Short, natural, and extended photoperiod response in BC2F4 lines of bread wheat with different photoperiod-1 (Ppd-1) alleles

Flowering is a critical period in the life cycle of flowering plant species, resulting in an irreversible commitment of significant resources. Wheat is photoperiod sensitive, flowering only when daylength surpasses a critical length; however, photoperiod insensitivity (PI) has been selected by plant breeders for >40 years to enhance yield in certain environments. Control of flowering time has been greatly facilitated by the development of molecular markers for the Photoperiod-1 (Ppd-1) homeoloci, on the group 2 chromosomes. In the current study, an allelic series of BC2F4 lines in the winter wheat cultivars 'Robigus' and 'Alchemy' was developed to elucidate the influence on flowering of eight gene variants from the B- and D-genomes of bread wheat and the A-genome of durum wheat. Allele effects were tested in short, natural, and extended photoperiods in the field and controlled environments. Across genetic background and treatment, the D-genome PI allele, Ppd-D1a, had a more potent effect on reducing flowering time than Ppd-B1a. However, there was significant donor allele effect for both Ppd-D1a and Ppd-B1a, suggesting the presence of linked modifier genes and/or additional sources of latent sensitivity. Development of Ppd-A1a BC2F4 lines derived from synthetic hexaploid wheat provided an opportunity to compare directly the flowering time effect of the A-genome allele from durum with the B- and D-genome variants from bread wheat for the first time. Analyses indicated that the reducing effect of Ppd-A1a is comparable with that of Ppd-D1a, confirming it as a useful alternative source of PI.

Chiasma-based genetic maps of chromosome 21

The available cytogenetic data on meiotic chiasmata have been used to construct sex-specific genetic maps, showing the genetic distances and recombination fractions along the length of 21q. The male maps are based on direct observations of spermatocytes, while the female maps are derivations related to the increased chromosome length in oocytes. The male chiasma data have also been used as a frame of reference for ordering and positioning loci on the physical map with D21S110 as a fixed point.

Coding region single nucleotide polymorphism in the barley low-pI, alpha-amylase gene Amy32b

Barley alpha-amylase variability influences the quality of barley grain in the brewing, feed and food industries. alpha-Amylase proteins are encoded by multigene families in cereals, and this study focused on the barley Amy32b gene. We identified coding region single nucleotide polymorphism (cSNP) and insertion/deletion variation in DNA sequences, which resulted in amino acid substitution and stop codon formation, respectively. The substitution affected the beta1 strand in domain C, whereas the stop codon removed the beta5 strand. Possible effects of these changes on the protein are discussed. A cSNP in the coding region of the Amy32b gene was used as a specific marker to map Amy32b loci on chromosome 7H.

Allelic variation at the linked AP1 and PhyC loci in hexaploid wheat is associated but not perfectly correlated with vernalization response

Vernalization requirement is an important trait in temperate crop plants such as wheat and must be considered when selecting varieties for cultivation under different climatic conditions. To determine the growth habit of wheat varieties, plants need to be grown under different vernalization regimes, a lengthy but necessary process for breeders involved in crossing winter with spring germplasm. If haplotypes can be associated with growth habit, then molecular marker assays that are reliable, cheap, and quick can be developed to assist in the selection of plants with the desired phenotype. We have analyzed 81 accessions that have different vernalization requirements and putative different origins of spring habit for sequence variation at the Apetala1 (AP1) locus, which underlies Vrn-1, and at the linked Phytochrome C (PhyC) locus. Good correspondence was found between the AP1 genotype and the PhyC haplotype for 77 of the 81 accessions. Two varieties displayed a recombination event between the AP1 and PhyC loci, and one variety carried a recombinant PhyC gene. In addition, one variety carried an apparent AP1 winter allele, but displayed the Vrn-A1 spring habit. The PhyC haplotype for this variety also indicated the presence of a Vrn-A1 spring allele. Our data suggest that both the AP1 promoter region and PhyC SNPs can be used as diagnostic markers for vernalization response at the vrn-A1 locus, but that neither are perfect tags.

