Homoeologous Chromosome Pairing Research Articles

The origin of unusual chromosome constitutions among newly formed allopolyploids

Wide hybridization followed by spontaneous chromosome doubling of the resulting hybrids plays an important role in plant speciation. Such chromosome doubling is usually accomplished via unreduced gametes produced by altered meiosis, the so-called 'meiotic restitution'. Unreduced gametes are expected to carry somatic chromosome numbers and constitutions. However, it has been shown recently that new allopolyploids often carry unusual chromosome constitutions which include compensating and noncompensating nulli-tetrasomies and monotrisomies, and translocations of homoeologues. We have reanalyzed meiotic divisions in a wheat-rye hybrid by in situ probing with labeled DNA focusing on deviations from the standard pattern of meiotic restitution. In a typical first division restitution in a wide hybrid, there is no chromosome pairing, univalents separate sister chromatids in anaphase I, and there is no meiosis II. Here we illustrate that occasional pairing of homoeologous chromosomes in metaphase I, combined with separation of sister chromatids of univalents, generates diads with compensating nulli-disomies and associated translocations of homoeologues. Similarly, precocious metaphase I migration to the poles of some undivided univalents generates a wide range of noncompensating simple and complex nulli-disomies in the gametes. Both alterations to the standard pattern of meiotic restitution tend to maintain the somatic chromosome numbers in the gametes; chromosome constitutions are variable but mostly genetically balanced. This source of variation among progeny may be an important factor contributing to greater success of natural allopolyploids.

Elucidation of intergenomic recombination and chromosome translocation: meiotic evidence from interspecific hybrids of Lilium through GISH analysis

Intergenomic exchange has been found to be a normal phenomenon in sexual polyploids. In order to distinguish whether such exchanges are derived from intergenomic recombination or translocation, 13 genotypes of an interspecific hybrid, which were previously used as parents to generate sexual polyploids, were selected for a detailed meiosis analysis. In all genotypes under study, variable numbers of bivalents (0–12) resulting from homoeologous pairing were observed. But in two genotypes (006001-6 and 006001-13), a multivalent which was either a quadri- or a trivalent, as well as a bivalent involving two Asiatic chromosomes, was observed. Two Asiatic chromosomes in 006001-6 which were non-homologous were always found to be associated together, and formed trivalent or quadrivalent with chromosomes from homoeologous genome. This indicated that there was a duplication between these two Asiatic chromosomes. Such a duplication might have resulted from the segregation of a chromosomal translocation between two non-homologous chromosomes in the Asiatic parent ‘Connecticut King’ which was transmitted to the progeny (006001-6). With the exception of two genotypes, in 11 genotypes that formed variable frequencies of bivalents, the homoeologous chromosome pairing and chiasma formation were similar to that between homologous chromosomes. After from the analysis of anaphase I stages it was evident that the expected types of chiasma formation involving non-sister chromatids gave rise to two strand single, two strand double, three strand double, four strand double and multiple exchanges. Whereas these events resulted from locus specific homoeologous exchanges, the translocations resulted from an aberrant form of non-homologous chromosomal exchange of segments. Elucidation of such differences is only possible through the analysis of meiosis using GISH.

Cryptic homoeology analysis in species and hybrids of genus Zea

Cryptic intergenomic pairing of genus Zea was induced by the use of a diluted colchicine solution in order to elucidate the phylogenetic relations and differentiation of the homoeologous genomes. Results indicate that in species and hybrids with 2n = 20, there was chromosome pairing between the homoeologous A and B genomes with a maximum of 5IV, with the exception of Zea diploperennis and their interspecific hybrids where cryptic homoeologous chromosome pairing was not induced. In almost all 2n = 30 hybrids, observed cryptic pairing increased to a maximum of 10III although Z. mays × Z. mays with 2n = 30 did not show significant differences between treated and untreated materials. Pairing was also observed in species and hybrids with 2n = 40, in which a maximum of 10IV was observed, with the exception of Z. mays with 2n = 40 where treated and untreated cells did not differ significantly.

