Journal of Heredity Advance Access published online on April 8, 2007
Journal of Heredity, doi:10.1093/jhered/esm011
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Brief Communications |
Meiotic Restitution in Wheat Polyhaploids (Amphihaploids): A Potent Evolutionary Force
United States Department of Agriculture—Agricultural Research Service, Northern Crop Science Laboratory, Fargo, ND 58105-5677
Address correspondence to Prem Jauhar at the address above, or e-mail: prem.jauhar{at}ndsu.edu.
Polyploidy is well recognized as a major force in plant speciation. Among the polyploids in nature, allopolyploids are preponderant and include important crop plants like bread wheat, Triticum aestivum L. (2n = 6x = 42; AABBDD genomes). Allopolyploidy must result through concomitant or sequential events that entail interspecific or intergeneric hybridization and chromosome doubling in the resultant hybrids. To gain insight into the mechanism of evolution of wheat, we extracted polyhaploids of 2 cultivars, Chinese Spring (CS) and Fukuhokomugi (Fuko), of bread wheat by crossing them with maize, Zea mays L. ssp. mays. The derived Ph1-polyhaploids (2n = 3x = 21; ABD) showed during meiosis mostly univalents, which produced first-division restitution (FDR) nuclei that in turn gave rise to unreduced (2n) male gametes with 21 chromosomes. The haploids on maturity set some viable seed. The mean number of seeds per spike was 1.45 ± 0.161 in CS and 2.3 ± 0.170 in Fuko. Mitotic chromosome preparations from root tips of the derived plantlets revealed 2n = 42 chromosomes, that is, twice that of the parental polyhaploid, which indicated that they arose by fusion of unreduced male and female gametes formed by the polyhaploid. The Ph1-induced univalency must have produced 2n gametes and hence bilateral sexual polyploidization and reconstitution of disomic bread wheat. These findings highlight the quantum jump by which bread wheat evolved from durum wheat in nature. Thus, bread wheat offers an excellent example of rapid evolution by allopolyploidy. In the induced polyhaploids (ABD) that are equivalent of amphihaploids, meiotic phenomena such as FDR led to regeneration of parental bread wheat, perhaps a simulation of the evolutionary steps that occurred in nature at the time of the origin of hexaploid wheat.
Polyploidy—a condition with multiple sets of chromosomes—has long been recognized as a dominant factor in plant speciation, particularly among angiosperms (Leitch and Bennett 1997; Soltis DE and Soltis PS 1999; Jauhar 2006). An estimated 50–70% of angiosperms have undergone one or more events of polyploidization (Stebbins 1971; Lewis 1980; Masterson 1994; Wendel 2000). Allopolyploidy resulting from interspecific or intergeneric hybridization, coupled with concomitant or subsequent chromosome doubling, can lead to sexual polyploidization and has been instrumental in the formation of many of our most important cereal, forage, oilseed, and fiber crops. Chromosome doubling, resulting from fusion of 2n male and female gametes formed by an interspecific hybrid (amphihaploid—an interspecific or intergeneric hybrid, with half the genomic contribution of each parent), could directly lead to the origin of a fertile amphidiploid and hence a crop species. Such a sexual polyploidization may be the predominant source of allopolyploids in nature (Harlan and de Wet 1975; Jauhar et al. 2000; Jauhar 2003a). Several adaptive advantages of allopolyploidy are outlined by Comai (2005).
Durum or macaroni wheat (Triticum turgidum L., 2n = 4x = 28; AABB) and bread wheat (Triticum aestivum L. 2n = 6x = 42; AABBDD) are important cereals used for human consumption worldwide. Tetraploid durum wheat is a predecessor of hexaploid bread wheat (Figure 1). The donors of its 2 genomes, A and B, were respectively Triticum urartu Tumanian (Nishikawa 1983; Dvo
ák et al. 1993) and Aegilops speltoides Tausch (Sarkar and Stebbins 1956; Wang et al. 1997; Dvoák 1998), although some controversy surrounds the B-genome donor. The 2 diploid progenitors hybridized in nature about half a million years ago (Huang et al. 2002) and gave rise to tetraploid wild emmer wheat (T. turgidum var. dicoccoides Körn), presumably in one step as a result of functioning of unreduced (2n) gametes in the BA hybrid (amphihaploid) (Step 1 in Figure 1). Because the B-genome donor is believed to have acted as the female parent (Wang et al. 1997), its genome is indicated first in the hybrid BA. Because corresponding (homoeologous) chromosomes of the constituent genomes are closely related and hence capable of pairing with one another, a chromosome pairing regulator, called Pairing homoeologous gene, Ph1 arose as a mutation on the long arm of chromosome 5B at the time of origin of emmer wheat. The Ph1 gene is effective in suppressing homoeologous chromosome pairing, thereby ensuring diploid-like meiosis and disomic inheritance in polyploid wheats (Riley and Chapman 1958; Sears and Okamoto 1958; Jauhar et al. 1991; Jauhar and Joppa 1996). Another cycle of spontaneous hybridization took place (Step 2 in Figure 1), and hexaploid wheat resulted from tetraploid wheat by acquiring a third genome, DD, from another diploid goat grass (Aegilops tauschii Coss., 2n = 2x = 14, DD genome) (McFadden and Sears 1946), about 8000 years ago (Huang et al. 2002).