Synaptic behaviour of hexaploid wheat haploids with different effectiveness of the diploidizing mechanism

Haploids of three cultivars of Triticum aestivum (Thatcher, Chris, and Chinese Spring) were obtained from crosses with Zea mays. The level of chromosome pairing at metaphase I and the synaptic behaviour at prophase I was studied. There were differences in the meiotic behaviour of the haploids from different cultivars. Thatcher and Chris haploids had significantly higher levels of pairing at metaphase I than Chinese Spring haploids. This metaphase I pairing was correlated with higher levels of synapsis achieved in the Thatcher and Chris prophase I nuclei than in the Chinese Spring nuclei. Variation in the effectiveness of the diploidizing mechanism among cultivars of wheat is proposed to have a genetic origin and the role of the Ph1 locus in the different haploids is discussed.

Characterisation of a barley (Hordeum vulgare L.) homologue of the Arabidopsis flowering time regulator GIGANTEA

Barley cDNA and genomic clones homologous to the Arabidopsis flowering time regulator GIGANTEA were isolated. Genetic mapping showed that GIGANTEA is present as a single copy gene in barley (3HS) and rice (1S), while two copies are present in maize (3S and 8S) at locations consistent with previous comparative mapping studies. Comparison of the barley peptide with rice and Arabidopsis gave 94% and 79% similarity, respectively. Northern and semi-quantitative RT-PCR analysis of the barley gene (HvGI) showed the presence of a single mRNA species, with a peak of expression between 6 h and 9 h after dawn in short days (8 h light) and a peak 15 h after dawn in long days (16 h light). This behaviour is similar to that seen in Arabidopsis and rice, showing that sequence and expression pattern were well conserved. A lack of correspondence with the map positions of QTL affecting flowering time (heading date) suggests that variation at HvGI does not provide a major source of adaptive variation in photoperiod response.

Development of cost-effective Hordeum chilense DNA markers: molecular aids for marker-assisted cereal breeding

Hordeum chilense is a potential source of useful genes for wheat breeding. The use of this wild species to increase genetic variation in wheat will be greatly facilitated by marker-assisted introgression. In recent years, the search for the most suitable DNA marker system for tagging H. chilense genomic regions in a wheat background has lead to the development of RAPD and SCAR markers for this species. RAPDs represent an easy way of quickly generating suitable introgression markers, but their use is limited in heterogeneous wheat genetic backgrounds. SCARs are more specific assays, suitable for automatation or multiplexing. Direct sequencing of RAPD products is a cost-effective approach that reduces labour and costs for SCAR development. The use of SSR and STS primers originally developed for wheat and barley are additional sources of genetic markers. Practical applications of the different marker approaches for obtaining derived introgression products are described.

Utility of barley and wheat simple sequence repeat (SSR) markers for genetic analysis of Hordeum chilense and tritordeum

A selection of 36 wheat and 35 barley simple sequence repeat markers (SSRs) were studied for their utility in Hordeum chilense. Nineteen wheat and nineteen barley primer pairs amplified consistent H. chilense products. Nine wheat and two barley SSRs were polymorphic in a H. chilense mapping population, producing codominant markers that mapped to the expected homoeologous linkage groups in all but one case. Thirteen wheat and 10 barley primer pairs were suitable for studying the introgression of H. chilense into wheat because they amplified H. chilense products of distinct size. Analysis of wheat/ H. chilense addition lines showed that the H. chilense products derived from the expected homoeologous linkage groups. The results showed that wheat and barley SSRs provide a valuable resource for the genetic characterization of H. chilense, tritordeums and derived introgression lines.

Rapid verification of wheat-Hordeum introgressions by direct staining of SCAR, STS, and SSR amplicons

A range of single tagged site (STS), simple sequence repeat (SSR), and sequence-characterized amplified region (SCAR) markers were screened for their utility in detecting Hordeum vulgare and H. chilense chromosomes in a wheat background. PCR conditions were optimized for specific amplification of the targeted sequences and to avoid cross-species amplification. Two H. vulgare derived STSs, six H. vulgare derived SSRs, and nine H. chilense derived SCARs were usable for the detection of five H. vulgare and three H. chilense chromosomes by direct ethidium bromide staining of the PCR products in test tubes, avoiding the more costly and time-consuming DNA electrophoresis step. The practical application of the method is illustrated by the identification of a monotelosomic substitution of H. vulgare chromosome 6HS in tritordeum and a monosomic addition of H. chilense chromosome 6Hch in durum wheat.

