Complete characterization of wheat–alien metaphase I pairing in interspecific hybrids between durum wheat (Triticum turgidum L.) and jointed goatgrass (Aegilops cylindrica Host)

The pattern of homoeologous metaphase I (MI) pairing has been fully characterized in durum wheat × Aegilops cylindrica hybrids (2n = 4x = 28, ABC^cD^c) by an in situ hybridization procedure that has permitted individual discrimination of every wheat and wild constituent genome. One of the three hybrid genotypes examined carried the ph1c mutation. In all cases, MI associations between chromosomes of both species represented around two-third of total. Main results from the analysis are as follows (a) the A genome chromosomes are involved in wheat–wild MI pairing more frequently than the B genome partners, irrespective of the alien genome considered; (b) both durum wheat genomes pair preferentially with the D^c genome of jointed goatgrass. These findings are discussed in relation to the potential of genetic transference between wheat crops and this weedy relative. It can also be highlighted that inactivation of Ph1 provoked a relatively higher promotion of MI associations involving B genome.     

Wheat-alien metaphase I pairing of individual wheat genomes and D genome chromosomes in interspecific hybrids between Triticum aestivum L. and Aegilops geniculata Roth

Homoeologous metaphase I (MI) pairing of Triticum aestivum × Aegilops geniculata hybrids (2n = 5× = 35, ABDU^gM^g) has been examined by an in situ hybridization procedure permitting simultaneous discrimination of A, B, D and wild genomes. The seven D genome chromosomes (and their arms, except for 6D and 7D) plus some additional wheat chromosomes were also identified. Wheat-wild MI associations represented more than 60% of total, with an average ratio of 5:1:12 for those involving the A, B and D genomes, respectively. A remarkable between-chromosome variation for the level of wheat-wild genetic exchange is expected within each wheat genome. However, it can be concluded that 3DL and 5DL are the crop genome locations with the highest probability of being transferred to Ae. geniculata. Hybrids derived from the ph2b wheat mutant line showed increased MI pairing but identical pattern of homoeologous associations than those with active Ph2.     

Homoeologous relationships of Triticum sharonense chromosomes to T. aestivum

 Homoeologous pairing at metaphase I was analyzed in standard-type, ph2b, and ph1b hybrids of Triticum aestivum (common, bread or hexaploid wheat) and T. sharonense in order to establish the homoeologus relationships of T. sharonense chromosomes to hexaploid wheat. Chromosomes of both species, and their arms, were identified by C-banding. Normal homoeologous relationships for the seven chromosomes of the S^sh genome, and their arms, were revealed, which implies that no apparent chromosome rearrangement occurred in the evolution of T. sharonense relative to wheat. All three types of hybrids with low-, intermediate-, and high-pairing level showed preferential pairing between A-D and B-S^sh. A close relationship of the S^sh genome to the B genome of bread wheat was confirmed, but the results provide no evidence that the B genome was derived from T. sharonense. Data on the pairing between individual chromosomes of T. aestivum and T. sharonense provide an estimate of interspecific homoeologous recombination. Received: 14 October 1996 / Accepted: 25 October 1996     

Homoeologous relationships of Aegilops speltoides chromosomes to bread wheat

 Homoeologous pairing at metaphase I was analysed in the standard-type, ph2b and ph1b hybrids of Triticum aestivum (AABBDD) and Aegilops speltoides (SS). Data from relative pairing affinities were used to predict homoeologous relationships of Ae. speltoides chromosomes to wheat. Chromosomes of both species, and their arms, were identified by C-banding. The Ae. speltoides genotype carried genes that induced a high level of homoeologous pairing in the three types of hybrids analyzed. All arms of the seven chromosomes of the S genome showed normal homoeologous pairing, which implies that no apparent chromosome rearrangements occurred in the evolution of Ae. speltoides relative to wheat. A pattern of preferential pairing of two types, A-D and B-S, confirmed that the S genome is very closely related to the B genome of wheat. Although this pairing pattern was also reported in hybrids of wheat with Ae. longissima and Ae. sharonensis, a different behaviour was found in group 5 chromosomes. In the hybrids of Ae. speltoides, chromosome 5B-5S pairing was much more frequent than 5D-5S, while these chromosome associations reached similar frequencies in the hybrids of Ae. longissima and Ae. sharonensis. These results are in agreement with the hypothesis that the B genome of wheat is derived from Ae. speltoides. Received: 8 January 1998 / Accepted: 4 February 1998     