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Bread wheat and its predecessor durum wheat offer excellent examples of rapid evolution by allopolyploidy resulting from sexual polyploidization. With the availability of efficient tools of extracting haploids via hybridization with maize, Zea mays L. ssp. mays (Laurie and Reymondie 1991; Almouslem et al. 1998; Jauhar 2003b), it is possible to go back on the evolutionary ladder and obtain durum and bread wheat polyhaploids (a polyhaploid is a haploid derived from a polyploid species like wheat; however, the terms "polyhaploid" and "haploid" are used interchangeably in this paper) and then regenerate from them, respectively, tetraploid durum and hexaploid bread wheat. Earlier, we reported on synthetic haploids (2n = 2x = 14; AB genomes) of durum wheat and regenerated from them tetraploid durum (Jauhar et al. 2000). This paper describes maize-induced bread wheat haploids (2n = 3x = 21; ABD genomes) and spontaneous derivation of the parental species by chromosome doubling via sexual polyploidization.
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Materials and Methods |
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Two bread wheat cultivars, Chinese Spring (CS) and Fukuhokomugi (Fuko), were used for extracting polyhaploids by crossing them with maize. These cultivars are highly crossable, Fuko being one of the most crossable cultivars we have studied (Jauhar 1995; Jauhar and Chibbar 1999). Methods of emasculation, pollination, and postpollination treatments standardized earlier (Jauhar 2003b) were used.
Somatic chromosome counts were used to verify the haploid status of the plantlets obtained from crosses with maize. Meiotic stages were studied, especially with a view to unravel phenomena that led to meiotic restitution, 2n gamete formation, and subsequently to seed set and production of disomic bread wheat. Techniques of staining somatic and meiotic chromosomes described earlier (Jauhar et al. 1999; Jauhar 2003b) were used.
The haploid plants were grown to maturity in the greenhouse. Seed set on the CS and Fuko haploid plants was calculated on a per-plant and per-spike basis. The significance of difference in seed set was tested using the t-test procedure of SAS Statistical Analysis System, Cary, NC. The seeds were germinated in petri dishes, and somatic chromosomes studied from root tips.
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Results |
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Using wheat x maize hybridization, we produced 12 Ph1-polyhaploids in the wheat cultivar CS and 20 in Fuko. These haploids were studied meiotically, especially with reference to restitution events (Figures 2A–D), leading to the formation of unreduced gametes that in turn produced seed set. Both CS and Fuko haploids had Ph1, which was effective in suppressing homoeologous chromosome pairing. Thus, most of these haploids showed 21 univalents (Figure 2A), although occasionally 1 or 2 bivalents were observed. The univalents showed disjunctional abnormalities such as univalent division, multipolar separations, laggards, and micronuclei.
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In about 4–10% of the pollen mother cells (PMCs), the univalents aligned themselves in the center (plate) at meiotic metaphase I (Figure 2A). At late metaphase I or early anaphase I, the univalents split into chromatids (Figure 2B) that, in most cases, failed to move to poles and consequently formed first-division restitution (FDR) nuclei (Figure 2C). With no chromosome pairing and hence no reduction in chromosome number, the FDR nuclei contained 21 chromosomes (the somatic number of the parent polyhaploid), which then underwent a normal equational (mitotic) division and formed dyads (Figure 2D) instead of the tetrads formed by normal disomic wheat plants. The dyads gave rise to microspores with 21 chromosomes, the gametic number of the hexaploid parent. These unreduced gametes produced by polyhaploids must have functioned like the normal haploid gametes formed by disomic wheat plants. These observations, in concert with observations on viable seed set (Tables 1 and 2), indicate that analogous events occurred in megasporocytes and produced functional, unreduced female gametes.
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The fusion of the male and female gametes obviously resulted in seed set. Seed set on 6 CS haploids and 12 Fuko haploids is given in Tables 1 and 2, respectively. Fuko haploids were more vigorous and tillered profusely, with several spikes producing viable seed. These haploids (Table 2) produced significantly more seed than the CS haploids (P < 0.0187) (Table 1). Higher seed set on Fuko haploids than in CS haploids may be a function of Fuko's higher crossability, as observed in several crosses (Jauhar 1995). Fuko may well be more crossable with maize than is CS. Genotypic differences in ability to produce haploids are known (Almouslem et al. 1998).
Forty-two somatic chromosomes were observed in root tips of seed-derived plantlets of bread wheat. Clearly, the seeds resulted from precise duplication of chromosome number due to fusion of precisely unreduced 21-chromosome male and female gametes.