Molecular characterization of transforming plasmid rearrangements in transgenic rice reveals a recombination hotspot in the CaMV 35S promoter and confirms the predominance of microhomology mediated recombination

The characterization of plasmid-genomic DNA junctions following plant transformation has established links between DNA double-strand break repair (DSBR), illegitimate recombination and plasmid DNA integration. The limited information on plasmid-plasmid junctions in plants comes from the dicot species tobacco and Arabidopsis. We analyzed 12 representative transgenic rice lines, carrying a range of transforming plasmid rearrangements, which predominantly reflected microhomology mediated illegitimate recombination involving short complementary patches at the recombining ends. Direct end-ligation, in the absence of homology between the recombining molecules, occurred only rarely. Filler DNA was found at some of the junctions. Short, purine-rich tracts were present, either at the junction site or in the immediate flanking regions. Putative DNA topoisomerase I binding sites were clustered around the junctions. Although different regions of the transforming plasmid were involved in plasmid-plasmid recombination, we showed that a 19 bp palindromic sequence, including the TATA box of the CaMV 35S promoter, acted as a recombination hotspot. The purine-rich half of the palindromic sequence was specifically involved at the recombination junctions. This recombination hotspot is located within the 'highly recombinogenic' region of the full-length CaMV RNA that has been shown to promote viral recombination in dicot plants. Clustering of plasmid recombination events in this highly recombinogenic region, even in the absence of viral enzymes and other cis-acting elements proves that the plant cellular machinery alone is sufficient to recognize and act on these viral sequences. Our data also show the similarity between mechanisms underlying junction formation in dicot and monocot plants transformed using different procedures.

Towards rice genome scanning by map-based AFLP fingerprinting

Map-based DNA fingerprinting with AFLP markers provides a fast method for scanning the rice genome. Three hundred AFLP markers identified with ten primer combinations were mapped in two rice populations. The genetic maps were aligned and almost full coverage of the rice genome was obtained. The transferability of AFLP markers between indica x japonica and indica x indica crosses was tested. The chromosomes were divided into DNA Fingerprint Linkage Blocks (DFLBs) defined by specific AFLP markers. Using these blocks, the degree of similarity or divergence within specific chromosome regions was calculated for nine varieties. Applications of map-based fingerprinting for biodiversity studies and maker-assisted selection are discussed.

Transgene organization in rice engineered through direct DNA transfer supports a two-phase integration mechanism mediated by the establishment of integration hot spots

Organization of transgenes in rice transformed through direct DNA transfer strongly suggests a two-phase integration mechanism. In the "preintegration" phase, transforming plasmid molecules (either intact or partial) are spliced together. This gives rise to rearranged transgenic sequences, which upon integration do not contain any interspersed plant genomic sequences. Subsequently, integration of transgenic DNA into the host genome is initiated. Our experiments suggest that the original site of integration acts as a hot spot, facilitating subsequent integration of successive transgenic molecules at the same locus. The resulting transgenic locus may have plant DNA separating the transgenic sequences. Our data indicate that transformation through direct DNA transfer, specifically particle bombardment, generally results in a single transgenic locus as a result of this two-phase integration mechanism. Transgenic plants generated through such processes may, therefore, be more amenable to breeding programs as the single transgenic locus will be easier to characterize genetically. Results from direct DNA transfer experiments suggest that in the absence of protein factors involved in exogenous DNA transfer through Agrobacterium, the qualitative and/or quantitative efficiency of transformation events is not compromised. Our results cast doubt on the role of Agrobacterium vir genes in the integration process.

