Occurrence of 2n gametes in the F1 hybrids of Oriental × Asiatic lilies (Lilium): Relevance to intergenomic recombination and backcrossing

Cytological modes of the origin of 2n gametes were investigated in six different genotypes of F₁ hybrids between Oriental and Asiatic (OA) lilies (Lilium, 2n = 2x = 24). Chromosome pairing between the parental genomes was very low, the average frequency range from 0.3 to 1.2 bivalents per cell among the genotypes. Within a genotype the frequency of bivalents varied from 0 to 6 in some cases. The normally occurring haploid pollen grains were totally sterile. In contrast, in different genotypes, variable percentages of 2n pollen were found and shown to be fertile as estimated from pollen germination. A cytological analysis of Metaphase I and subsequent stages of meiosis using genomic in situ hybridization (GISH) revealed that there was intergenomic recombination between the alien genomes. Following Metaphase I stage, three different types of abnormal cytological events led to the formation of 2n pollen: (i) Post-Metaphase I division (PMI), (ii) Post-Metaphase II division (PMII) and (iii) Asymmetric Cytokinesis of the pollen mother cell followed by chromosome division. All three cytological events led to first division restitution (FDR) gametes. Based on in vitro pollen germination it was proved for two genotypes that 2n pollen was viable only during the first day of anthesis. It was possible to use 2n pollen successfully for backcrossing. Implications of 2n pollen for intergenomic recombination in BC₁ progenies are discussed.     

Progenies of allotriploids of Oriental × Asiatic lilies (Lilium) examined by GISH analysis

With the aim of utilizing allotriploid (2n = 3x = 36) lily hybrids (Lilium) in introgression breeding, different types of crosses were made. First, using diploid Asiatic lilies (2n = 2x = 24), reciprocal crosses (3x − 2x and 2x − 3x) were made with allotriploid hybrids (AOA) obtained by backcrosses of F₁ Oriental × Asiatic hybrids (OA) to Asiatic cultivars (A). Secondly, the AOA allotriploids were crossed with allotetraploid (OAOA, 2n = 4x = 48), in 3x − 4x combination. Finally, the AOA allotriploids where crossed to 2n gamete producer F₁ OA hybrids (3x − 2x (2n)). Two types of triploids were used as parents in the different types of crosses, derived from: (a) mitotic polyploidization and (b) sexual polyploidization. Ploidy level of the progeny was determined by estimating the DNA values through flowcytometry as well as chromosome counting. The aneuploid progeny plants from 3x − 2x and reciprocal crosses had approximate diploid levels and in 3x − 4x crosses and 3x − 2x (2n) the progeny had approximate tetraploid levels. Balanced euploid gametes (x, 2x and 3x) were formed in the AOA genotypes. Recombinant chromosomes were found in the progenies of all crosses, except in the case of 2x − 3x crosses through genomic in situ hybridization (GISH) analyses. Recombinant chromosomes occurred in the F₁ OA hybrid when the triploid AOA hybrid was derived through sexual polyploidization, but not through mitotic polyploidization with two exceptions. Those recombinant chromosomes were transmitted to the progenies in variable frequencies.     

Production of 2n pollen of Asiatic hybrid lilies by nitrous oxide treatment

Tetraploid varieties of lilies have superior agronomic traits such as large flowers and resistance to physiological disorders. In the present study, we attempted to induce 2n pollen of Asiatic hybrid lilies by arresting the meiotic process with nitrous oxide (N₂O) gas. To determine which meiotic stage is optimal for induction of 2n pollen, plants with attached buds at different meiotic stages were treated with N₂O for 24 h in a pressure-tolerant cylinder. A few 2n pollen grains were induced using plants with anthers in prophase I, whereas mixed pollen grains of differing size were produced using plants undergoing meiotic metaphase predominantly in anthers. Although normal lily pollen grains are elliptical, nitrous oxide exposure induced giant pollen grains that appeared spherical. Flow cytometry analysis showed that the giant pollen grains were diploid. When mixed pollen that included normal and giant pollen was crossed to tetraploid cultivars, the resulting seedlings were tetraploid and aneuploid, indicating that the giant pollen grains were diploids that could generate tetraploid seedlings through fusion to diploid eggs supplied from a tetraploid female parent. Thus, treatment with N₂O is useful for the production of 2n lily pollen and may provide a new approach for tetraploid lily breeding.     