Development of wheat–<Emphasis Type="Italic">Lophopyrum elongatum</Emphasis> recombinant lines for enhanced sodium ‘exclusion’ during salinity stress

Lophopyrum elongatum (tall wheatgrass), a wild relative of wheat, can be used as a source of novel genes for improving salt tolerance of bread wheat. Sodium ‘exclusion’ is a major physiological mechanism for salt tolerance in a wheat–tall wheatgrass amphiploid, and a large proportion (~50%) for reduced Na⁺ accumulation in the flag leaf, as compared to wheat, was earlier shown to be contributed by genetic effects from substitution of chromosome 3E from tall wheatgrass for wheat chromosomes 3A and 3D. Homoeologous recombination between 3E and wheat chromosomes 3A and 3D was induced using the ph1b mutant, and putative recombinants were identified as having SSR markers specific for tall wheatgrass loci. As many as 14 recombinants with smaller segments of tall wheatgrass chromatin were identified and low-resolution breakpoint analysis was achieved using wheat SSR loci. Seven recombinants were identified to have leaf Na⁺ concentrations similar to those in 3E(3A) or 3E(3D) substitution lines, when grown in 200 mM NaCl in nutrient solution. Phenotypic analysis identified recombinants with introgressions at the distal end on the long arm of homoeologous group 3 chromosomes being responsible for Na⁺ ‘exclusion’. A total of 55 wheat SSR markers mapped to the long arm of homoeologous group 3 markers by genetic and deletion bin mapping were used for high resolution of wheat–tall wheatgrass chromosomal breakpoints in selected recombinants. Molecular marker analysis and genomic in situ hybridisation confirmed the 524-568 recombinant line as containing the smallest introgression of tall wheatgrass chromatin on the distal end of the long arm of wheat chromosome 3A and identified this line as suitable for developing wheat germplasm with Na⁺ ‘exclusion’.     

GISH reveals different levels of meiotic pairing with wheat for individual <Emphasis Type="Italic">Aegilops biuncialis</Emphasis> chromosomes

The Triticum aestivum — Aegilops biuncialis (2n=4x=28; U^bU^bM^bM^b) disomic addition lines 2M^b, 3M^b, 7M^b and 3U^b 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 2M^b, 3M^b and 3U^b, but not in the case of 7M^b. The wheat-Aegilops pairing frequency decreased in the following order: 2M^b > 3M^b > 3U^b > 7M^b, 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.     

Multidisciplinary approach to genome analysis in the diploid species,Thinopyrum bessarabicum and Th. elongatum (Lophopyrum elongatum), of the Triticeae

Summary The J and E genome species of the Triticeae are invaluable sources of salt tolerance. The evidence concerning the phyletic relatedness of the J genome of diploid Thinopyrum bessarabicum and the E genome of diploid Th. elongatum (=Lophopyrum elongatum) is discussed. Low level of chromosome pairing between J and E at different ploidy levels, suppression of J-E pairing by the Ph1 pairing regulator that inhibits homoeologous pairing, complete sterility of the diploid hybrids (JE), karyotypic divergence of the two genomes, differences in total content and distribution of heterochromatin along their chromosomes, and marked differences in gliadin proteins, isozymes, 5S DNA, and rDNA indicate that J and E are distinct genomes. Well-defined biochemical markers have been identified in the two genomes and may be useful in plant breeding. The level of distinction between J and E is comparable to that among the universally accepted homoeologous genomes A, B, and D of wheat. Therefore, the J and E genomes are homoeologous and not homologous, although some workers continue to call them homologous. The previous workers' data on chromosome pairing in diploid hybrids and/ or karyotypic differences in the conventionally stained chromosomes do not provide sufficient evidence for the proposed merger of J and E genomes (and, hence, of the genera Thinopyrum and Lophopyrum) specifically and for establishing genome relationships generally. Extra precautions should be exercised before changing the designation of an established genome and before merging two genera. A uniform, standardized system of genomic nomenclature for the entire Triticeae is proposed, which should benefit cytogeneticists, plant breeders, taxonomists, and evolutionists.Cooperative investigations of the USDA-Agricultural Research Service and the Utah Agricultural Experiment Station, Logan, UT 84322, USA. Approved as Journal Paper no. 3832     