Rapid reorganization of resistance gene homologues in cereal genomes

We used conserved domains in the major class (nucleotide binding site plus leucine-rich repeat) of dicot resistance (R) genes to isolate related gene fragments via PCR from the monocot species rice and barley. Peptide sequence comparison of dicot R genes and monocot R-like genes revealed shared motifs but provided no evidence for a monocot-specific signature. Mapping of these genes in rice and barley showed linkage to genetically characterized R genes and revealed the existence of mixed clusters, each harboring at least two highly dissimilar R-like genes. Diversity was detected intraspecifically with wide variation in copy number between varieties of a particular species. Interspecific analyses of R-like genes frequently revealed nonsyntenic map locations between the cereal species rice, barley, and foxtail millet although tight collinear gene order is a hallmark of monocot genomes. Our data suggest a dramatic rearrangement of R gene loci between related species and implies a different mechanism for nucleotide binding site plus leucine-rich repeat gene evolution compared with the rest of the monocot genome.

QTL analysis to study the association between leaf size and abscisic acid accumulation in droughted rice leaves and comparisons across cereals

Plants accumulate abscisic acid (ABA) under droughted conditions. Genetic variation in the accumulation of ABA in deteched and partially dehydrated leaves of rice has previously been reported, and this was found to be associated with variation in leaf size (smaller leaves made more ABA). Correlation analysis failed to distinguish clearly between a causal relationship between the two traits and close genetic between loci controlling the traits. Here we present a detailed genetic analysis of ABA accumulation in detached and partially dehydrated rice leaves. using a population of F2 plants generated from the lowland x upland cross IR20 (high-ABA) x 63-83 (low-ABA) which was mapped with RFLP and AFLP markers. Several highly significant quantitative trait loci (QTLs) for ABA accumulation and leaf weight were identified. Only one of the minor QTLs for ABA accumulation (accounting for only 4% of the phenotypic variance) was coincident with any QTLs for leaf size such that the high-ABA allele was associated with smaller leaves. This analysis, therefore, showed that the association previously found between ABA accumulation and leaf size was probably largely due to genetic linkage and not to a direct effect of leaf size on ABA accumulation or vice versa. Because of the importance of ABA accumulation in regulating responses of plants to drought stress and the effects of plant size on the rate of development of stress, QTLs for drought-induced ABA accumulation, leaf size and tiller number were compared between rice and wheat. In particular, a possible location in rice was sought for a homoeologue of the major wheat vernalization responsive gene, Vrn1, as this gene is also associated with major effects on leaf size, tiller number and ABA accumulation in wheat. The likelihood of homoeologous loci regulating ABA accumulation, leaf size and tiller number in the two crops is discussed.

Comparative genetics of flowering time

Analysis of genes controlling flowering time (heading date) contributes to our understanding of fundamental principles of plant development and is of practical importance because of the effects of flowering time on plant adaptation and crop yield. This review discusses the extent to which plants may share common genetic mechanisms for the control of flowering time and the implications of such conservation for gene isolation from the major cereal crops. Gene isolation may exploit the small genome of rice in map-based approaches, utilizing the conservation of gene order that is revealed when common DNA markers are mapped in different species. Alternatively, mechanisms may be conserved within plants as a whole, in which case genes cloned from the model dicot Arabidopsis thaliana provide an alternative route.

Comparative genetics of flowering time

Analysis of genes controlling flowering time (heading date) contributes to our understanding of fundamental principles of plant development and is of practical importance because of the effects of flowering time on plant adaptation and crop yield. This review discusses the extent to which plants may share common genetic mechanisms for the control of flowering time and the implications of such conservation for gene isolation from the major cereal crops. Gene isolation may exploit the small genome of rice in map-based approaches, utilizing the conservation of gene order that is revealed when common DNA markers are mapped in different species. Alternatively, mechanisms may be conserved within plants as a whole, in which case genes cloned from the model dicot Arabidopsis thaliana provide an alternative route.

