphase II and anaphase II, as described for potato (Mok et al. 1975), alfalfa (Vorsa and Bingham 1979), and red clover (Parrott and Smith 1984). The high correlation between the expected 2n pollen frequencies obtained using the parallel and tripolar spindle frequencies (ps) and the observed 2n pollen frequencies (r = 0.89**) strongly suggests that the spindle disorientation is responsible for the 2n pollen formation in tetraploid I. batatas.
Twenty-one genotypes with 2n pollen production presented dyads, triads, and tetrads simultaneously, while in 23 genotypes, only dyads and tetrads were found. According to Mok and Peloquin (1975a), this occurs when premature cytokinesis, either after telophase I (PC I) or during prophase II (PC II), rather than parallel spindles, is the mechanism involved in 2n pollen formation. It was not possible to observe if PC I and/or PC II were responsible for the exclusive dyad production in every genotype, but 3 of the 22 genotypes with 2n pollen (81.2-6, 81.2-7, and 81.2-13) and exclusive dyad production were cytologically evaluated during meiosis (Table 2). The analysis of these three genotypes showed that they also produced parallel spindles at metaphase II, although this may not be the only cause of dyad formation. This result suggested that in genus Ipomoea, the absence of triads does not indicate that premature cytokinesis is the only mechanism responsible for 2n pollen formation, but that parallel spindles are also involved. In this particular case, the absence of triads in genotypes 81.2-6, 81.2-7, and 81.2-13 and low triad frequencies in genotype 81.2-56 were due to the low tripolar spindle frequency (see genotype 56).
Ramanna (1979) reevaluated the cytological mechanisms of 2n pollen production proposed by Mok and Peloquin (1975b). He suggested that fused spindles (fs) at metaphase II is the unique genetically controlled mechanism that undoubtedly leads to 2n pollen formation. Fused spindles unequivocally produce 2n pollen grains, but they were scarce in our plant material. Our results strongly suggest that not only fused spindles, but also parallel spindles are involved in 2n pollen formation in Ipomoea. The similarity between the spindle disorientation in these 4x I. batatas genotypes and those in potato and alfalfa (Vorsa and Bingham 1979), as well as the joint observation of dyads and triads in our material, led us to suggest that the 2n pollen we observed corresponded to the FDR type. The potential of FDR 2n pollen type to transmit heterozygosity from a diploid hybrid to a tetraploid hybrid has been demonstrated in both theory and practice in potato (Mendiburu and Peloquin 1977; Peloquin 1983; Peloquin et al. 1999).
The 2n pollen production in Ipomoea is frequent. In an evaluation of 2x accessions of I. trifida, Orjeda et al. (1990) found that 20% of 494 genotypes studied produced 2n pollen at different frequencies. The maximum 2n pollen production in a single individual was 34%. Jones (1990), studying 2n pollen production in accession 81.2, reported a maximum 2n pollen production of 74% in a single genotype. Several other reports on 2n gametes in this genus indicate that this is not an isolated event, but a common phenomenon (Freyre 1989; Iwanaga et al. 1991; Jones 1990; Orjeda et al. 1990). We found 2n pollen production in most of the 4x genotypes (93.2%) we evaluated. Their production frequencies varied between 0.3% and 30%, except for two that had very high production (81.2-13 at 67% and Ch 7.3-1 at 84%).
The use of 2n pollen has been shown to be efficient in increasing the ploidy level of any 2x or 3x collection to the 6x level, and thus in facilitating the introgression of wild germplasm into the cultivated sweet potato (Freyre 1989; Orjeda et al. 1991). However, it remains uncertain that 2n pollen can be efficiently used for gene flow from the 4x to the 6x level in Ipomoea.
We analyzed the ploidy of 12 families derived from 4x x 4x (2n) crosses to evaluate the efficiency of 2n pollen in polyploidization. All 60 hybrid genotypes (five per family) deviated from the expected 6x. Similar results were obtained by Orjeda (1995), working with hybrids derived from 4x x 8x crosses [in these crosses the gametes had the same ploidy combination, 2x x 4x, as gametes in 4x x 4x (2n) crosses]. All seed produced in Orjeda's study were 4x or 8x, suggesting the existence of barriers to crossability between the 4x and 8x genotypes. In our study, the unexpected occurrence of 4x in the 4x x 4x (2n) progeny suggested that either the 2n gametes of the male parent were not viable (prezygotic barriers) or ploidy and/or structural genomic differences (postzygotic barriers) exist.
The endosperm balance number (EBN) proposed by Johnston et al. (1980) has been tentatively proposed to explain the absence of 6x progeny from crosses with a ploidy combination of 2x x 4x in Ipomoea (Orjeda 1995). The EBN hypothesis bases the success of intra- and interspecific crosses on a female:male EBN ratio in the endosperm of 2:1 (Johnston et al. 1980), which will have normal endosperm development and will give rise to viable seeds in the hybrid plant. A deviation from this ratio of EBN maternal to paternal would suppress endosperm development. In the crosses analyzed in the present work, it is clear that the male and female n = 2x gametes are fertile and genotype compatible, otherwise no 4x progeny would have been obtained. The absence of both viable seeds in crosses with high 2n pollen producers (81.2-13 and Ch 7.3-1) and hexaploid progeny in crosses with low to intermediate 2n pollen producers suggested that the male 2n gametes contributed twice the EBN that a normal n gamete does, making an unviable EBN ratio in the endosperm of 2:2 (ploidy ratio of endosperm/embryo of 8x/6x or 4x/3x) instead of the desirable 2:1 (ploidy ratio of endosperm/embryo of 3x/2x) required for embryo development.
Although our results are not conclusive they are supported by previous studies of mating designs based on the use of 2n gamete-producing diploid and tetraploid parents to develop interploid hybrids in Ipomoea (Iwanaga et al. 1991; Jones 1990; Orjeda et al. 1991). The control of endosperm development in Ipomoea needs to further study, since knowing the EBNs would make it possible to predict the success or failure of crosses between species with different ploidies in this genus.
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Footnotes |
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Corresponding Editor: Reid G. Palmer
Received November 16, 2001
Accepted March 29, 2002
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References |
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