不同细胞质的普通小麦"中国春"(Triticum aestivum var.Chinese Spring)与小偃麦杂交F1代的育性与减数分裂行为的研究

15个不同细胞质“中国春”小麦与八倍体小偃麦杂交 ,杂种F1减数分裂的染色体行为表明 :普通小麦与天蓝偃麦草的F或E组染色体之间存在着部分同源关系 ;D2 型细胞质促进部分同源染色体配对、但却抑制同源染色体配对 ;Sv 型细胞质对同源染色体或部分同源染色体的配对均有抑制作用 ;G型细胞质促进同源染色体配对。1 5个不同细胞质“中国春”小麦与六倍体小偃麦杂交 ,F1结实率很低 ,减数分裂中期的染色体行为混乱 ,单价体过多 ,或许意味着在天蓝偃麦草 (Elytrigiain termedium)与长穗偃麦草 (E .elongatum)的E组染色体之间存在着很大差别。随着回交代数的增加 ,选出G型、D2 型、Mt 型、Mu 型等细胞质雄性不育的八倍体小偃麦品系 ,其中D2 型细胞质八倍体小偃麦具有光周期敏感性雄性不育的特征 ;G型细胞质“远中 3”育性正常 ,表明八倍体小偃麦“远中 3”的E组染色体中存在G型胞质的育性恢复基因。     

Transfer of Ph I genes promoting homoeologous pairing from Triticum speltoides to common wheat

Diploid-like chromosome pairing in polyploid wheat is controlled by several Ph (pairing homoeologous) genes with major and minor effects. Homoeologous pairing occurs in either the absence of these genes or their inhibition by genes from other species (Ph ^I genes). We transferred Ph ^I genes from Triticum speltoides (syn Aegilops speltoides) to T. aestivum, and on the basis of further analysis it appears that two duplicate and independent Ph ^I genes were transferred. Since Ph ^I genes are epistatic to the Ph genes of wheat, homoeologous pairing between the wheat and alien chromosomes occurs in the F₁ hybrids. Using the Ph ^I gene stock, we could demonstrate homoeologous pairing between the wheat and Haynaldia villosa chromosomes. Since homoeologous pairing occurs in F₁ hybrids and no cytogenetic manipulation is needed, the Ph ^I gene stock may be a versatile tool for effecting rapid and efficient alien genetic transfers to wheat.Contribution no. 93-435-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, KS 66506-5502, USA     

Exploring the tertiary gene pool of bread wheat: sequence assembly and analysis of chromosome 5Mg of Aegilops geniculata

Next‐generation sequencing (NGS) provides a powerful tool for the discovery of important genes and alleles in crop plants and their wild relatives. Despite great advances in NGS technologies, whole‐genome shotgun sequencing is cost‐prohibitive for species with complex genomes. An attractive option is to reduce genome complexity to a single chromosome prior to sequencing. This work describes a strategy for studying the genomes of distant wild relatives of wheat by isolating single chromosomes from addition or substitution lines, followed by chromosome sorting using flow cytometry and sequencing of chromosomal DNA by NGS technology. We flow‐sorted chromosome 5M^g from a wheat/Aegilops geniculata disomic substitution line [DS5M^g (5D)] and sequenced it using an Illumina HiSeq 2000 system at approximately 50 × coverage. Paired‐end sequences were assembled and used for structural and functional annotation. A total of 4236 genes were annotated on 5M^g, in close agreement with the predicted number of genes on wheat chromosome 5D (4286). Single‐gene FISH indicated no major chromosomal rearrangements between chromosomes 5M^g and 5D. Comparing chromosome 5M^g with model grass genomes identified synteny blocks in Brachypodium distachyon, rice (Oryza sativa), sorghum (Sorghum bicolor) and barley (Hordeum vulgare). Chromosome 5M^g‐specific SNPs and cytogenetic probe‐based resources were developed and validated. Deletion bin‐mapped and ordered 5M^g SNP markers will be useful to track 5M‐specific introgressions and translocations. This study provides a detailed sequence‐based analysis of the composition of a chromosome from a distant wild relative of bread wheat, and opens up opportunities to develop genomic resources for wild germplasm to facilitate crop improvement.     

