Chromosome Pairing In Hybrids Research Articles

DArTseq-based analysis of genomic relationships among species of tribe Triticeae

Precise utilization of wild genetic resources to improve the resistance of their cultivated relatives to environmental growth limiting factors, such as salinity stress and diseases, requires a clear understanding of their genomic relationships. Although seriously criticized, analyzing these relationships in tribe Triticeae has largely been based on meiotic chromosome pairing in hybrids of wide crosses, a specialized and labourious strategy. In this study, DArTseq, an efficient genotyping-by-sequencing platform, was applied to analyze the genomes of 34 Triticeae species. We reconstructed the phylogenetic relationships among diploid and polyploid Aegilops and Triticum species, including hexaploid wheat. Tentatively, we have identified the diploid genomes that are likely to have been involved in the evolution of five polyploid species of Aegilops, which have remained unresolved for decades. Explanations which cast light on the progenitor of the A genomes and the complex genomic status of the B/G genomes of polyploid Triticum species in the Emmer and Timopheevi lineages of wheat have also been provided. This study has, therefore, demonstrated that DArTseq genotyping can be effectively applied to analyze the genomes of plants, especially where their genome sequence information are not available.

Wheat cytogenetics and cytogenomics: the present status

The chromosome number of hexaploid wheat (2n = 6x = 42) was first worked out in 1918 and was followed by research on intergeneric and interspecific hybridization, mainly in Japan under the leadership of Hitoshi Kihara. Starting in 1935, Ernie Sears in USA produced a large collection of aneuploids including nullisomics, monosomics, trisomics, tetrasomics, ditelocentrics and nullisomic–tetrasomic (NT) lines. In parallel, through a study of meiotic chromosome pairing in hybrids produced through interspecific and intergeneric crosses, progenitors of the A and D genomes of 6x wheat were determined with certainty; the progenitor of B genome was difficult and still not known with certainty. The diploidizing system and the Ph1 locus restricting meiotic chromosome pairing to only between homologous (but not between homoeologous) chromosomes was also initially discovered by Okamoto (Wheat Inf Serv 5:6, 1957), and was subsequently studied in more detail by Ralph Riley and his group at Cambridge, UK during late 1950s. With the establishment of International Triticeae Mapping Initiative (ITMI) and the development of ITMI mapping population in 1990, first genetic maps were produced for all the seven homoeologous groups involving 21 wheat chromosomes by 1995. High-density genetic maps were developed later through large-scale development of SNPs through next generation sequencing (NGS) technology. During 1990s, TR Endo and BS Gill at Kansas State University, USA also produced > 400 chromosome deletion stocks that were extensively used for the development of physical maps for all the 21 wheat chromosomes. More recently, during the last 12 years (2005–2017), cytogenomics research was conducted, which included separation of longest chromosome 3B and the 40 chromosome arms for the remaining 20 chromosomes through flow sorting and their utilization in the development of BAC libraries that were used for developing BAC-based physical maps. These maps were used in generating chromosome survey sequences, and assembly of the whole genome sequence of 6x wheat Chinese Spring. During 2017–2018, whole genome sequences for a wild emmer tetraploid wheat (T. turgidum) and those for the two diploid species (T. urartu, Ae. tauschii) representing the donors of A and D sub-genomes of 6x wheat, were also published. For hexaploid wheat, estimates of a pangenome with ~ 140,000 genes and that of the core genome with ~ 80,000 genes also became available. These resources are already being extensively utilized and will continue to be utilized throughout the world for several decades not only for basic research, but also for the improvement of wheat crop to feed the growing world population.

Multiple and asymmetrical origin of polyploid dog rose hybrids (Rosa L. sect. Caninae (DC.) Ser.) involving unreduced gametes.