RFLP mapping of the barley homeotic mutant lax-a

The lax-a homeotic mutant of barley has flowers in which lodicules are replaced by stamens (giving five stamens per flower). RFLP mapping of an F2 population from a Bonus lax-a (1) x H. spontaneum cross showed that the mutation was on the short arm of chromosome 7(5H), closely linked to the centromere. An additional F2 population was used to show that the lax-a mutation gave the five-stamen phenotype in all flowers of 6-rowed spikes and that hoods were elevated and reduced in size in lax-a/Hooded double-mutant plants.

Conservation of fine-scale DNA marker order in the genomes of rice and the Triticeae

DNA markers distribute over large chromosomal regions exhibit conservation of order (collinearity) in different cereal species, but it is not known whether this is maintained on a finer scale, i.e. < or = 2 cM. To address this, sets of two or more genetically linked DNA markers were localised to yeast artificial chromosomes containing rice DNA inserts. Linkage analysis of these DNA markers in barley revealed complete correspondence with their genetic order in rice, the distance between linked sequences on rice chromosomes being < 1.6 cM or < or = 1 + 10(6) bp (1 Mb). Thus, DNA markers separated in this range are collinear in rice, barley and, by inference, other members of the Triticeae. These results are discussed with respect to the use of rice as a key system for the isolation of cereal genes.

RFLP mapping of five major genes and eight quantitative trait loci controlling flowering time in a winter × spring barley (Hordeum vulgare L.) cross

A genetic map of 92 RFLP loci and two storage protein loci was made using 94 doubled-haploid lines from a cross between the winter barley variety Igri and the spring variety Triumph. The markers were combined with data from two field experiments (one spring sown and one autumn (fall) sown) and a glasshouse experiment to locate a total of 13 genes (five major genes and eight quantitative trait loci (QTL)) controlling flowering time. Two photoperiod response genes were found; Ppd-H1 on chromosome 2(2H)S regulated flowering time under long days, while Ppd-H2 on chromosome 5(1H)L was detected only under short days. In the field experiments Ppd-H1 strongly affected flowering time from spring and autumn sowings, while Ppd-H2 was detected only in the autumn sowing. The glasshouse experiment also located two vernalization response genes, probably Sh and Sh2, on chromosomes 4(4H)L and 7(5H)L, respectively. The vernalization response genes had little effect on flowering time in the field. Variation in flowering time was also affected by nine additional genes, whose effects were not specifically dependent on photoperiod or vernalization. One was the denso dwarfing gene on chromosome 3(3H)L. The remaining eight were QTLs of smaller effect. One was located on chromosome 2(2H), one on 3(3H), one on 4(4H), one on 7(5H), two on 6(6H), and two on 1(7H). Model fitting showed that the 13 putative genes, and their interactions, could account for all the observed genetical variation from both spring and autumn sowings, giving a complete model for the control of flowering time in this cross.Key words: barley, Hordeum vulgare, flowering time, photoperiod, vernalization, mapping.

Extraction and genetic control of two new water-soluble proteins of mature barley seed

This paper describes the genetic control of two new water-soluble proteins in barley. Water-soluble proteins (WSPs) of mature barley seed form part of the albumin/globulin class of seed proteins. They can be extracted from hand-milled grain with water, though some WSPs are more efficiently extracted with a solution of 10 mM dithiothreitol. Polymorphisms for WSPs were detected in isoelectric focusing gels incorporating various ampholine combinations. Two new controlling genes (Wsp4 and Wsp5) have been identified and located using wheat/barley chromosome addition lines and barley doubled haploids. Wsp4 is located on chromosome 2 (2H), and Wsp5 was found to be tightly linked to Wsp2 on the long arm of chromosome 7 (5HL). Segregation of a sixth gene (Wsp6) is also described, but this has not been mapped. The results are discussed with respect to other previously mapped Wsp loci.

The distribution of RFLP markers on chromosome 2(2H) of barley in relation to the physical and genetic location of 5S rDNA

The 5S rDNA locus on the long arm of barley chromosome 2(2H) was genetically mapped in two crosses in relation to 30 other RFLP loci. Comparison of the genetic maps with the previously published physical position of the 5S rDNA, determined by in-situ hybridization, showed that there was a marked discrepancy between physical and genetic distance in both crosses, with recombination being less frequent in the proximal part of the arm. Pooled information from the present study and other published genetic maps showed that at least 26 of the 44 (59%) RFLPs that have been mapped on 2(2H)L lie distal to the 5S rDNA locus even though this region is only 27% of the physical length of the arm. The distribution of RFLP markers is significantly different from expected (P < 0.01), implying that the low-copy sequences used for RFLP analysis occur more frequently in distal regions of the arm and, or, that sequences in distal regions are more polymorphic.