Localization of high level of sequence conservation and divergence regions in cotton

In a previous study, we observed that the variations in chromosome size are due to uneven expansion and contraction by comparing the structures and sizes of a pair of homoeologous high-resolution cytogenetic maps of chromosomes 12A and 12D in tetraploid cotton. To reveal the variation at the sequence level, in the present paper, we sequenced two pairs of homoeologous bacterial artificial chromosomes derived from high- to low-variable genomic regions. Comparisons of their sequence variations confirmed that the highly conserved and divergent sequences existed in the distal and pericentric regions, e.g., high- and low-variable genome size regions in these two pairs of cotton homoeologous chromosomes. Sequence analysis also confirmed that the differential accumulation of Gossypium retrotransposable gypsy-like element (Gorge3) accounted for the main contributions for the size difference between the pericentric regions. By fluorescence in situ hybridization analysis, we found that Gorge3 has a bias distribution in the A(T)/A proximal regions and is associated with the heterochromatin along the chromosomes in the entire Gossypium genome. These results indicate that, between A(T)/A and D(T)/D genomes, the distal and pericentric regions usually possess high level of sequence conservation and divergence, respectively, in cotton.

Comparison of homoeologous chromosome pairing between hybrids of wheat genotypes Chinese Springph1band Kaixian-luohanmai with rye

The ph-like genes in the Chinese common wheat landrace Kaixian-luohanmai (KL) induce homoeologous pairing in hybrids with alien species. In the present study, meiotic phenotypic differences on homoeologous chromosome pairing at metaphase I between hybrids of wheat genotypes Chinese Spring ph1b (CSph1b) and KL with rye were studied by genomic in situ hybridization (GISH). The frequency of wheat-wheat associations was higher in CSph1b×rye than in KL×rye. However, frequencies of wheat-rye and rye-rye associations were higher in KL×rye than in CSph1b×rye. These differences may be the result of different mechanisms of control between the ph-like gene(s) controlling homoeologous chromosome pairing in KL and CSph1b. Wheat-wheat associations were much more frequent than wheat-rye pairing in both hybriods. This may be caused by lower overall affinity, or homoeology, between wheat and rye chromosomes than between wheat chromosomes.

Homoeologous chromosome pairing between the A and B genomes of Musa spp. revealed by genomic in situ hybridization

Most cooking banana and several desert bananas are interspecific triploid hybrids between Musa acuminata (A genome) and Musa balbisiana (B genome). In addition, M. balbisiana has agronomical characteristics such as resistance to biotic and abiotic stresses that could be useful to improve monospecific acuminata cultivars. To develop efficient breeding strategies for improving Musa cultivars, it is therefore important to understand the possibility of chromosome exchange between these two species. A protocol was developed to prepare chromosome at meiosis metaphase I suitable for genomic in situ hybridization. A series of technical challenges were encountered, the main ones being the hardness of the cell wall and the density of the microsporocyte's cytoplasm, which hampers accessibility of the probes to the chromosomes. Key parameters in solving these problems were addition of macerozyme in the enzyme mix, the duration of digestion and temperature during the spreading phase. This method was applied to analyse chromosome pairing in metaphase from triploid interspecific cultivars, and it was clearly demonstrated that interspecific recombinations between M. acuminata and M. balbisiana chromosomes do occur and may be frequent in triploid hybrids. These results provide new insight into Musa cultivar evolution and have important implications for breeding.

Integration of physical and genetic maps in apple confirms whole-genome and segmental duplications in the apple genome

A total of 355 simple sequence repeat (SSR) markers were developed, based on expressed sequence tag (EST) and bacterial artificial chromosome (BAC)-end sequence databases, and successfully used to construct an SSR-based genetic linkage map of the apple. The consensus linkage map spanned 1143 cM, with an average density of 2.5 cM per marker. Newly developed SSR markers along with 279 SSR markers previously published by the HiDRAS project were further used to integrate physical and genetic maps of the apple using a PCR-based BAC library screening approach. A total of 470 contigs were unambiguously anchored onto all 17 linkage groups of the apple genome, and 158 contigs contained two or more molecular markers. The genetically mapped contigs spanned ∼421 Mb in cumulative physical length, representing 60.0% of the genome. The sizes of anchored contigs ranged from 97 kb to 4.0 Mb, with an average of 995 kb. The average physical length of anchored contigs on each linkage group was ∼24.8 Mb, ranging from 17.0 Mb to 37.73 Mb. Using BAC DNA as templates, PCR screening of the BAC library amplified fragments of highly homologous sequences from homoeologous chromosomes. Upon integrating physical and genetic maps of the apple, the presence of not only homoeologous chromosome pairs, but also of multiple locus markers mapped to adjacent sites on the same chromosome was detected. These findings demonstrated the presence of both genome-wide and segmental duplications in the apple genome and provided further insights into the complex polyploid ancestral origin of the apple.

