In tergeneric crosses with Fragaria and Potentilla. II. Crosses between the progeny of Fragaria moschata x Potentilla fruticosa and the original parents

Summary An aneuploid hybrid (2n=23) of Fragaria moschata (2n=42) and Potentilla fruticosa (2n=14) was backcrossed with pollen of both parents, separately and combined in a pollen mixture. Seven vigorous progeny were obtained. The origin of the exeptional chromosome numbers, 2n=44, 49, 63, 63, 65, 67, 67, is discussed, and it is shown that each of the numbers could be produced by the fertilisation of unreduced and double unreduced gametes of the hybrid by normally reduced gametes of one of the parental species.     

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.     

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.     

Unreduced gamete in interspecific hybrids in the Festuca/Lolium complex

Self-sterile interspecific hybrids between diploid Lolium perenne and either Festuca arundinacea or F. gigantea (both hexaploids) set some seeds when subjected to a heavy pollen cloud from the amphiploids derived from the hybrids. All the progeny were octoploid and were formed from the fertilization of unreduced gametes in the F₁ hybrid by amphiploid pollen. The significance of the results in relation to breeding efforts within the Lolium/Festuca complex is discussed.     

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.     

Breeding for quantitative resistance to potato cyst nematode (Globodera pallida) in tetraploid potatoes using dihaploids and unreduced gametes

Summary Dihaploids were produced from tetraploids resistant to potato cyst nematode (Globodera pallida (Stone)). High levels of resistance were found in the dihaploids and three were used to produce tetraploid progenies by crossing them with susceptible tetraploid cultivars. One dihaploid, PDH505, produced more highly resistant offspring than the other two, PDHs 417 and 418. The latter gave progenies whose levels of resistance were similar to those obtained from susceptible dihaploids crossed with resistant tetraploids.The differences between the progenies of the resistant dihaploids were probably due to different modes of unreduced gamete formation (PDH505 producing gametes by first division restitution (FDR) and PDHs 417 and 418 by second division restitution (SDR)) although cytological studies would be necessary to confirm this. The methods by which dihaploids could be utilised in a tetraploid potato breeding programme are discussed in relation to the mode of unreduced gamete formation.     

Spontaneous haploids in durum wheat: their cytogenetic characterization

Summary Durum or macaroni wheat (Triticum turgidum L., 2n = 4x = 28; AABB genomes) arose as a natural hybrid between two wild species, Aegilops speltoides Tausch (2n = 2x = 14; BB genome) and Triticum urartu Tumanian (2n = 2x = 14; AA genome). The two progenitors hybridized in nature about half a million years ago and gave rise to tetraploid wheat, presumably in one step as a result of functioning of unreduced gametes in their hybrid BA (amphihaploid). It is easily possible to go back on the evolutionary scale and obtain durum haploids BA, and then regenerate tetraploid durum plants from them. Interestingly, such a reversion to haploidy does occur in nature as well, although at a very low frequency. This article reports on the occurrence of two spontaneous durum haploids and describes their chromosomal characteristics. The haploids (euhaploids, to be precise) had 14 somatic chromosomes, which, on fluorescent genomic in situ hybridization (fl-GISH), could be distinguished as 7 A-genome and 7 B-genome chromosomes. At meiosis, only 2.3 and 2.7% of the chromosomes paired in the two haploids, because of the presence of the homoeologous pairing-suppressor gene, Ph1. The Ph1-induced lack of pairing is a prerequisite for chromosome doubling through the formation of unreduced gametes that give rise to tetraploid durum wheats.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.     

Interspecific hybridization of a multiploid mutant of durum wheat with rye and Triticum monococcum L. results in pentaploid hybrids

The multiploid mutant of durum wheat is a genotype that produces unreduced gametes. Our objective was to test the recovery of pentaploid hybrids in crosses of the mutant with rye and Triticum monococcum L. Compared with check crosses, the mutant had a two‐third reduction in percent seed set for rye crosses, but had only a slight decrease in crossability with T. monococcum. Pentaploid hybrids were associated with plump seeds of the mutant/rye cross, and with shrivelled seeds of the mutant/T. monococcum cross. We suggest that the endosperm balance number hypothesis explains the association of pentaploid hybrids with endosperm type. This association made for easy recovery of pentaploid hybrids from crosses to both species. Mature, plump seeds from the mutant/rye cross were germinated and pentaploid hybrids were recovered. One pentaploid hybrid was recovered for every 50.5 and 15.1 florets pollinated with rye and T. monococcum, respectively. Unreduced gametes in the multiploid mutant will facilitate interspecific hybridization by reducing the time to produce pentaploid plants.     