Metaphase I bound arms frequency and genome analysis in wheat-Aegilops hybrids

Summary Meiotic associations of different wheat-Aegilops variabilis and wheat-Ae. kotschyi hybrid combinations with low and high homoeologous pairing were analyzed at metaphase I. Five types of pairing involving wheat and Aegilops genomes were identified by using C-banding. A genotype that seems to promote homoeologous pairing has been found in Ae. variabilis var. cylindrostachys. Its effect is detectable in the low pairing hybrids but not in the high ones. Pairing affinity has been analyzed on the basis of metaphase I associations in the low and high homoeologous pairing hybrids, and in bivalents and multivalents in the high pairing hybrids. The results indicate that the amount of bound arms of each type of identifiable association relative to the total associations formed (relative contribution) was not maintained, either between the different levels of pairing (low and high) or between different meiotic configurations (bivalents and multivalents). These findings seem to indicate that quantifications of genomic relationships based on the amount of chromosome pairing at metaphase I must be carefully done in this type of hybrid combinations.     

Construction of Agropyron Gaertn. genetic linkage maps using a wheat 660K SNP array reveals a homoeologous relationship with the wheat genome

Agropyron Gaertn. (P genome) is a wild relative of wheat that harbours many genetic variations that could be used to increase the genetic diversity of wheat. To agronomically transfer important genes from the P genome to a wheat chromosome by induced homoeologous pairing and recombination, it is necessary to determine the chromosomal relationships between Agropyron and wheat. Here, we report using the wheat 660K single nucleotide polymorphism (SNP) array to genotype a segregating Agropyron F₁ population derived from an interspecific cross between two cross‐pollinated diploid collections ‘Z1842’ [A. cristatum (L.) Beauv.] (male parent) and ‘Z2098’ [A. mongolicum Keng] (female parent) and 35 wheat–A. cristatum addition/substitution lines. Genetic linkage maps were constructed using 913 SNP markers distributed among seven linkage groups spanning 839.7 cM. The average distance between adjacent markers was 1.8 cM. The maps identified the homoeologous relationship between the P genome and wheat and revealed that the P and wheat genomes are collinear and relatively conserved. In addition, obvious rearrangements and introgression spread were observed throughout the P genome compared with the wheat genome. Combined with genotyping data, the complete set of wheat–A. cristatum addition/substitution lines was characterized according to their homoeologous relationships. In this study, the homoeologous relationship between the P genome and wheat was identified using genetic linkage maps, and the detection mean for wheat–A. cristatum introgressions might significantly accelerate the introgression of genetic variation from Agropyron into wheat for exploitation in wheat improvement programmes.     

Homoeologous recombination in the presence of <Emphasis Type="Italic">Ph1</Emphasis> gene in wheat

A crossover (CO) and its cytological signature, the chiasma, are major features of eukaryotic meiosis. The formation of at least one CO/chiasma between homologous chromosome pairs is essential for accurate chromosome segregation at the first meiotic division and genetic recombination. Polyploid organisms with multiple sets of homoeologous chromosomes have evolved additional mechanisms for the regulation of CO/chiasma. In hexaploid wheat (2n = 6× = 42), this is accomplished by pairing homoeologous (Ph) genes, with Ph1 having the strongest effect on suppressing homoeologous recombination and homoeologous COs. In this study, we observed homoeologous COs between chromosome 5M^g of Aegilops geniculata and 5D of wheat in plants where Ph1 was fully active, indicating that chromosome 5M^g harbors a homoeologous recombination promoter factor(s). Further cytogenetic analysis, with different 5M^g/5D recombinants, showed that the homoeologous recombination promoting factor(s) may be located in proximal regions of 5M^g. In addition, we observed a higher frequency of homoeologous COs in the pericentromeric region between chromosome combination of rec5M^g#2S·5M^g#2L and 5D compared to 5M^g#1/5D, which may be caused by a small terminal region of 5DL homology present in chromosome rec5M^g#2. The genetic stocks reported here will be useful for analyzing the mechanism of Ph1 action and the nature of homoeologous COs.     