Polyploidy and hybridization are important factors for generating diversity in plants. The species-rich dog roses ( Rosa sect. Caninae ) originated by allopolyploidy and are characterized by unbalanced meiosis producing polyploid egg cells (usually 4 x ) and haploid sperm cells (1 x ). In extant natural stands species hybridize spontaneously, but the extent of natural hybridization is unknown. The aim of the study was to document the frequency of reciprocal hybridization between the subsections Rubigineae and Caninae with special reference to the contribution of unreduced egg cells (5 x ) producing 6 x offspring after fertilization with reduced (1 x ) sperm cells. We tested whether hybrids arose by independent multiple events or via a single or few incidences followed by a subsequent spread of hybrids. Population genetics of 45 mixed stands of dog roses across central and south-eastern Europe were analysed using microsatellite markers and flow cytometry. Hybrids were recognized by the presence of diagnostic alleles and multivariate statistics were used to display the relationships between parental species and hybrids. Among plants classified to subsect. Rubigineae , 32 % hybridogenic individuals were detected but only 8 % hybrids were found in plants assigned to subsect. Caninae . This bias between reciprocal crossings was accompanied by a higher ploidy level in Rubigineae hybrids, which originated more frequently by unreduced egg cells. Genetic patterns of hybrids were strongly geographically structured, supporting their independent origin. The biased crossing barriers between subsections are explained by the facilitated production of unreduced gametes in subsect. Rubigineae . Unreduced egg cells probably provide the highly homologous chromosome sets required for correct chromosome pairing in hybrids. Furthermore, the higher frequency of Rubigineae hybrids is probably influenced by abundance effects because the plants of subsect. Caninae are much more abundant and thus provide large quantities of pollen. Hybrids are formed spontaneously, leading to highly diverse mixed stands, which are insufficiently characterized by the actual taxonomy.

Experimental hybridization and chromosome pairing in Kosteletzkya (Malvaceae, Malvoideae, Hibisceae), and possible implications for phylogeny and phytogeography in the genus

Kosteletzkya C. Presl, 1835 (Malvaceae, Malvoideae, Hibisceae) includes 17 species, all but two of which are about evenly distributed between Africa and the northern Neotropics. Fifteen of the species were brought into cultivation and used in a hybridization program in an attempt to shed light on evolutionary and phytogeographic relationships in the genus. Chromosome pairing (x = 19) at meiosis was examined in 51 of the 56 interspecific hybrids that were produced, and the seven New World species, all diploids, were found to exhibit nearly complete pairing among themselves, indicating that they share a genome. By contrast the three African diploids showed low levels of chromosome pairing in crosses among themselves, leading to the recognition here of three distinct genomes, newly designated A, B and G. The African B-genome diploid, Kosteletzkya buettneri Gürke, 1889, was found to share its genome with the New World species. Four other African species are known to be tetraploids and a fifth, a hexaploid. The results of chromosome pairing in hybrids among all of the African species at all ploidy levels, plus the discovery of a spontaneously tetraploidized experimental intergenomic African diploid hybrid, suggest that three of the four tetraploids and the single hexaploid might all be allopolyploids built on the three known extant genomes. The fourth tetraploid paired poorly or moderately with these three genomes. Results are consistent with the hypothesis that Kosteletzkya arose in Africa, radiated at the diploid level, underwent natural interspecific hybridization, produced two tiers of allopolyploids, and at some more recent time dispersed a B-genome diploid to the New World where it underwent another radiation at the diploid level. Structural features of the fruits suggest adaptations for passive distribution by animals, potentially over long distances.

Chromosome pairing in hybrids between hexaploid bread wheat and tetraploid crested wheatgrass (Agropyron cristatum)

To transfer drought and cold tolerance from crested wheatgrass, Agropyron cristatum (2n = 4x = 28; PPPP genomes) to bread wheat, Triticum aestivum (2n = 6x = 42; AABBDD), intergeneric hybrids (2n = 5x = 35; ABDPP) were synthesized. The F, hybrids were perennial like the male wheatgrass parent. Their morphology was almost intermediate between the two parents. A low-pairing (LP) hybrid and a high-pairing (HP) hybrid were studied. The LP hybrid, with apparently functional Phi (the pairing regulator that suppresses homoeologous pairing), had a mean of 0.14 III + 1.16 ring II + 3.17 rod II + 25.91 I, with a c (the mean arm-pairing frequency) of 0.206. If A, cristatum were an autotetraploid, its haploid complement (PP) in the hybrid should form approximately 7 II. The mean of only 4.33 II (of which about 1.0 bivalent probably involved the A, B and D genomes of wheat) would suggest a certain degree of divergence between the two P genomes. The HP hybrid had 0.03 chain IV + 0.47 III + 2.98 ring II + 5.89 rod II + 15.73 I, with c= 0.460. Such a high pairing probably involved both autosyndesis (pairing within the ABD component and within the PP component of the ABDPP hybrid) and allosyndesis (pairing between the parental complements), and could have occurred only if Phl was partially inactivated. Homoeologous pairing of chromosomes of P with wheat chromosomes seems sufficient to facilitate the transfer of desirable traits of A. cristatum into wheat.