The cytological instability of neoallopolyploids suggesting a potent way for DNA introgression: The case of synthetic hexaploid wheat × Aegilops peregrina

SHW-L1 is a newly synthetic hexaploid wheat, which has a genomic combination analogous to that of natural common wheat. In the present study, intergeneric F(1) hybrids of SHW-L1 - Aegilops peregrina AS24 were produced without embryo rescue. Among the 31 F(1) hybrid plants observed, 20 had the expected chromosome number 2n = 35 and were euploids (genome ABDUS(I)). The remainder 11 plants were aneuploids with 33, 34, 36 and 37 chromosomes, indicating that the neoallohexaploid SHW-L1 was cytologically instable and produced aneuhaploid gametes with 19, 20, 22 and 23 chromosomes, respectively. The hybrid plant lacking chromosome 5B carrying Ph1 showed a very high level of homoeologous chromosome pairing. The absence of Ph gene provides a potent cytological mechanism for DNA introgression among homoeologous chromosomes between newly synthetic wheat and wild species.

Evidence for non-disomic inheritance in a Citrus interspecific tetraploid somatic hybrid between C. reticulata and C. limon using SSR markers and cytogenetic analysis

Artificial tetraploid somatic hybrids have been developed for sterile triploid citrus breeding by sexual hybridization between diploid and tetraploid somatic hybrids. The genetic structure of diploid gametes produced by tetraploid genotypes depends on the mode of chromosome association at meiosis. In order to evaluate tetraploid inheritance in a tetraploid interspecific somatic hybrid between mandarin and lemon, we performed segregation studies using cytogenetic and single sequence repeat molecular markers. Cytogenetic analysis of meiosis in the somatic hybrid revealed 11% tetravalents and 76% bivalents. Inheritance of the tetraploid hybrid was analyzed by genotyping the triploid progeny derived from a cross between a diploid pummelo and the tetraploid somatic hybrid, in order to derive genotypes of the meiospores produced by the tetraploid. A likelihood-based approach was used to distinguish between disomic, tetrasomic, and intermediate inheritance models and to estimate the double reduction rate. In agreement with expectations based the cytogenetic data, marker segregation was largely compatible with tetrasomic and inheritance intermediate between disomic and tetrasomic, with some evidence for preferential pairing of homoeologous chromosomes. This has important implications for the design of breeding programs that involve tetraploid hybrids, and underscores the need to consider inheritance models that are intermediate between disomic and tetrasomic.

Nucleotide diversity maps reveal variation in diversity among wheat genomes and chromosomes.