Nonstructural Chromosome Differentiation among Wheat Cultivars, with Special Reference to Differentiation of Chromosomes in Related Species

Wheat cultivar Chinese Spring (Triticum aestivum L. em. Thell.) was crossed with cultivars Hope, Cheyenne and Timstein. In all three hybrids, the frequencies of pollen mother cells (PMCs) with univalents at metaphase I (MI) were higher than those in the parental cultivars. No multivalents were observed in the hybrids, indicating that the cultivars do not differ by translocations. Thirty-one Chinese Spring telosomic lines were then crossed with substitution lines in which single chromosomes of the three cultivars were substituted for their Chinese Spring homologues. The telosomic lines were also crossed with Chinese Spring. Data were collected on the frequencies (% of PMCs) of pairing of the telesomes with their homologues at MI and the regularity of pairing of the remaining 20 pairs of Chinese Spring chromosomes in the monotelodisomics obtained from these crosses. The reduced MI pairing in the intercultivar hybrids was caused primarily by chromosome differentiation, rather than by specific genes. Because the differentiation involved a large part of the chromosome complement in each hybrid, it was concluded that it could not be caused by structural changes such as inversions or translocations. In each case, the differentiation appeared to be unevenly distributed among the three wheat genomes. It is proposed that the same kind of differentiation, although of greater magnitude, differentiates homoeologous chromosomes and is responsible, together with structural differentiation, for poor chromosome pairing in interspecific hybrids.     

Crossability of Brassica tournefortii and B. rapa, and morphology and cytology of their F1 hybrids

The crossability between Brassica tournefortii (TT, 2n = 20) and Brassica rapa (AA, 2n = 20) and the cytomorphology of their F₁ hybrids were studied. Hybrids between these two species were obtained only when B. tournefortii was involved as a female parent. The hybrid plants were intermediate for most of the morphological attributes and were found to be free from white rust under field conditions. The F₁ plants showed poor pollen fertility, although occasional seed set was achieved from open pollination. Self-pollination or backcrosses did not yield any seeds in these plants. The occurrence of chromosome association ranging from bivalents (0–7), trivalents (0–2) to a rare quadrivalent (0–1) in the dihaploid hybrids indicates pairing between the T and A genomes. The homoeologous pairing coupled with seed set in the F₁ plants offer an opportunity for interspecific gene transfers from B. tournefortii to B. rapa and vice-versa through interspecific hybridization. Received: 3 July 2000 / Accepted: 22 September 2000     

Isolation and identification of Triticum aestivum L. em. Thell. cv Chinese Spring-T. peregrinum Hackel disomic chromosome addition lines

Analyses of RFLPs, isozymes, morphological markers and chromosome pairing were used to isolate 12 Triticum aestivum cv Chinese Spring (genomes A, B, and D)-T. peregrinum (genomes S^v and U^v) disomic chromosome addition lines. The evidence obtained indicates that each of the 12 lines contains an intact pair of T. peregrinum chromosomes. One monosomic addition line, believed to contain an intact 6S^v chromosome, was also isolated. A CS-7U^v chromosome addition line was not obtained. Syntenic relationships in common with the standard Triticeae arrangement were found for five of the seven S^v genome chromosomes. The exceptions were 4S^v and 7S^v. A reciprocal translocation exists between 4S¹ and 7S¹ in T. longissimum and evidence was obtained that the same translocation exists in T. peregrinum. In contrast, evidence for syntenic relationships in common with the standard Triticeae arrangements were found for only one U^v chromosome of T. peregrinum.; namely, chromosome 2U^v. All other U^v genome chromosomes are involved in at least one translocation, and the same translocations were found in the U genome of T. umbellulatum. Evidence was also obtained indicating that the centromeric regions of 4U and 4U^v are homoeologous to the centromeric regions of Triticeae homoeologous group-6 chromosomes, that the centromeric regions of 6U and 6U^v are homoeologous to the centromeric regions of group-4 chromosomes, and that 4U and 4U^v are more closely related overall to Triticeae homoeologous group-6 chromosomes than they are to group-4 chromosomes.     