Chromosome pairing in hybrids between hexaploid bread wheat and tetraploid crested wheatgrass (Agropyron cristatum)

Chromosome pairing in hybrids between hexaploid bread wheat and tetraploid crested wheatgrass (Agropyron cristatum)

Effect of the ph1b mutant on chromosome pairing in hybrids between Dasypyrum villosum and Triticum aestivum

AbstractChromosome pairing was analysed in F1 hybrids of the wheat cultivar ‘Chinese Spring’ (CS) and its ph1b mutant (CSphlb) with Dasypyrum villosum. On average, 1.61 chromosomes per cell paired in the hybrid CS ×D. villosum, but 14.43 in the hybrid CS ph1b×D. villosum. Genomic fluorescence in situ hybridization (GISH) revealed three types of homoeologous association between wheat (W) and D. villosum (D) chromosomes (W‐D, D‐W‐W and D‐W‐D) in pollen mother cells of the CS ph1b×D. villosum hybrid, and only one type (W‐W), in the CS ×D. villosum hybrid. Both F1 hybrids were self‐sterile. The seed set of the backcross of CS ×D. villosum with CS was 6.67% and that of CS ph1b×D. villosum with CS or CS ph1b was only 0.45%. The chromosome number of BC1 plants varied from 48 to 72. Translocations of chromosome segments or entire arms between wheat and D. villosum chromosomes were detected by GISH in the BC1 plants from the backcross of CS ph1b×D. villosum to CS ph1b.

Chromosomes of Villadia and Altamiranoa (Crassulaceae)

Villadia, ranging from Texas to Peru with some 25 species, has a rather distinctive thyrsoid to spicate inflorescence, and we keep it as a genus separate from Sedum. Twenty species show every gametic chromosome number from 9 to 17 and also 20-22 and higher. Chromosome pairing in hybrids shows that the species differ by many translocations and that species with 21 or lower are effectively diploid. More specialized species tend to have fewer and larger chromosomes, suggesting that through time translocations have rearranged the ancestral genome into fewer units. We suspect that relocated genes may be programmed differently, affecting phenotype. Thus Villadia is like Echeveria in having a remarkably long descending series of evidently diploid chromosome numbers. Altamiranoa, often included in Villadia, with about 15 species from Mexico south, more closely resembles Sedum in its broadly cymose inflorescence. It appears polyphyletic, with no clear boundary from Sedum, and we disperse its species in Sedum. The ten species studied have gametic numbers from 20 to 29 that probably are effectively diploid, with a few higher and probably polyploid. Again, chromosome pairing in hybrids shows that the species differ by many translocations. Putative relatives in Sedum section Leptosedum have n = 26 to 31. Thus cytologically as well as morphologically Altamiranoa has remained more similar than Villadia to its Sedum relatives.

Cytogenetic analysis of Kengyilia mutica (Keng) J. L. Yang, Yen and Baum (Poaceae: Triticeae)

Kengyilia mutica (Keng) Yang, Yen et Baum is a hexaploid perennial grass of the tribe Triticeae native to western central China. The analyzer species with known genomic constitution used to produce interspecific hybrids with the target taxon were Roegneria kamoji Ohwi (StStHHYY), K. hirsuta (Keng) J. L. Yang, Yen et Baum (PPStStYY) and K. rigidula (Keng) J. L. Yang, Yen et Baum (PPStStYY). Analysis of metaphase I pairing configurations in the F1 hybrids indicates that K. mutica possesses the P, St and Y genomes, with only minor structural rearrangements. Chromosome pairing in hybrids supports the inclusion of K. mutica in the genus Kengyilia.