BackgroundA genome-wide assessment of nucleotide diversity in a polyploid species must minimize the inclusion of homoeologous sequences into diversity estimates and reliably allocate individual haplotypes into their respective genomes. The same requirements complicate the development and deployment of single nucleotide polymorphism (SNP) markers in polyploid species. We report here a strategy that satisfies these requirements and deploy it in the sequencing of genes in cultivated hexaploid wheat (Triticum aestivum, genomes AABBDD) and wild tetraploid wheat (Triticum turgidum ssp. dicoccoides, genomes AABB) from the putative site of wheat domestication in Turkey. Data are used to assess the distribution of diversity among and within wheat genomes and to develop a panel of SNP markers for polyploid wheat.ResultsNucleotide diversity was estimated in 2114 wheat genes and was similar between the A and B genomes and reduced in the D genome. Within a genome, diversity was diminished on some chromosomes. Low diversity was always accompanied by an excess of rare alleles. A total of 5,471 SNPs was discovered in 1791 wheat genes. Totals of 1,271, 1,218, and 2,203 SNPs were discovered in 488, 463, and 641 genes of wheat putative diploid ancestors, T. urartu, Aegilops speltoides, and Ae. tauschii, respectively. A public database containing genome-specific primers, SNPs, and other information was constructed. A total of 987 genes with nucleotide diversity estimated in one or more of the wheat genomes was placed on an Ae. tauschii genetic map, and the map was superimposed on wheat deletion-bin maps. The agreement between the maps was assessed.ConclusionsIn a young polyploid, exemplified by T. aestivum, ancestral species are the primary source of genetic diversity. Low effective recombination due to self-pollination and a genetic mechanism precluding homoeologous chromosome pairing during polyploid meiosis can lead to the loss of diversity from large chromosomal regions. The net effect of these factors in T. aestivum is large variation in diversity among genomes and chromosomes, which impacts the development of SNP markers and their practical utility. Accumulation of new mutations in older polyploid species, such as wild emmer, results in increased diversity and its more uniform distribution across the genome.

A Tal-PhI wheat genetic stock facilitates efficient alien introgression

The genetic stock Chinese Spring PhI (refer as CS-PhI hereafter) with the Ph I gene (Ph gene inhibitor, introduced from a high-pairing accession of Aegilops speltoides) was crossed as a male parent to a Tai-gu genic male-sterile wheat with the Tal gene and backcrossed once to CS-PhI to develop the Tal-PhI genetic stock. The Tal-PhI stock, CS-PhI as well as Chinese Spring (CS), were test-crossed using Aegilops peregrina (syn. Aegilops variabilis) to test the effectiveness of the Ph I gene inducing homoeologous chromosome pairing in the Tal-PhI stock. At meiotic I(MI) of pollen mother cells (PMCs), the average chromosome pairing of the TC1 testcrosses of TalCSPhIC08–4/Ae. peregrina, TalCSPhIC11–5/Ae. peregrina and CS/Ae. peregrina was 26.81 I + 3.64 II + 0.33 Open image in new window + 0.07 III + 0.01 IV, 24.75 I + 4.58 II + 0.25 Open image in new window + 0.21 III + 0.07 IV and 32.25 I + 1.38 II, respectively. These data indicated that the Ph I gene could promote homoeologous chromosome pairing in the Tai-gu male sterile background. In order to verify the introgression of the chromatin from Ae. speltoides into CS in the three CS-PhI lines, a total of 79 SSR primers located on group-3 chromosomes were used for PCR analysis using the genomic DNA of CS, Ae. speltoides and the three CS-PhI lines. Two markers Xwmc505 -110 and Xwmc674 -160 amplified S genome-specific fragments from Ae. speltoides in lines PhIC04–13 and PhIC11–5, providing molecular evidence for the introgression of Ae. speltoides-specific chromatin in the two lines. Furthermore, some 3D-specifc SSR markers were missing in line PhIC04–13, indicating there might be a deletion of chromosome 3D in this line. In further experiments, a BC1 population was produced from the cross of Tal-PhI/Triticum durum-Dasypyrum villosum amphiploid (AABBVV)//CS-PhI, and the progenies were further screened for chromosome recombination by GISH analysis. A homozygous translocation line T5VS·5VL-5DL was identified from the BC1F2 by genomic in situ hybridization, C-banding and SSR analysis. The results of the present study demonstrate the Tal-PhI is a useful genetic stock for transferring alien genes into common wheat.