Direct evidence for high level of autosyndetic pairing in hybrids of Thinopyrum intermedium and Th. ponticum with Triticum aestivum

 Genomic in situ hybridization (GISH) was used to distinguish autosyndetic from allosyndetic pairing in the hybrids of Thinopyrum intermedium and Th. ponticum with Triticum aestivum cv ‘Chinese Spring’ (CS). All hybrids showed high autosyndetic pairing frequencies among wheat chromosomes and among Thinopyrum chromosomes. The high autosyndetic pairing frequencies among wheat chromosomes in both hybrids suggested that Th. intermedium and Th. ponticum carry promoters for homoeologous chromosome pairing. The higher frequencies of autosyndetic pairing among Thinopyrum chromosomes than among wheat chromosomes in both hybrids indicated that the relationships among the three genomes of Th. intermedium and among the five genomes of Th. ponticum are closer than those among the three genomes of T. aestivum. Received: 19 September 1996 / Accepted: 18 April 1997     

Induced Homoeologous Pairing for Transfer of Useful Variability for High Grain Fe and Zn from <Emphasis Type="Italic">Aegilops kotschyi</Emphasis> into Wheat

Biofortification of bread wheat by the transfer of useful variability of high grain Fe and Zn from Aegilops kotschyi through induced homoeologous pairing is the most feasible approach to alleviate micronutrient malnutrition worldwide. Deficiency of chromosome 5B in interspecific hybrids allows homoeologous pairing and recombination of chromosomes of wheat with those of the related species. The interspecific hybrid plants without 5B chromosome showed much higher chromosome pairing than did the plants with 5B. The F₁ plants without 5B chromosome were selected and repeatedly backcrossed with wheat cultivar PBW343. The chromosome number of BC₂F₁ plants ranged from 43 to 60 with several univalents and multivalents. Molecular markers and GISH analysis confirmed the introgression of U/S chromosomes of Ae. kotschyi and their fragments in wheat. The BC₂F₂ plants showed up to 125 % increase in Fe and 158 % increase in Zn compared to PBW343 with Lr24 and Yr36. Induced homoeologous pairing in the absence of 5B was found to be an effective approach for transfer of useful variability for enhanced grain Fe and Zn content for biofortification of wheat for high grain micronutrient content.     

Genotypic variation in tetraploid wheat affecting homoeologous pairing in hybrids with Aegilops peregrina.

The Ph1 gene has long been considered the main factor responsible for the diploid-like meiotic behavior of polyploid wheat. This dominant gene, located on the long arm of chromosome 5B (5BL), suppresses pairing of homoeologous chromosomes in polyploid wheat and in their hybrids with related species. Here we report on the discovery of genotypic variation among tetraploid wheats in the control of homoeologous pairing. Compared with the level of homoeologous pairing in hybrids between Aegilops peregrina and the bread wheat cultivar Chinese Spring (CS), significantly higher levels of homoeologous pairing were obtained in hybrids between Ae. peregrina and CS substitution lines in which chromosome 5B of CS was replaced by either 5B of Triticum turgidum ssp. dicoccoides line 09 (TTD09) or 5G of Triticum timopheevii ssp. timopheevii line 01 (TIMO1). Similarly, a higher level of homoeologous pairing was found in the hybrid between Ae. peregrina and a substitution line of CS in which chromosome arm 5BL of line TTD140 substituted for 5BL of CS. It appears that the observed effect on the level of pairing is exerted by chromosome arm 5BL of T turgidum ssp. dicoccoides, most probably by an allele of Ph1. Searching for variation in the control of homoeologous pairing among lines of wild tetraploid wheat, either T turgidum ssp. dicoccoides or T timopheevii ssp. armeniacum, showed that hybrids between Ae. peregrina and lines of these two wild wheats exhibited three different levels of homoeologous pairing: low, low intermediate, and high intermediate. The low-intermediate and high-intermediate genotypes may possess weak alleles of Ph1. The three different T turgidum ssp. dicoccoides pairing genotypes were collected from different geographical regions in Israel, indicating that this trait may have an adaptive value. The availability of allelic variation at the Ph1 locus may facilitate the mapping, tagging, and eventually the isolation of this important gene.     

The genotypic control of homoeologous chromosome association in Lolium temulentum × Lolium perenne interspecific hybrids

Two contrasting genotypes of Lolium perenne and two inbred lines of L. temulentum were examined with regard to their effect on homoeologous chromosome pairing in interspecific hybrids derived from them. Substantial differences in chiasma frequency were observed between the hybrid progeny of the different parental types. The background genes involved were found to operate in the presence and in the absence of B chromosomes. The combination of A chromosome genes present in some of the 0B hybrids was found to result in a considerable suppression of chiasma formation at the diploid level, and the restriction of pairing to strict homologues at the tetraploid level. It appears, therefore, that genes are present within the diploid species of the genus Lolium which are capable of performing a function similar to that of the Ph locus in wheat.