N2O induces mitotic polyploidization in anther somatic cells and restores fertility in sterile interspecific hybrid lilies

Fertile plants undergoing male gametogenesis can be treated with nitrous oxide (N₂O) gas to obtain 2n male gametes. N₂O treatment is also expected to restore the fertility of interspecific hybrids through meiotic restitution or mitotic amphidiploidization. However, this technique has few applications to date, and it is un-known how N₂O treatment restores fertility in sterile hybrids. To establish optimal N₂O treatment conditions and determine its cytological mechanism of action, we treated various sized floral buds with N₂O gas at different anther developmental stages from fertile and sterile hybrid lilies. N₂O treatment using the optimal 1–4 mm floral buds induced mitotic polyploidization of male archesporial cells to produce 2n pollen in fertile hybrid lilies. In sterile hybrid lilies, N₂O treatment doubled the chromosome number in male archesporial cells followed by homologous chromosome pairing and normal meiosis in pollen mother cells (PMC), resulting in restoration of pollen fertility. Backcrossing the resultant fertile pollen to Lilium × formolongi produced many triploid BC₁ plants. Thus N₂O treatment at the archesporial cell proliferating stage effectively overcame pollen sterility in hybrid lilies, resulting in fertile, 2n pollen grains that could produce progeny. The procedure presented here will promote interspecific or interploidy hybridization of lilies.     

Occurrence, inheritance and use of reproductive mutants in alfalfa improvement

The widespread occurrence of 2ngametes (i.e. gametes with the somatic chromosome number) in the Medicago sativa-coerulea-falcata complex supports the concept that gene flow from diploid to tetraploid species occurs continuously in nature and plays a key role in alfalfa evolution. Breeders realized early that gene transfer between ploidy levels via 2n pollen and 2n eggs would have had potential use in cultivated alfalfa improvement. Cytological investigations provided insights into the types of meiotic abnormalities responsible for the production of 2n gametes. Alterations were defined as genetically equivalent to first (FDR) or second division restitution(SDR) mechanisms. For breeding purposes,data have proven that 2n gametes of the FDR type are more advantageous than those obtained by SDR for transferring parental heterozygosity and retaining epistatic interactions. The use of diploid meiotic mutants that produce 2ngametes is now recognized as one of the most effective methods available for exploiting heterosis and introgressing wild germplasm traits into cultivated tetraploid alfalfa via unilateral (USP) and bilateral sexual polyploidization (BSP) schemes. Both2n egg and 2n pollen producers could be used for direct gene transfer from wild diploid relatives into cultivated alfalfa by means of 2x-4x and4x-2x crosses. Although data have shown that forage yield improvement can be achieved when plants are sexually tetraploidized, problems related to reduced plant fertility and seed production remain largely unexplained. Apomixis has the potential of cloning plants through seed and thus provides a unique opportunity for developing superior tetraploid cultivars with permanently fixed heterosis and epistatic effects. A main goal in alfalfa breeding could be the introduction of functional apomixis (i.e. Apomeiosis and parthenogenesis) in cultivated alfalfa stocks. In the future, the efficiency of alfalfa breeding programs based on the use of reproductive mutants could be improved by direct selection at the genotype level using RFLPs and PCR-based markers. Suitable DNA markers and detailed linkage maps of alfalfa mutants should help to discover apomictic mutants and address basic genetic issues such as the extent of genomicre combination in polyploid hybrids and the effect of sexual polyploidization on heterosis. Molecular markers have recently been used in alfalfa for studying the inheritance of 2n gamete formation and identifying polymorphisms associated to genes involved in meiotic abnormalities. Molecular tagging of 2n egg and 2n pollen formation not only should explain the genetic control and regulation of these traits, but may also be an essential step towards marker-assisted selection of 2n gamete producers and implementation of USP and BSP breeding schemes. Future perspectives include strategies for the map-based cloning of genomic DNA markers,and screening of EST mini-libraries related to flowers at different developmental stages from meiotic mutants and wild-type scan lead to the identification of mRNAs and thus of candidate genes that control 2n gamete formation in alfalfa. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relevance of unilateral and bilateral sexual polyploidization in relation to intergenomic recombination and introgression in <Emphasis Type="Italic">Lilium</Emphasis> species hybrids