Intergeneric hybridization of Diplotaxis erucoides with Brassica napus. I. cytogenetic analysis of F1 and BC1 progeny

Summary Intergeneric hybrids (F₁) Diplotaxis erucoides (DeDe) x Brassica napus (AACC) and the first backcross to B. napus (BC₁) have been obtained through in vitro culture of excised ovaries. The chromosome numbers of F₁ and BC₁ plants proved the occurrence of unreduced gametes. The study of metaphase I chromosome pairing showed that autosyndesis in De genome and allosyndesis between De and A/C genomes might exist. The male fertility of the F₁ plants was low. Some male-sterile plants were found in F₁ and BC₁ progeny. The possibilities of creating addition lines B. napus-D. erucoides and of obtaining a new cytoplasmic male sterility in B. napus are discussed.     

Spontaneous triploids in forage kale,Brassica oleracea var. acephala

Summary On screening samples of seed of commercial kale for spontaneous haploids, unexpectedly large numbers of triploid seeds were found in the small seed fractions. These are presumed to have arisen from unreduced gametes and may be similar in origin to diploid maternals, more usually obtained via interspecific pollination.     

波斯小麦×节节麦杂种F_1直接形成双二倍体的细胞遗传学研究

波斯小麦(Triticum carthlicum,AABB)与节节麦(Aegilops tauschii,DD)杂交,杂种 F_1出现可育性。不同组合的自交结实率21.25—39.72%。细胞学研究表明,可育性是由于杂种 F_1通过两种途径形成了未减数配子:第一,一部分花粉母细胞第一次分裂消失,只发生第二次分裂。即21条染色体集中到赤道面上之后,姊妹染色单体均等分离并移向     

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.     

Numerically unreduced (2n) gametes and sexual polyploidization in Actinidia

This is the first report on the production of numerically unreduced (2n) gametes in Actinidia species. Their occurrence was apparent from direct observations of microporogenesis in 2x, 3x, 4x and 6x plants and was inferred from ploidy analysis of progenies from 4x-2x and 2x-4x crosses. Both male and female plants were found to produce 2n gametes. 2n gamete producers were detected only in interploidy crosses. The observed frequency of 2n gametes was either similar to or towards the lower end of the ranges reported for other genera. The role of sexual polyploidization in the evolution of the genus is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The occurrence of 2n-pollen and the origin of sexual polyploids in dihaploid roses (Rosa hybrida L.)

Haploidisation by in situ parthenogenesis of 4x R. hybrida resulted in the production of some dihaploid roses (2n=2x=14) able to produce viable pollen. A cytological study of microsporogenesis revealed that, although the first meiotic reductional division occurred normally, the second (equational) division was characterised by frequent abnormalities which concerned spindle formation and led to unreduced gametes of First Division Restitution (FDR) type. Analysis of the hybrid progeny of a parthenogenetically derived male fertile dihaploid plant revealed a selective advantage of the 2n-pollen, especially in the case of an hybridization with a tetraploid female parent. Moreover, crosses carried out among dihaploid partners always resulted in hybrids with ploidy levels≥ 3x. This revised version was published online in July 2006 with corrections to the Cover Date.     

Trifolium ambiguum as a source of variation in the breeding of white clover

Summary Hybrids between T. ambiguum and T. repens were successfully established after ovule culture. The hybrids were male sterile but one of the hybrids produced a single seed when backcrossed to T. repens, while the other produced a significantly greater number of seeds. The majority of the backcross hybrids from the latter had 48 chromosomes, being produced from the fertilisation of unreduced gametes of the F1 hybrid by T. repens pollen. Regular meiosis in the BC1 hybrid resulted in the production of balanced gametes containing the full complement of T. repens chromosomes and the polyhaploid set of T. ambiguum chromosomes. There was no difficulty in establishing a substantial population of BC2 hybrids. The meiotic behaviour of the BC1 hybrid indicated that it should be possible to transfer genes from T. ambiguum into white clover T. repens. The significance of the results in terms of the use of alien variation in the improvement of white clover is discussed.     