The agronomic performance of wheat doubled haploid lines derived from wheat x maize crosses

The agronomic performance of 9 doubled haploid (DH) lines of Chinese Spring, 6 DH lines of Hope, 14 DH lines of the single chromosome substitution line Chinese Spring (Hope 5 A) and their respective parents was analyzed under field conditions. Seventeen Chinese Spring DH lines derived from wheat x Hordeum bulbosum crosses were also included for comparison. No significant variation was detected in either population of Chinese Spring DH lines and neither DH population differed from its parent. The Hope DH lines differed significantly for tiller biomass, spikelet number per ear, ear grain weight and 50-grain weight. However, all the variation could be attributed to the poor performance of only one line. Chinese Spring (Hope 5 A) DH lines showed significant variation for ear emergence time, but this was probably due to genetic heterogeneity in the parental stock. Overall, the results suggest that most DH lines produced by the wheat x maize method resemble their wheat parent, and that the variation induced in DH production is likely to be similar to that found in DHs from wheat x Hordeum bulbosum crosses.

The timing of chromosome elimination in hexaploid wheat × maize crosses

Early seed development in crosses between the hexaploid wheat genotype 'Chinese Spring' and the maize genotype 'Seneca 60' was studied to determine the timing of elimination of the maize chromosomes. Elimination of one or more maize chromosomes occurred in about 70% of zygotic mitoses. Metaphase nuclei from two-celled embryos had 5 to 10 maize chromosomes, most of which were lost during the second cell division. About half the metaphase nuclei from four-celled embryos had no maize chromosomes, and the remainder had one to five. Anaphase or telophase nuclei from four-celled embryos showed no maize chromosomes in about half the cells and one or more pairs of lagging maize daughter chromosomes in the remainder. No maize chromosomes were seen in metaphase preparations from embryos with eight or more cells. These data strongly suggest that all maize chromosomes were lost during the first three cell-division cycles in most embryos. All embryos with four or more cells had micronuclei, showing that embryo development was dependent on fertilization. The only primary endosperm metaphase obtained in the experiment had 42 wheat and 10 maize chromosomes and the presence of micronuclei in most developing endosperms showed that at least 85% were of hybrid origin. Few endosperm nuclei were formed in comparison with self-pollinated 'Chinese Spring' caryopses, and many were abnormal. The implications of the results for wheat haploid production and gene transfer from maize to wheat are discussed.Key words: wheat, maize, wide hibridization, chromosome elimination.

The frequency of fertilization in wheat × pearl millet crosses

Wheat × pearl millet crosses were studied to determine whether fertilization occurred and whether any resulting hybrids were karyotypically stable. Crosses between the hexaploid wheat genotype 'Chinese Spring' (kr1, kr2) and the pearl millet genotype 'Tift 23BE' gave fertilization in 28.6% of the 220 florets pollinated. Chromosome counts from zygotes at metaphase confirmed the hybrid origin of the embryos. Three had the expected F1 combination of 21 wheat and 7 pearl millet chromosomes and a fourth had 21 wheat and 14 pearl millet chromosomes. The expected F1 chromosome complement was also found in a primary endosperm mitosis. The hybrid embryos were karyotypically unstable and probably lost all the pearl millet chromosomes in the first four cell division cycles. Similar results were obtained using two other wheat genotypes. Crosses between the hexaploid wheat genotype 'Highbury', which differs from 'Chinese Spring' in having alleles for reduced crossability with rye and Hordeum bulbosum at the Kr1 and Kr2 loci, and 'Tift 23BE' gave fertilization in 32% of analyzed florets. This was not significantly different from the frequency found in 'Chinese Spring', indicating that 'Tift 23BE' was insensitive to the action of the Kr genes. Crosses between the tetraploid wheat genotype 'Kubanka' and 'Tift 23BE' gave fertilization in 48% of florets. The potential of pearl millet for wheat haploid production is discussed.Key words: wheat, pearl millet, wide hybridization, chromosome elimination.