Sexual polyploids were induced in diploid (2n = 2x = 24) interspecific F1 hybrids of Longiflorum × Asiatic (LA) and Oriental × Asiatic (OA) Lilium hybrids by backcrossing to Asiatic (AA) parents as well as by sib-mating of the F1 LA hybrids. A majority of the BC1 progenies were triploid and the progenies from sib-mating were tetraploid or near tetraploids. Genomic in situ hybridization (GISH) technique was applied to assess the intergenomic recombination in the BC1 populations of LA and OA hybrids obtained after unilateral sexual polyploidization. A total of 63 LA (LA × AA and AA × LA) and 53 OA hybrids were analysed. LA hybrids were originated through the functioning of 2n gametes either as 2n eggs or 2n pollen while those of OA hybrids originated through functional 2n pollen of diploid OA genotype. In both type of crosses, a majority of the progenies had originated through First Division Restitution (FDR) mechanism of functional 2n gamete either with or without a cross over. However, there were nine LA- and four OA-genotypes where Indeterminate Meiotic Restitution (IMR) was the mechanism of 2n gamete formation. Based on GISH, total amount of introgression of Longiflorum and Oriental genome into Asiatic genome was determined. Most of the BC progenies exhibited recombination and the amount of recombination was higher in LA hybrids as compared to OA hybrids. Intergenomic recombination was also determined cytologically in the 16 plants of sib-mated LA hybrids where both parents had contributed 2n gametes. Based on these results the nature of interspecific lily hybrids obtained from uni- and bilateral sexual polyploidization leading to allotriploid and allotetraploid formation is discussed in the context of introgression and intergenomic recombination.     

Effect of selection and temperature stress on the production of 2n gametes in Lotus tenuis

Five Lotus tenuis genotypes showing 2n pollen production greater than 5% and producing 4x progenies in 4x(male sterile Lotus corniculatus) × 2x crosses were found in different natural populations. Meiotic analyses of these plants showed some second metaphases with disoriented spindles, i.e. parallel and tripolar spindles which resulted in diads and triads, respectively. Because useful genes for winter growth and resistance to powdery mildew from L. tenuis can be introgressed into L. corniculatus via 2n gametes, a programme of selection was started to increase the occurrence of 2n gamete production in L. tenuis. Controlled crosses were performed between genotypes producing 2n gametes and the progenies were checked for 2n pollen production and meiotic aberration, following the same methodologies as used for the parental plants. Although in some progeny plants the frequency of 2n gamete production increased remarkably, the overall progeny level of 2n gamete production remained unchanged. This suggested that different genes for meiotic aberration exist in natural populations and/or differential seed-setting ability exists between genotypes. Diallel crosses between selected genotypes confinned this hypothesis. Evaluation of 2n pollen production of clones grown under different temperature regimes (warm vs. temperate) showed that, in L. tenuis, the effects of genotype, temperature and genotype × temperature interaction are significant. In particular, high temperature increases the level of 2n gamete production in L. tenuis.     