Genome structure affects the rate of autosyndesis and allosyndesis in AABC, BBAC and CCAB Brassica interspecific hybrids

Gene introgression into allopolyploid crop species from diploid or polyploid ancestors can be accomplished through homologous or homoeologous chromosome pairing during meiosis. We produced trigenomic Brassica interspecific hybrids (genome complements AABC, BBAC and CCAB) from the amphidiploid species Brassica napus (AACC), Brassica juncea (AABB) and Brassica carinata (BBCC) in order to test whether the structure of each genome affects frequencies of homologous and homoeologous (both allosyndetic and autosyndetic) pairing during meiosis. AABC hybrids produced from three genotypes of B. napus were included to assess the genetic control of homoeologous pairing. Multi-colour fluorescent in situ hybridisation was used to quantify homologous pairing (e.g. A-genome bivalents in AABC), allosyndetic associations (e.g. B-C in AABC) and autosyndetic associations (e.g. B-B in AABC) at meiosis. A high percentage of homologous chromosomes formed pairs (97.5-99.3%), although many pairs were also involved in autosyndetic and allosyndetic associations. Allosyndesis was observed most frequently as A-C genome associations (mean 4.0 per cell) and less frequently as A-B genome associations (0.8 per cell) and B-C genome associations (0.3 per cell). Autosyndesis occurred most frequently in the haploid A genome (0.75 A-A per cell) and least frequently in the haploid B genome (0.13 B-B per cell). The frequency of C-C autosyndesis was greater in BBAC hybrids (0.75 per cell) than in any other hybrid. The frequency of A-B, A-C and B-C allosyndesis was affected by the genomic structure of the trigenomic hybrids. Frequency of allosyndesis was also influenced by the genotype of the B. napus paternal parent for the three AABC (B. juncea × B. napus) hybrid types. Homoeologous pairing between the Brassica A, B and C genomes in interspecific hybrids may be influenced by complex interactions between genome structure and allelic composition.

GISH reveals different levels of meiotic pairing with wheat for individual Aegilops biuncialis chromosomes

The Triticum aestivum — Aegilops biuncialis (2n=4x=28; UbUbMbMb) disomic addition lines 2Mb, 3Mb, 7Mb and 3Ub were crossed with the wheat cv. Chinese Spring ph1b mutant genotype in order to induce homoeologous pairing, with the final goal of introgressing Ae. biuncialis chromatin into cultivated wheat. Wheat-Aegilops homoeologous chromosome pairing was studied in metaphase I of meiosis in the F1 hybrid lines. Using U and M genomic probes, genomic in situ hybridization (GISH) demonstrated the occurrence of wheat-Aegilops homoeologous pairing in the case of chromosomes 2Mb, 3Mb and 3Ub, but not in the case of 7Mb. The wheat-Aegilops pairing frequency decreased in the following order: 2Mb > 3Mb > 3Ub > 7Mb, which may reflect differences in the wheat-Aegilops homoeologous relationships between the examined Aegilops chromosomes. The selection of wheat-Aegilops homoeologous recombinations could be successful in later generations.

Allosyndetic recombinants of theAegilops peregrina-derivedLr59translocation in common wheat

With 2 figures and 2 tables Abstract The wild relatives constitute a valuable source of rust resistance genes that can be utilized in wheat breeding. However, translocation of desirable genes through chromosome engineering inevitably results in co-transfer of deleterious wild species chromatin. An attempt was made to replace such redundant alien chromatin on the Lr59 translocation through homoeologous chromosome pairing and crossing over in the absence of Ph1. Strong segregation distortion resulted in the recovery of an unexpectedly high frequency of resistant recombinants. Eight of these retained comparatively short, distal segments of foreign chromatin, including Lr59. The foreign chromatin that remained in the latter plants was characterized with the use of twelve anonymous AFLP loci, the data of which suggested reduced homoeology with 1AL that could have been the result of a sub-terminal, paracentric inversion. Crossing over within an inversion loop may have resulted in a low frequency of genetically unbalanced gametes. It will therefore be necessary to develop near-isogenic lines of the eight recombinants and to do field evaluations in order to identify those that retained the shortest, balanced translocations.

Chromosome pairing of individual genomes in tall fescue (Festuca arundinacea Schreb.), its progenitors, and hybrids with Italian ryegrass (Lolium multiflorum Lam.)