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Discussion |
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Allopolyploid species are natural, stable hybrids that enjoy the benefits of perpetual hybridity and genetic luxuries afforded by polyploidy-endowed genetic redundancy. Functioning of meiotically unreduced gametes in the newly formed interspecific hybrids leads to evolutionarily successful polyploidization, which results in new species with different genomes. Sexual polyploidization coupled with genetically regulated chromosome pairing promotes the rapid founding of successful allopolyploid species in cereals (Jauhar 2003a, 2006; Ceoloni and Jauhar 2006) and fescues (genus Festuca) (Jauhar 1975a,b, 1993) that are preponderant in nature. Both durum and bread wheat offer excellent examples of such a cataclysmic evolution by allopolyploidy.
Bread wheat evolved in nature as a result of 2 quantum jumps. First, durum wheat resulted from one event of spontaneous hybridization between 2 wild species, followed by chromosome doubling, acquiring 2 genomes, BB and AA (Step 1 shaded portion in Figure 1). Then, bread wheat resulted from another, similar cycle of spontaneous hybridization, gaining an additional genome, DD, from Ae. tauschii (Step 2 shaded portion in Figure 1). These 2 quantum jumps of evolution that produced first durum wheat and then bread wheat can be elucidated using synthetic haploids. Because Ph1 is hemizygous effective and its single dose is fully functional in polyhaploids of bread wheat (Jauhar et al. 1991) and durum wheat (Jauhar et al. 1999), these polyhaploids form mostly univalents. Earlier, Jauhar et al. (2000) showed a mechanism(s) responsible for the formation of 14-chromosome gametes in durum haploids, subsequent seed set, and the regeneration of tetraploid durum.
The 21 univalents of bread wheat polyhaploids gave rise to FDR nuclei which formed unreduced gametes with 21 chromosomes—the somatic number of the parent polyhaploids. Because seed-derived plantlets showed 2n = 42 chromosomes, it is safe to conclude that there was precise doubling of chromosomes by the fusion of precisely unreduced male and female gametes formed in the parent polyhaploids. It is likely that as a result of hybridization between durum wheat and Ae. tauschii, a triploid hybrid BAD was first formed in nature, which then produced unreduced male and female gametes that on fusion produced hexaploid seed, reconstituting bread wheat. The seed set on the BAD amphihaploid formed in nature was probably low as observed in the maize-induced haploids of 2 wheat cultivars (Tables 1 and 2). However, even a few seeds would be good enough to produce disomic wheat that was probably picked up and cultivated through human intervention. These observations on seed set in wheat haploids parallel the events in durum haploids (amphihaploids BA) that set seed (Jauhar et al. 2000; Matsuoka and Nasuda 2004). This may lead to the conclusion that genetic regulation of pairing impacts not only fertility of an existing allopolyploid but also its propensity to successfully form and perpetuate higher forms of allopolyploidy.
It may be argued that 2n gametes could have functioned in both progenitors, thereby producing an instant amphidiploid that became bread wheat. However, the formation of 2n gametes in the diploid progenitor Triticum tauschii would perhaps be improbable because 2n gametes occur very rarely, if at all, in diploid species. Thus, chromosome doubling most plausibly occurred via fusion of unreduced gametes in the BAD hybrid (amphihaploid) because such gametes are known to occur in interspecific and intergeneric hybrids but not in diploid parent species (Ceoloni and Jauhar 2006; Mujeeb-Kazi 2006). The reproductive behavior of the BAD haploids seems to exemplify the types of natural events that can create allopolyploids.
Because meiotic phenomena leading to formation of unreduced gametes occurred in Ph1-haploids of durum wheat which lacked pairing (Jauhar et al. 2000), it was inferred that Ph1-induced lack of pairing is a prerequisite for the occurrence of meiotic restitution and sexual polyploidization. That is understandable because Ph1-induced lack/failure of pairing would be conducive to formation of univalents and FDR. Without this regulatory mechanism, the A, B, and D genomes would have converged during the several centuries they have been together in the same cellular environment (see also Jauhar 2006). Meiotic restitution leads to chromosome doubling and hence fertility in several amphihaploids (interspecific and intergeneric hybrids) in the Triticeae involving wheat and its progenitors or allied species (Xu and Joppa 2000; Jauhar 2003a, 2006; Matsuoka and Nasuda 2004). These findings lend support to Jauhar's hypothesis (2003a) that 3 factors, namely, sexual reproduction, allopolyploidy, and genetic control of chromosome pairing, jointly constitute a perfect recipe for rapid or cataclysmic evolution in nature (see also Jauhar 2006). Here, we have shown in induced bread wheat polyhaploids the meiotic phenomena that result in parental bread wheat, essentially a simulation of the evolutionary steps that occurred at the time of origin of these polyploid wheats. The occurrence of meiotic restitution in interspecific or intergeneric hybrids (amphihaploids), involving wheat and other grasses, is a potent force in the evolution of the Triticeae.
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Acknowledgments |
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I acknowledge the excellent technical help of Terrance Peterson and Kara Burt in this study, and of Emilie Hanson in the preparation of the manuscript. I am also grateful to Dr David Stelly for critically reading the manuscript and for giving several useful suggestions. Mention of trade names or commercial products in this publication is solely to provide specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
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Footnotes |
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Corresponding Editor: Reid Palmer
Received July 26, 2006
Accepted November 16, 2006
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