Induction of 2n pollen in tulips by arresting the meiotic process with nitrous oxide gas

Triploid tulips have agronomically desirable traits such as vigorous growth and large flower size, but only a portion of all cultivated tulips is triploid. To apply 2n pollen to polyploid breeding of tulips, the polyploidizing agent, nitrous oxide gas (N₂O), was applied to bulbs. In tulips, meiosis in anthers occurs inside the bulbs from mid- to late-October. When meiosis in anthers (excised from bulbs) reached metaphase I, we treated other bulbs of the same clones with N₂O for 24–48 h. Most of the treated plants produced pollen grains with a wide-ranging or bimodal size distribution, indicating a mixture of n, 2n and aneuploid pollen grains. The use of pollen containing a relatively high proportion of giant pollen grains tended to yield larger numbers of triploids in the progeny. The number of giant pollen grains could be increased when N₂O-treated pollen grains were suspended in 10% sucrose and then sieved through a nylon mesh. Very few polyploids were observed in some cross combinations, even those involving pollen with a relatively high proportion of giant grains. Even so, this low polyploid yield most likely is due to a triploid block, because the capsules obtained in the crosses of the diploid×N₂O-treated plants contained some abnormal seeds, which were mostly triploid. Embryo culture was useful in rescuing abnormal embryos. The present study reveals that 2n pollen can be produced at high frequency using N₂O during tulip breeding.     

Induction of 2n pollen formation in <Emphasis Type="Italic">Begonia</Emphasis> by trifluralin and N<Subscript>2</Subscript>O treatments

In this study, treatments of both trifluralin (at 10, 100 and 1000 μM) and N₂O (in the form of gas under pressure) were applied to Begonia flower buds to induce the formation of 2n pollen. Three male fertile species (B. cucullata, B. subvillosa var. leptotricha and B. fischeri) and two male sterile hybrids (B. schmidtiana × B. cucullata and B. subvillosa var. leptotricha × B. cucullata) were treated. Pollen size, which is related to pollen DNA content, increased after both N₂O and trifluralin treatments, but the induction of large pollen was genotype dependent. Trifluralin induced large pollen only in the male fertile species, while N₂O treatments induced fertile 2n pollen in the male sterile B. schmidtiana × B. cucullata. Cytological studies showed that trifluralin induced multinuclear monads that resulted in 4n gametes in stead of 2n gametes. In general, large pollen obtained after trifluralin treatments showed low germination capability, while large pollen obtained after N₂O treatments retained high germination capability. Seedlings with raised ploidy level could only be obtained after crosses were performed with large pollen obtained from N₂O treatments. Hence, N₂O treatments are preferable to the use of trifluralin to induce 2n gametes in Begonia.     

Meiotic restitution mechanisms and 2n pollen formation in a Solanum tuberosum dihaploid and in dihaploid × wild species hybrids*

A Solatium tuberosum dihaploid that proved to be a 2n pollen producer, exhibited two types of meiotic abnormality involved in 2n pollen formation: premature cytokinesis at prophase II and parallel/tripolar spindles in meiosis II. For this reason such a dihaploid was preferentially used as a seed parent in crosses with the diploid species S. tarijense, S. sanctae-rosae, and S. phureja. Diploid interspecific hybrids were evaluated for male fertility, for 2n pollen production and stability over three years and for meiotic nuclear restitution mechanisms. Pollen stainability ranged from 6 to 85% and 2n pollen from 5 to 35% among hybrids. The meiotic mechanism generating 2n pollen was that of spindle abnormality in meiosis II and consequent dyad/triad formation. High correlation was found between fused/parallel spindles and dyads.     