果树2n配子研究进展

植物在进化过程中能产生低频率的未减数分裂配子,即2n配子,它在果树育种方面发挥着重要的作用。本文详述了2n配子的主要研究概况、发生途径及影响因素,为其在植物育种、种质利用和种质创新等方面提供了重要的应用基础。同时,通过近年的研究成果,总结出诱导方法和人工分离纯化方法,不同品种可根据其不同的生理生化特性选择合适的方法。另外,也叙述了2n配子在果树方面的特殊应用潜力,即在有性多倍化、稳定传递杂合性、克服自交不亲和和克服胚乳平衡数障碍方面有其他育种方法无法比拟的优势。因此,2n配子育种将会是一种重要的育种途径。     

A re-examination of the mechanisms of 2n gamete formation in potato and its implications for breeding

Summary The claim that first division restitution gametes are produced through the unique meiotic abnormality of parallel spindle formation at anaphase II of microsporogenesis in Solanum species appears to be not justified. An examination of the phenomenon of meiotic nuclear restitution in fifty-six diploid Solanum clones revealed that the parallel orientation of spindles either at metaphase or anaphae II does not necessarily lead to dyad or unreduced microspore formation. However, the fused spindle formation at metaphase II of microsporogenesis invariably leads to dyad and unreduced microspore formation. The 2n gametes produced through fused spindles may be genetically equivalent to first division restitution gametes, and therefore, they may be useful in potato breeding. In addition to fused spindle formation, at least three other types of meiotic abnormalities that could lead to 2n gamete formation were also observed. Although any one, or two, of these mechanisms were predominantly observed in certain clones, it was by no means certain that other mechanisms did not operate in these clones at the same time, or at a later stage. Because of the great modifying influence of environment on the process of meiosis, and meiotic nuclear restitution, it might be erroneous to conclude that one or the other meiotic nuclear restitution mechanism alone operates at a time in a given clone. In order to elucidate the genetic basis of 2n gamete formation in potato, it is desirable (i) to answer whether there are exclusive genetic mechanisms for the formation of first or second division restitution gametes; (ii) to establish reliable cytological criteria for their detection; and (iii) to determine the effect of environmental factors on the occurrence of different mechanisms.     

Formation of 2n gametes in durum wheat haploids: Sexual polyploidization

Allopolyploidy, resulting from interspecific and intergeneric hybridization accompanied by sexual doubling of chromosomes, has played a major role in the evolution of crop plants that sustain humankind. The allopolyploid species, including durum wheat, bread wheat, and oat, have developed a genetic control of chromosome pairing that confers on them meiotic regularity (diploid-like chromosome pairing), and hence reproductive stability, and disomic inheritance. Being natural hybrids, they enjoy the benefits of hybridity as well as polyploidy that make them highly adaptable to diverse environments. Despite the complexities of sexual reproduction, it is widespread among plants and animals. Sexual polyploids are highly successful in nature. Sexual polyploidization is far more efficient than somatic chromosome doubling. Sexual polyploidization effected by functioning of unreduced (2n) gametes in the parental species or in their hybrids has been instrumental in producing our grain, fiber, and oilseed crops. Evidence is presented for the occurrence of sexual polyploidization in durum haploids. ThePh1-induced failure of homoeologous pairing is an important factor in the formation of first division restitution(FDR) nuclei and 2n gametes. The evolutionary and breeding significance of sexual polyploidization is discussed. It is emphasized that three factors, viz.,sexual reproduction, allopolyploidy, and genetic control of chromosome pairing,jointly constitute a perfect recipe for cataclysmic evolution in nature. This revised version was published online in July 2006 with corrections to the Cover Date.     

A tetraploid hybrid plant from 4x × 2x crosses in Vitis and its origin

To study the origin of unreduced (2n) gametes in diploid Vitis cultivars, we surveyed the occurrence of tetraploid hybrid seedlings from 40 interploid crosses with five tetraploid and seven diploid cultivars. A total of 250 seedlings from the interploid crosses were established through embryo culture and by seed sowing. In 20 2x × 4x crosses, no tetraploid hybrid seedlings were derived from 8,902pollinations. In 20 4x × 2x crosses, two tetraploid hybrid seedlings were obtained from 8,057 pollinations. Investigation of isozyme genotypes of the two tetraploid seedlings using three variable enzyme systems indicated that one of the two seedlings resulted from the union of a diploid egg with a 2n male gamete and that failure of second meiotic division resulted in the formation of the 2n male gamete. The percentage of giant pollen grains in `Muscat Bailey A', a pollen parent of the tetraploid seedling, was relatively high(about 5.9%) and 10.9% of the giant pollen grains germinated on agar medium. These results suggested that 2n pollen of diploid cultivars are useful for breeding tetraploid grape. This revised version was published online in August 2006 with corrections to the Cover Date.