Cytology and Morphology of Elymus hoffmanni (Poaceae: Triticeae): A New Species from the Erzurum Province of Turkey

Cytological and morphological characteristics of a proposed new species, Elymus hoffmanni Jensen & Asay, are described. This species is cytologically stable and is genomically similar to Elymus repens (L.) Gould (SSSSHH) (= Agropyron repens [L.] P. Beauv and Elytrigia repens [L.] Nevski) and the cultivar NewHy (SSSSHH), but morphologically different from E. repens. Elymus hoffmanni has longer leaves, shorter awns on the glumes, and less rhizome development, and it is taller than the cultivar NewHy. Elymus hoffmanni was crossed with E. repens and the cultivar NewHy hybrid wheatgrass (2n = 6x = 42). A natural hybrid between E. hoffmanni and the cultivar Hycrest crested wheatgrass (2n = 4x = 28, PPPP) was identified from open-pollinated progeny of E. hoffmanni. Meiosis of E. hoffmanni was characterized as an autoallohexaploids with a mean chromosome association of 1.63 I + 17.87 II + 0.35 III + 0.67 IV + 0.04 V per cell. Mean chromosome associations were 1.88 I + 19.12 II + 0.16 III 0.46 + IV per cell in the hybrids of E. repens x E. hoffmanni, 1.63 I + 19.43 II + 0.22 III + 0.31 IV + 0.01 V per cell in NewHy x E. hoffmanni hybrids, and 9.22 I + 12.74 II + 0.40 III + 0.02 IV in E. hoffmanni x Hycrest hybrids. Based on chromosome pairing in hybrids with E. repens and NewHy and its morphological uniqueness from E. repens, we propose E. hoffmanni as a new species originating from a primitive hybrid between E. repens and the bluebunch wheatgrasses (Pseudoroegneria spp.) of Turkey.

Fluorescent in situ hybridization — a useful aid to the introduction of alien genetic variation into wheat

Fluorescent in situ hybridization (FISH) of DNA to plant chromosomes has proved to be a powerful cytogenetic tool. The value of fluorescent in situ hybridization of total genomic DNA (GISH) of related species is demonstrated in the determination of wheat/alien chromosome pairing in hybrids. Its use for assessing the relative merits of the various genes that affect chromosome pairing is also shown. The ability of GISH to identify the presence in wheat of whole alien chromosomes or alien chromosome segments is illustrated. The potential of FISH for detecting repeated DNA sequences, low copy sequences and single copy genes is discussed.

Cytology and taxonomy of Elymus kengii, E. grandiglumis, E. alatavicus, and E. batalinii (Poaceae: Triticeae)

Elymus alatavicus (Drob.) A. Löve, E. batalinii (Krasn.) A. Löve, E. kengii (Keng) Tzvelev, and E. grandiglumis (Keng) A. Löve are rare hexaploid perennial grasses of the tribe Triticeae native to central Asia. This paper further describes the (i) genomic makeup of E. batalinii and E. alatavicus; (ii) chromosome pairing and fertility in hybrids between E. alatavicus or E. batalinii and E. kengii or E. grandiglumis; (iii) morphological variation among the taxa; and (iv) necessary taxonomic changes within Elymus necessitated by the above studies. On the basis of chromosome pairing in the hybrids E. batalinii × E. dentatus (Hook. f.) Tzvelev ssp. ugamicus (Drob.) Tzvelev, 10.21 I + 12.11 II + 0.18 III + 0.01 IV, and E. alatavicus × Pseudoroegneria tauri (Boiss. &Bal.) A. Löve, 13.10 I + 10.05 II + 0.52 III + 0.08 IV, the genomic formula for E. batalinii and E. alatavicus can be written as SSYYPP. Chromosome pairing in the hybrids E. alatavicus × E. grandiglumis, 9.37 I + 16.23 II + 0.06 III; E. kengii × E. batalinii, 10.25 I + 15.54 II + 0.21 III + 0.02 IV; and E. grandiglumis × E. batalinii, 9.15 I + 13.47 II + 1.09 III + 0.06 IV, combined with species morphology, supports the grouping of the above taxa in Elymus L. section Hyalolepis (Nevski) A. Löve rather than being separated in Elymus section Goulardia (Husnot) Tzvelev.Key words: genome, meiosis, chromosome pairing, interspecific hybrids, Elymus, Triticeae.