Relevance of sexual polyploidization for crop improvement – A review

Colchicine induced polyploids have not directly contributed for crop improvement in the past. On the other hand, the so-called natural polyploids, derived from the functioning of numerically unreduced(2n) gametes have been shown to be more relevant for crop improvement in many cases. Different types of cytological abnormalities during meiosis can give rise to 2n gametes and the genetic composition of these gametes is variable. Depending on the type meiotic abnormalities, various types of 2ngametes, such as first division restitution(FDR), second division restitution (SDR),indeterminate meiotic restitution (IMR) and post meiotic restitution (PMR) gametes,among others, have been described in recent years. For the improvement of autopolyploids such as potato, alfalfa,Vaccinium spp., and some of the fodder grasses, FDR gametes have been proved to be highly useful. However, the use of 2n gametes for the improvement of allopolyploid crops has received much less attention so far. Some of the investigations on allopolyploids, derived from Festuca-Lolium, Alstroemeria and Lilium species hybrids, have revealed that 2ngametes can be most useful for the introgression of alien genes and chromosomes into cultivars. An important feature of using sexual polyploidization in the case of allopolyploids is that introgression can be achieved through recombination due to genetic crossing-over between alien chromosomes as well as addition of alien chromosomes, which is extremely difficult or impossible to achieve in the case of colchicine induced allopolyploids. Because of the recent developments in the field of plant molecular biology, methods have become available for the analysis of 2ngametes and sexual polyploid progenies more accurately and to develop systematic breeding approaches. The methods include DNA in situ hybridization (GISH and FISH)and molecular mapping (AFLP, RFLP, RAPDs).In addition to providing basic information on the genetic and genome composition of the polyploid progenies, these methods can be potentially useful for a more efficient creation of desirable breeding material and cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Meiotic behavior in interspecific hybrids between Brachiaria ruziziensis and Brachiaria brizantha (Poaceae)

The meiotic behavior of two half-sib interspecific tetraploid (2n = 4x = 36) promising hybrids, a sexual and an apomictic one, from crosses B. ruziziensis and B. brizantha, was evaluated. Although chromosome paired predominantly as bivalents, a few tri- and quadrivalents were recorded. Results suggest that B. brizantha and B. ruziziensis are closely related and genetic recombination is expected in hybrids. Introgression of specific target genes from B. ruziziensis into B. brizantha and vice-versa may be foreseen. However, abnormalities such as irregular chromosome segregation, chromosome stickiness and abnormal cytokinesis reported in these hybrids affect pollen fertility. More than 65% of pollen grains are sterile. Since the distinctive cytological feature of these hybrids is abnormal cytokinesis, this fact suggests that both parental genomes are unable to coordinate their activities with regard to this cytological phenomenon. Deployment of such hybrids in the process of developing varieties is discussed.     

Occurrence of numerically unreduced (2n) gametes in Alstroemeria interspecific hybrids and their significance for sexual polyploidisation

The F1 hybrids of seven diploid Alstroemeria species (2n=2x=16) were investigated for the production of numerically unreduced (2n) gametes and their mode of origin. Based on a survey of 17 interspecific hybrid combinations,consisting of 119 genotypes, it was found that the F1 hybrids of Chilean-Brazilian species mostly produced first division restitution (FDR) 2n gametes. These F1 hybrids were self-pollinated in order to obtain F2 seeds, which was an indication that the F1 plants also produced 2neggs simultaneously. All the F2 progeny plants were typical allotetraploids, most of which formed 16 bivalents and a small proportion formed multivalents during metaphase I stages of meiosis. Through genomic in situ hybridisation (GISH) it was proved that multivalent formation in F2plants, derived from A. inodora ×A. pelegrina hybrid, was due to homoeologous recombination but not from reciprocal translocations. In order to test the segregation pattern of the recombinant chromosomes, an F3 population from one genotype, P6C49-6, was investigated. The recombinant chromosomes assorted independently from each other supporting the hypothesis that the segregation of chromosomes in ring quadrivalents did not behave like those in translocation heterozygotes. It was concluded that in allopolyploids of Alstroemeria,bilateral sexual polyploidisation could accomplish genetic recombination by both homoeologous crossing-over as well as through the assortment of chromosomes. This revised version was published online in July 2006 with corrections to the Cover Date.     