A diploid-like pairing system prevents meiotic irregularities and improves the efficiency of gamete production in allopolyploid species. While the nature of the system is known in some polyploid crops including wheat, little is known about the control of chromosome pairing in polyploid fescues (Festuca spp.). In this work we studied chromosome pairing in allohexaploid F. arundinacea, its progenitors F. pratensis and F. glaucescens, and two intergeneric hybrids Lolium multiflorum (2x) ×F. arundinacea (6x) and L. multiflorum (4x) ×F. glaucescens(4x). The use of genomic in situ hybridization (GISH) permitted the analysis of homoeologous chromosome pairing and recombination of different genomes involved. We detected a diploid-like pairing system in polyploid fescues F. arundinacea and F. glaucescens, the latter being one of the progenitors of F. arundinacea. The pairing control system was absent in the second progenitor F. pratensis. Detailed analysis of intergeneric hybrids confirmed the presumed haploinsufficiency of the fescue system, which resulted in homoeologous pairing between all component genomes. This indicates that introgression of any specific chromosome segment from one genome to another is possible in all genome combinations. Our results not only contribute to the quest to discover the nature of the system controlling chromosome pairing in polyploid fescues, but may also have serious implications for design of hybrid breeding schemes in forage grasses.

Development and characterization of Triticum aestivum– Aegilops kotschyi amphiploids with high grain iron and zinc contents

Synthetic amphiploids betweenTriticum aestivum(AABBDD) landrace Chinese Spring (PhI) and cultivar WL711 with different accessions ofAegilops kotschyi(UUSlSl) were developed through colchicine treatment of sterile hybrids. TheF1hybrids and amphiploid plants were intermediate between the parents for plant morphology and spike characteristics. Meiotic metaphase chromosome analysis of theF1hybrids (ABDUSl) showed the expected chromosome number (35) and very little but variable homoeologous chromosome pairing. The amphiploids (AABBDDUUSlSl), however, had variable frequency of univalents at meiotic metaphase-I. The SDS–PAGE of high molecular weight glutenin subunits of amphiploids along with the parents showed the presence and expression of all the parental genomes in the amphiploids. The amphiploids with seeds as large as that of wheat cultivars had higher grain, flag leaf and grain ash iron and zinc concentrations than the wheat parents and comparable with those of theirAe. kotschyiparents suggest thatAe. kotschyipossesses a distinctive genetic system for the micronutrient uptake, translocation and sequestration than the wheat cultivars. This could, however, be demonstrated unequivocally only with comprehensive data on biomass, grain yield and harvest index of theAegilopsdonors and the synthetic amphiploids, which is not feasible due to their shattering and hard threshing. The use of amphiploids for the transfer of high iron and zinc concentrations and development of alien addition and substitution lines in wheat is in progress.

Molecular mapping of genic male‐sterile genes ms15, ms5 and ms6 in tetraploid cotton

AbstractTwo genic male sterile (GMS) lines, Lang‐A conditioned by ms15 and Zhongkang‐A conditioned by ms5ms6 duplicate recessive genes in Gossypium hirsutum L., were chosen to map GMS genes. These two lines were crossed with Gossypium barbadense cv. ‘Hai7124’ to produce segregating populations. The ms15 gene was mapped on chromosome 12, and was flanked by two simple sequence repeat (SSR) markers, NAU2176 and NAU1278, with a genetic distance of 0.8 and 1.9 cM respectively. The ms5 and ms6 genes were mapped to one pair of homoeologous chromosomes, ms5 on chromosome 12 flanked by three SSR markers, NAU3561, NAU2176 and NAU2096, with genetic distances of 1.4, 1.8 and 1.8 cM, respectively, and ms6 on chromosome 26 flanked by two SSR markers, BNL1227 and NAU460, with a genetic distance of 1.4 and 1.7 cM respectively. These tightly linked markers with the ms15, ms5 and ms6 genes can be used in the marker‐assisted selection among segregating populations in a breeding programme, and provide the foundation for gene isolation by map‐based cloning for these three genes.

Supernumerary chromosomes in Alopecurus pratensis L.