Cytology, fertility, and morphology of Elymus kengii (Keng) Tzvelev and E. grandiglumis (Keng) A. Löve (Triticeae: Poaceae)

This study reports on the cytogenetics, fertility, mode of reproduction, and morphological variation of two perennial Triticeae grasses, Elymus kengii (Keng) Tzvelev and Elymus grandiglumis (Keng) A. Löve, from west central China. Both species are allohexaploids (2n = 42), self-fertile, and morphologically distinct on the basis of their plant color, glume length, and lemma and rachis vestiture. F1 hybrids between these two species are partially fertile and morphologically intermediate to their parents. Analysis of chromosome pairing in hybrids between E. grandiglumis or E. kengii and the following "analyzer" species, Psathyrostachys juncea (Fisch.) Nevski (NN), Psathyrostachys huashanica Keng (NN), Elymus lanceolatus (Scribn. &Smith) Gould (SSHH), Elymus dentatus (Hook. f.) Tzvelev ssp. ugamicus (Drob.) Tzvelev (SSYY), Elymus ciliaris (Trin.) Nevski (SSYY), Pseudoroegneria spicata (Pursh) A Löve (SS), and Pseudoroegneria tauri (Boiss. &Bal.) A. Löve (SSPP), suggested that both taxa contain the S, Y, and P genomes. This represents a new genome combination not previously reported and shows that the P genome from the crested wheatgrasses (Agropyron) has been involved in polyploid evolution within the. Triticeae.Key words: genome, meiosis, chromosome pairing, interspecific hybrids, Elymus, Triticeae.

Cytology and Morphology of Elymus pendulinus, E. pendulinus ssp. Multiculmis, and E. parviglume (Poaceae: Triticeae)

Elymus pendulinus (Nevski) Tzvelev, E. pendulinus (Nevski) Tzvelev ssp. multiculmis (Kitagawa) A. Love and E. parviglume (Keng) A. Love are perennial grasses of the tribe Triticeae inhabiting regions of central Asia. This manuscript reports: (1) mode of pollination (self- vs. cross-pollination); (2) meiotic behavior; (3) chromosome pairing in hybrids with various ''analyzer'' parents; and (4) morphological variation for the above taxa. All taxa investigated were capable of self-pollination. Bivalent frequencies of 13.98, 13.89, and 13.95, respectively, suggested that the taxa behave as allotetraploids composed of two different genomes. Mean bivalent frequencies in E. dentatus ssp. ugamicus (SSYY) x E. pendulinus, E. dentatus ssp. ugamicus x E. pendulinus ssp. multiculmis and E. parviglume x E. yezoensis (SSYY) averaged 12.57, 12.27, and 12.18 bivalents per cell, respectively. The genomic formula should be designated as SSYY, with each taxon having a slight modification resulting from evolutionary pressure...

An assessment of genome analysis based on chromosome pairing in hybrids of perennial Triticeae

Genome analysis based on chromosome pairing in diploid hybrids of perennial Triticeae species was examined along with those interpretations based on data of triploid and tetraploid hybrids and (or) species. The mean arm pairing frequency, c, of diploid hybrids with known genome combinations was used as a measure of chromosome homology. The mean trivalent frequency and the averaged sum of trivalents and quadrivalents were used as indices of pairing in triploids and tetraploids, respectively, to assess the validity of the diploid c values. The relative affinity, x value, of the respective triploid hybrids was also used to assess the c value of a diploid combination. Positive correlation coefficients were found between c values of diploid hybrids and trivalent frequencies in triploid hybrids (r = 0.97) as well as between the former and sums of trivalents and quadrivalents in tetraploid hybrids or amphidiploids (r = 0.92). A lower correlation coefficient (r = 0.65) was found for the comparison between diploid and amphidiploid hybrids involving annual Triticeae species. The c values of diploid combinations were negatively correlated (r = −0.79) with the x values of triploid hybrids. These relationships indicate that chromosome relatedness can be measured by the level of pairing in diploid hybrids of perennial Triticeae. It is also suggested that if the diploid hybrid has a c value of about 0.5, a conclusion regarding the genome relationship should be based on additional evidence, such as karyotype data. It is shown that assignment of genome symbols cannot be guided by the presence or absence of preferential pairing in polyploids. Genes regulating chromosome pairing pose problems for genome analysis but should not invalidate either genome analysis as a whole or that of diploid hybrids in particular.Key words: genome, karyotype, meiosis, hybrid, amphidiploid, triploid, diploid, Triticeae.