Formation of unreduced gametes is impeded by homologous chromosome pairing in tetraploid <Emphasis Type="Italic">Triticum turgidum</Emphasis> × <Emphasis Type="Italic">Aegilops tauschii</Emphasis> hybrids

It is believed that unreduced gametes with somatic chromosome numbers play a predominant role in natural polyploidization. Allohexaploid bread wheat originated from spontaneous hybridization of Triticum turgidum L. with Aegilops tauschii Coss. Unreduced gametes originating via meiotic restitution, including first-division restitution (FDR) and single-division meiosis (SDM), are well documented in triploid F₁ hybrids of T. turgidum with diploid Ae. tauschii (genomic constitution ABD, usually with 21 univalents in meiotic metaphase I). In this study, two T. turgidum lines known to carry genes for meiotic restitution were crossed to tetraploid Ae. tauschii. The resulting F₁ hybrids (genomes ABDD), had seven pairs of homologous chromosomes and regularly formed 14 univalents and seven bivalents at metaphase I. Neither FDR nor SDM were observed. The distribution of chromosome numbers among progeny obtained by self pollination and a backcross to T. turgidum showed the absence of unreduced gametes. These results suggest that high homologous pairing interfered with meiotic restitution and the formation of unreduced gametes. This may be related to asynchronous movement during meiosis between paired and unpaired chromosomes or to uneven distribution of chromosomes in anaphases, resulting in nonviable gametes.     

Meiotic studies of spontaneous hybrids of Amaranthus: genome analysis

The meiotic behaviour of 13 spontaneous interspecific F₁ hybrids of Amaranthus was studied. The hybrids between species with n= 16 chromosomes had 16 bivalents but varied considerably in pollen stainability (0–55%). These results suggest the existence of cryptic structural hybridity. The hybrids involving A. cruentus (n = 17) and species with n = 16 (A. caudatus and A. quitensis) always formed 15II+1 III with very low pollen stainability (5–7%). Further observations indicated that Amaranthus species are allotetraploids with basic numbers of x= 8 and x= 9 but exhibit x= 16 and x= 17 as secondary basic numbers, as demonstrated by (a) the frequent presence of 811 + 171 in the meiosis of the hybrid A. spinosus (n = 17) × A. hybridus (n= 16); and the occurrence of secondary associations between bivalents in MI. Genomic formulae are proposed for each species, on the basis of the meiotic behaviour of the hybrids studied.     

The transfer of triazine resistance from Brassica napus L. to B. oleracea L. III. First backcross to parental species

Summary Atrazine resistant Brassica napus × B. oleracea F₁ hybrids were backcrossed to both parental species. The backcrosses to B. napus produced seeds in both directions but results were much better when the F₁ hybrid was the pollen parent. Backcrosses to B. oleracea failed completely but BC₁s were rescued by embryo culture both from a tetraploid hybrid (2n = 4x = 37; A₁C₁CC) and sesquidiploid hybrids (2n = 3x = 8; A₁C₁C). Progeny of crosses between the tetraploid hybrid and B. oleracea had between 25 and 28 chromosomes. That of crosses between the sesquidiploid hybrid and B. oleracea had between 21 and 27. A few plants that had chromosome counts outside the expected range may have originated from either diploid parthenogenesis, unreduced gametes or spontaneous chromosome doubling during in vitro culture. Pollen stainability of the BC₁s ranged from 0% to 91.5%. All the BC₁s to B. oleracea were resistant to atrazine.     

2n Gamete formation in the genus Brachiaria (Poaceae: Paniceae)

Microsporogenesis of several Brachiaria species of the Brazilian collection at Embrapa Beef Cattle has been analyzed in detail. This paper reports abnormal cytokinesis in three accessions of three different species (Brachiaria humidicola, 2n = 4x = 36, Brachiaria decumbens, 2n = 4x = 36, and Brachiaria dura, 2n = 6x = 54). Chromosomes paired in bi-, tri-, and quadrivalents in these accessions, whereas chromosome segregation at meiosis I was characterized by exclusion of laggards as micronuclei. In a high number of meiocytes, the first sign of cytokinesis appeared only in metaphase II and did not divide the meiocyte into a dyad. Total absence of cytokinesis was also detected among meiocytes in the second division. Since in both cases the two metaphase plates were very close, they favored the rejoining of chromosome sets after anaphase II and formed a restitutional nucleus in telophase II. Second cytokinesis occurred after telophase II in most meiocytes. Monads, dyads, and triads with n or 2n nuclei were observed among meiotic products. The 2n gametes observed correspond to the first division restitution (FDR). The number of affected cells in each accession was variable, but the number of microspores with restitutional nucleus, including those scored in tetrads and the released ones, did not exceed 9%. Although polyploidy is common in the genus Brachiaria, its origin is still unclear. Current results suggest that 2n gametes may have contributed to the evolutionary history of the genus.     