The interspecific hybridization between A. pratensis (4x) and A. pratensis ssp. alpestris (4x) gave fertile F1 plants in high frequency. One of the two A. pratensis genomes (ApBp) is suggested to be homologous with one of the two genomes (AaBa) in A. p. ssp. alpestris. The other genomes in the two species are suggested to be at least partially homologous. Supernumerary chromosomes are found to reduce homoeologous chromosome pairing, giving 7.6% fewer meiotic irregularities in the F1's with 2n>28 compared with the F1's with 2n=28. In a field experiment, F1 outyielded parent A. pratensis by 69% and had 20.9% less outwintering over two years. In a diallel cross among 8 F1 plants, the 2n=28 F1's, compared with the 2n>28 F1's, had higher g.c.a. and lower s.c.a. for chromosome number and seed production, lower g.c.a. and higher s.c.a. for plant height and no significant differences for fodder yield. Two breeding procedures for stabilizing the heterotic effect are proposed and discussed. One deals with selection among F2 and following generations, and the other is concerned with colchicine-induced chromosome doubling of F1 to get an 8x amphidiploid. Genome balance and recombination effects in the first procedure and use of supernumerary chromosomes and optimum ploidy level in the second procedure are discussed as main problems for developing a new stable synthetic variety.

Analysis of the meiosis in the F1 hybrids of Longiflorum × Asiatic (LA) of lilies (Lilium) using genomic in situ hybridization

Longiflorum and Asiatic lilies of the genus Lilium of the family Liliaceae are two important groups of modern lily cultivars. One of the main trends of lily breeding is to realize introgression between these groups. With cut style pollination and embryo rescue, distant hybrids between the two groups have been obtained. However, the F(1) hybrids are highly sterile or some of them could produce a small number of 2n gametes, and their BC(1) progenies are usually triploids. Dutch lily breeders have selected many cultivars from these BC(1) progenies based on their variation. It is presumably suggested that such variation could be caused by intergenomic recombination and abnormal meiosis during gamete formation in F(1) hybrids of Longiflorum x Asiatic (LA) hybrids in Lilium. Therefore, the meiotic process of ten F(1) LA hybrids was cytologically investigated using genomic in situ hybridization and traditional cytological methods in the present research. The results showed that: at metaphase I, the homoeologous chromosome pairing among different F(1) hybrids ranged from 2.0 to 11.4 bivalents formed by homoeologous chromosomes per pollen mother cell (PMC), and very few multivalents, and even very few bivalents were formed by two chromosomes within one genome rather than homoeologous chromosomes in some PMCs; at anaphase I, all bivalents were disjoined and most univalents were divided. Both the disjoined bivalents (half-bivalents) and the divided univalents (sister chromatids) moved to the opposite poles, and then formed two groups of chromosomes; because the two resulting half-bivalents retained their axes in the cell undisturbed, many crossover types, including single crossovers, three strand double crossovers, four strand double crossovers, four strand triple crossovers, and four strand multiple crossovers between the non-sister chromatids in the tetrads of bivalents, were clearly inferred by analyzing the breakpoints on the disjoined bivalents. The present investigation not only explained the reason for sterility of the F(1) LA hybrids and the variation of their BC(1) progenies, but also provided a new method to analyze crossover types in other F(1) interspecific hybrids as well.

Cytological studies of a dihaploid and hybrid from intergeneric cross Elymus shandongensis × Triticum aestivum

One dihaploid (n=2x=14) and one true hybrid (2n=5x=35) were produced from the intergeneric cross Elymus shandongensis X Triticum aestivum. The dihaploid was a result of selective elimination of T. aestivum chromosomes early in embryo development. Meiotic chromosome behaviour was studied in the parents, dihaploid, and hybrid. E. shandongensis (2n=4x=28) and the hexaploid wheat formed only bivalents at metaphase I. Some homoeologous chromosome pairing with an average of 0.68 bivalents, ranging from zero to four, and a very low frequency of trivalents was observed in the dihaploid. This reveals a low affinity between S and Y genomes of E. shandongensis. A haplo-insufficient (hemizygous ineffective) genetic control of homoeologous chromosome pairing may also exist in the species. Homologous chromosome association in the hybrid (0.25 chiasmata/cell) was distinctly lower than in the dihaploid (0.73 chiasmata/cell). This indicates that chromosome pairing suppressors, e.g., the Ph gene system, in T. aestivum could affect homoeologous chromosome pairing between the genomes of wild species.