Genome analysis in higher plants

Gordon Kimber introduced the workshop topics that were designed to illustrate several facets of genome analysis in higher plants. Kimber's presentation covered genome analysis by chromosome pairing in hybrids. He emphasized that this is the only method in which at least some DNA is sampled along the entire length of the chromosomes. Moreover, originally based on total pairing, genome analysis has now been extended to include numerical assessments of the patterns of pairing. Genome analysis is most accurate in those hybrids where there is pairing competition; for example in diploid x allopolyploid hybrids. There are various models of pairing at increasing levels of ploidy that can be used to calculate relative measures of affinity among the genomes. In the ensuing discussion,, Kimber pointed out that genes affecting chromosome pairing do not alter the results of genome analysis as they influence overall pairing, but not the pattern of pairing. Limitations to genome analysis were also discussed. For example, chromosome pairing in diploid hybrids cannot be considered a reliable indication of relationship because of lack of competition for synaptic partners. Similarly, plants with small chromosomes are also unsuitable for genome analysis as differences between rod and ring bivalents cannot be distinguished. Second speaker, B. S. Gill, spoke on the use of heterochromatin banding and in situ hybridization in assaying subgenomic affinities. These techniques provide additional and complementary information to genome analysis by chromosome pairing. The subgenomic affinities that can be assayed include individual chromosome and chromosome arm homologies and specific DNA sequence homologies. Individual chromosome homologies have been assessed by banding analysis of metaphase I configurations in meiocytes. Somatic chromosome banding analysis provides additional information on the amount, location, and structure of heterochromatin. In situ hybridization has been used to study the amount and location of specific DNA sequences in different genomes. Such marked chromosomes can be traced in natural and experimental introgressive hybridization studies. Southern blotting analysis with defined DNA sequences (rDNA, as an example) at different 'stringencies of hybridization can be used to discern evolutionary relationship among taxa. The questions involved strategies for cloning of DNA sequences for molecular cytogenetics analysis. The third speaker, K. Tsunewaki described the role of plasmon analysis as a means of identifying cytoplasm donor to polyploid species. Cytoplasm of various Aegilops species have been substituted with the nucleus of Triticum aestivum. From the phenotypes of such alloplasmic lines, it has been possible to trace diploid donor of cytoplasm of polyploid Triticum and Aegilops species. More recently, restriction digestion analysis of chloroplast and mitochondrial DNA has been used for assaying cytoplasmic affinities. Some of the problems with the use of plasmon analysis in phylogenetic affinities were discussed. First of all, there is intraspecific variation in both mitochondrial and chloroplast DNAs. Another potential problem is the male transmission of cytoplasmic organelles, although in wheat no such male transmission has been documented. The final speaker, D. F. Weber, discussed the use of restriction fragment length polymorphisms (RFLPs) in corn improvement. He elaborated on the methodology and applications of RFLP analysis. In addition to their use as molecular markers, RFLPs are also useful in uncovering single gene or small chromosomal duplications or other aberrations that cannot be detected by pairing analysis. About 400 RFLP loci have been mapped in corn. About 20 % of the loci were found to be duplicated either on the same chromosome or were dispersed on many chromosomes. The A-B translocations were useful in chromosome arm assignment of RFLP loci, in mapping of centromeres, and in aligning RFLP linkage maps with chromosome maps and conventional linkage maps. There was considerable discussion on the advancement of RFLP maps in different taxa. It was suggested that there should be a coordination of the work on RFLP maps and on comparative gene mapping. Annual meetings of the Genetics Society of Canada or America could provide forums for joint meetings of RFLP workers in plants. The workshop ended on a note of thanks by Kimber to participating speakers and the audience. About 60 people participated and the workshop was considered a success.

The effect of different doses of Ph1 on chromosome pairing in hybrids between tetraploid Agropyron elongatum and common wheat

Chromosome pairing at first meiotic metaphase was studied in F1 hybrids between tetraploid cytotypes of Agropyron elongatum and common wheat lines of the cultivar Chinese Spring, carrying zero, one, and two doses of Ph1. The bivalentization gene system of A. elongatum could not compensate for the absence of Ph1: hybrids deficient for this gene exhibited pairing between the Agropyron E1 and E2 chromosomes, between the wheat A, B, and D chromosomes, and between the Agropyron and the wheat chromosomes. In hybrids with one or two doses of Ph1, pairing was restricted to the Agropyron E1 and E2 chromosomes. It was concluded that E1 and E2 are distant homologues, thus further supporting the autoploidy nature of tetraploid A. elongatum. The genomic relationships in other polyploid species of the genus Agropyron is discussed in the light of this evidence.Key words: chromosome pairing, Triticum, common wheat, Agropyron.