Intergeneric hybrid between Brassica napus and Diplotaxis harra through ovary culture and the cytogenetic analysis of their progenies

Brassica napus (2n = 38) and Diplotaxis harra (2n = 26) were used to investigate gene transfer from D. harra to B. napus. Intergeneric F₁ hybrids (dihaploid 2n = 32 chromosomes) were obtained through ovary culture. The chromosome associations in the first meiotic division was (0–2)_III + (2–10)_II + (12–28)_I. Many seeds were harvested in the F₁ hybrid after backcrossing with B. napus, and from open pollination of the F₁ hybrid. Somatic chromosome numbers of BC₁ and hybrid plants varied from 2n = 26 to 52. In the first meiotic division, high frequencies of bivalent association and relatively low pollen fertility were observed. BC₂ plants generated from the BC₁ plants with 2n = 38 chromosomes, 69.6% showed 2n = 38 chromosomes. Many aneuploids with addition and deletion of chromosomes were also obtained. A bridge plant between B. napus and D. harra with 2n = 32 chromosomes should be valuable material for the breeding of brassica crops.     

Frequent occurrence of unreduced gametes in <Emphasis Type="Italic">Triticum turgidum</Emphasis>–<Emphasis Type="Italic">Aegilops tauschii</Emphasis> hybrids

Spontaneous chromosome doubling via union of unreduced (2n) gametes has been thought to be the way that common wheat (Triticum aestivum L.) was originated from the hybridization of T. turgidum L. with Ae. tauschii Cosson. Previous works have observed unreduced gametes in F₁ hybrids of Ae. tauschii with six of the eight T. turgidum subspecies. It is not clear, however, whether the formation of these unreduced gametes is a norm in the F₁ hybrids. In the present study, we tried to answer this question by assessing the occurrence frequency of unreduced gametes in 115 T. turgidum–Ae. tauschii hybrid combinations, involving 76 genotypes of seven T. turgdium subspecies and 24 Ae. tauschii accessions. Our data show that these hybrid combinations differed significantly (P ≤ 0.01, F = 11.40) in selfed seedset, an indicator for production of unreduced gametes. This study clearly showed that meiotic restitution genes are widely distributed within T. turgidum. However, significant differences were found between as well as within T. turgidum subspecies and in the interaction of the T. turgidum genotypes with those of Ae. taushii. The possible application of the meiotic restitution genes from T. turgidum in production of double haploids is also discussed.     

Unreduced 3x gamete formation of allotriploid hybrid derived from the cross of Primula denticulata (4x)?×?P. rosea (2x) as a causal factor for producing pentaploid hybrids in the backcross with pollen of tetraploid P. denticulata

A triploid hybrid, which was obtained from interspecific crosses between tetraploid Primula denticulata (2n = 4x = 44) and P. rosea (2n = 2x = 22), successfully produced 11 plants by backcrossing with pollen of tetraploid P. denticulata. Analysis of ploidy level using flow cytometry and chromosome counting in the 11 BC₁ plants revealed that all progeny had much larger DNA contents and chromosome number than both parents. In this triploid-tetraploid (3x–4x) crossing, progeny was predominantly true or near pentaploid presumably produced by the fertilization between true or near triploid female gamete produced from triploid hybrid and diploid pollen of tetraploid P. denticulata. These results suggest that unreduced (3x) or near triploid female gametes were partially produced by single step meiosis, either first-division restitution or second-division restitution process. The zygotes formed by the fertilization between true or near triploid egg produced by single step meiosis in triploid hybrid and diploid pollen produced by normal meiosis of tetraploid P. denticulata might be the only survivors in embryogenesis.