The production of haploid wheat plants from wheat x maize crosses

Hybrid embryos from hexaploid wheat x maize crosses rapidly lose the maize chromosomes to produce haploid wheat embryos. Such embryos almost always aborted when left to develop on the plant, and only 1 was recovered from 2440 florets (0.17% of the expected number). Embryos had greater viability in spikelet culture, 47 (26.5% of the expected number) being recovered from 706 ovaries. Thirty-two of these embryos germinated to give green plants, 31 of which were haploid (21 wheat chromosomes) and 1 of which was euploid (42 wheat chromosomes). Spikelet culture enabled 17.1% of the expected number of embryos to be recovered as haploid plants, a 100-fold improvement on allowing embryos to develop in vivo. Ten haploid plants of 'Chinese Spring' (kr1, kr2), 13 plants of 'Chinese Spring (Hope 5A)' (kr1, Kr2), and 8 of 'Hope' (Kr1, Kr2) were recovered. The potential of wheat x maize crosses for wheat haploid production and for gene transfer from maize to wheat is discussed.

The effect of the crossability loci Kr1 and Kr2 on fertilization frequency in hexaploid wheat x maize crosses

Dominant alleles of the Kr1 and Kr2 genes reduce the crossability of hexaploid wheat with many alien species, including rye and Hordeum bulbosum, with Kr1 having the greater effect. However, a cytological study of wheat ovaries fixed 48 h after pollination showed that the wheat genotypes 'Highbury' (kr1, Kr2) and 'Chinese Spring (Hope 5B)' (kr1, kr2) were crossable with 'Seneca 60' maize, fertilization occurring in 14.4 and 30.7% of embryo sacs respectively. The latter figure was similar to the 29.7% fertilization found in 'Chinese Spring' (kr1, kr2). Most embryo sacs in which fertilization occurred contained an embryo but lacked an endosperm and where an endosperm was formed it was usually highly aberrant. All three wheat x maize combinations were karyotypically unstable and rapidly eliminated maize chromosomes to produce haploid wheat embryos.

Wheat × maize hybridization

Cytogenetic evidence is presented that the cross between hexaploid bread wheat (Triticum aestivum, 2n = 42) and maize (Zea mays, 2n = 20) results in a hybrid zygote with one complete haploid chromosome set from each parent. Maize chromosomes are subsequently eliminated. This sytem has potential for wheat haploid production and may also allow segments of maize DNA, including transposable elements, to be transferred to wheat.Key words: wide crosses, wheat, maize, chromosome elimination, haploids.

Further studies on bivalent chiasma frequency in human males with normal karyotypes

Previously unpublished data on the chiasma frequency of individual bivalents identified by a triple staining technique are presented for four males. The total autosomal cell chiasma frequency and sex chromosome univalence frequency are also given for these males and for three others. All seven males had apparently normal 46,XY karyotypes and normal spermatogenesis. The extent of inter-individual variation in cell and bivalent chiasma frequency and the gross relationship between chromosome length and chiasma frequency are discussed.

Further studies on chiasma distribution and interference in the human male

Some unusual patterns of chiasma distribution were noted in a preliminary investigation of meiosis in an infertile male with an apparently normal mitotic karyotype and a normal mean autosomal cell chiasma frequency. A detailed investigation of chiasma distribution on all 22 autosomes revealed that several chromosomes showed a significant change in chiasma distribution and/or mean inter-chiasma distance in comparison with previously published controls. These findings are discussed in relation to the general patterns of chiasma localization in the human male and the role of interference.

Chiasma derived genetic lengths and recombination fractions: a 46, XY, t(9; 10) (p22; q24) reciprocal translocation

Chiasma distribution data on chromosomes 1, 2 and 9 from a reciprocal translocation carrier with a 46, XY, t(9; 10) (p22; q24) karyotype were used to calculate genetic distances and recombination fractions for chromosome segments corresponding to the major mitotic bands and for intervals between the centromeres and points at 10% intervals along the chromosome arms. These values were compared with those from control males with normal karyotypes. The translocation showed a marked increase in crossing-over in one specific region of chromosome 9 and, in addition, there was evidence of interchromosomal effects in chromosomes 1 and 2.

The location of the major bands on chromosome 1 at diakinesis in the human male and the relationship between banding pattern and chiasma localization

Q-banded chromosome 1 bivalents from six human males were measured in order to determine the locations of the major band borders. Chiasma position was also recorded in these bivalents in order to determine whether chiasmata preferentially occurred in Q-bright regions, Q-dark regions or in the interfaces between. The results indicated that the locations of the major bands of chromosome 1 were very similar at diakinesis and at mitotic prometaphase and that chiasma distribution was not governed by the banding pattern of the chromosome.

A direct cytogenetic technique for assessing the rate of first meiotic non-disjunction in the human male by the analysis of cells at metaphase II

The successful application of a triple staining technique incorporating quinacrine mustard fluorescence, lacto-propionic orcein staining and C-banding to metaphase II cells in the human male is described. This procedure overcomes the major technical difficulties associated with the analysis of these cells, enables unambiguous chromosome counts to be made and allows the majority of cells to be karyotyped. Preliminary results on two hundred cells from six men with apparently normal karyotypes are presented.

Studies on chiasma frequency and distribution in two fertile men carrying reciprocal translocations; one with a t(9;10) karyotype and one with a t(Y;10) karyotype

The frequency and distribution of chiasmata was investigated in two fertile carriers of reciprocal translocations, one with a 46,XY,t(9;10)(p22;q24) karyotype and one with a 46,X,-Y,+der(Y),t(Y;10)(q12;q24) karyotype. In both cases the chromosomes involved in the translocation showed an increase in chiasma frequency in comparison to karyotypically normal controls and in both cases this increase was localised, affecting only one interstitial segment of each translocation quadrivalent. In the t(9;10) case chiasmata appeared in substantial numbers in a novel location, the proximal two thirds of 9p, while in the t(Y;10) case chiasmata appeared in a conventional location, the medial region of 10q, but at an increased frequency. Furthermore there was evidence for inter-chromosomal effects in the t(9;10) case.

Chiasma derived genetic lengths and recombination fractions: chromosomes 2 and 9

An investigation of chiasms distribution in chromosomes 1, 2 and 9 in the human male has shown that each arm consistently has a characteristic and highly non-random distribution of recombination. The chiasma frequencies of chromosome regions corresponding to the major mitotic bands have been used to construct genetic maps under the assumption that there is no chromatid interference or chiasma movement and no difference between meiotic and mitotic band positions. This paper presents genetic lengths and recombination fractions for these bands and for combinations of bands in chromosomes 2 and 9. Our results are particularly useful for relating genetic distance to recombination fraction and for dealing with long stretches of chromosome which cannot easily be analysed by any other techniques.

Chiasma derived genetic maps and recombination fractions: chromosome 1

Chiasma distribution data from six human males have been used to calculate genetic distance and recombination for chromosome 1. Estimates given for each major chromosome band are valid only under the assumption that there is no chromatid interference, no chiasma movement, and no differential chromosome contraction between mitosis and meiosis.

Chiasma frequency and distribution in a sample of human males: chromosomes 1, 2, and 9

Chiasma frequency and the distribution of chiasmata within chromosomes 1, 2, and 9 were investigated in a total of seven males with presumptively normal meiosis. With the exception of one unusual individual there was little variation among the males examined for these characteristics. The exceptional male had a reduced chiasma frequency, which, rather than being considered abnormal, was thought to represent the lower end of the normal range of variability. A statistical analysis of the histograms of chiasma distribution confirmed the impression of overall similarity, and we conclude that genetic maps of these chromosomes may now be constructed without too much disturbance from differences between individuals. The maps will be presented separately in a subsequent paper.