A reassessment of genome relationships between Thinopyrum bessarabicum and T. elongatum of the Triticeae

Chromosome pairing and chiasma frequency in diploid (2n = 2x = 14; JE genomes), amphidiploid (2n = 4x = 28; JJEE), and triploid (2n = 3x = 21; JJE) hybrids between Thinopyrum bessarabicum (2n = 2x = 14; JJ) and T. elongatum (2n = 2x = 14; EE) were analyzed. The diploid hybrids (JE) showed a mean pairing of < 0.01V + 0.30IV + 0.28III + 4.98II + 1.97I with 8.36 chiasmata per cell. The pairing was rather poor, most bivalents being rod-shaped; some were clearly hetero-morphic and loosely paired (probably pseudochiasmate). The diploid hybrids were sterile, showing the reproductive isolation of the parental species. The JJE triploid had a mean chromosome configuration of < 0.01VI + 0.06IV + 1.53III + 5.46II + 5.20I with a chiasma frequency of 13.45 per cell. Chromosomes of the duplicated genome JJ showed preferential pairing, forming mostly ring bivalents with two or even three chiasmata each, as in the T. bessarabicum parent; most chromosomes of the E genome remained as univalents. Thus, the E genome chromosomes offered little synaptic competition to the chromosomes of the duplicated JJ genome. The degree of preferential pairing was even stronger in the JJEE amphidiploids, which predominantly showed bivalent pairing with up to 14 ring bivalents in some cells. They had a mean pairing of 0.01VI + 0.55IV + 0.26III + 11.75II + 1.42I; the mean quadrivalent frequency per cell varied from 0.10 to 1.53. Thus J and E genomes essentially maintained their meiotic integrity at the 4x level. This pattern of chromosome pairing in hybrids at different ploidies and the sterility of diploid hybrids show that J and E are distinct genomes and that there is little justification for merging them, as suggested by previous workers. The J and E are homoeologous at best. The merger of Lophopyrum (E genome) with the genus Thinopyrum (J genome) would be improper. Although the J and E genomes are close enough to permit some intergenomic gene flow, which may be exploited in plant breeding, they are certainly not close enough to have the same genomic designation. The JJEE amphidiploids are meiotically stable and may be a useful source of genes for wheat improvement.Key words: genome, meiosis, chromosome pairing, phylogenetic relationships, Thinopyrum, interspecific hybrid, autoallo-triploid, amphidiploid.

Chromosome pairing in hybrids of ph1b mutant wheat with rye

Metaphase I pairing was studied in five ph1b mutant wheat × rye hybrids to verify the presence of translocations between homoeologous chromosomes in ph1b mutant wheat and to establish the pairing homoeology between wheat and rye chromosomes. Three 5B-deficient ABDR hybrids with standard chromosome structure were used as controls. Chromosomes 1R and 5R of rye and most wheat chromosomes, as well as their arms, were identified by means of C-banding. The presence of 5BS in ph1b hybrids raised the overall pairing level. The pattern of pairing between wheat chromosomes in ph1b hybrids, as in 5B-deficient hybrids, was characterized by the occurrence of preferential pairing between chromosomes of the A and D genomes in most homoeologous groups. The existence of a double translocation involving 4BL, 5AL, and 7BS in common wheat was confirmed. Deviation from the standard pairing pattern suggested the existence of a translocation involving 1BL and 1DL in one ph1b ABDR plant and another translocation involving 3AL and 3DL in three other ph1b hybrids. In ph1b hybrids, wheat – rye pairing was relatively frequent for 1RL, 5RL, and an arm of a metacentric rye chromosome, probably 2R, that is homoeologous to 2BL, and the homoeologous arms of 2A and 2D. The existence of a translocation involving 5RL and 4RL in rye was confirmed.Key words: homoeologous, homologous, 5B-deficient, translocations, C-banding.

Cytology and Reproductive Behavior of Paspalum equitans, P. ionanthum, and Their Hybrids with Diploid and Tetraploid Cytotypes of P. cromyorrhizon

Paspalum equitans is a sexual, cross-pollinated diploid (2n = 2x = 20) with normal meiosis. It has some potential for apomixis because one or two aposporous embryo sacs were observed beside the meiotic sac in 18.2% of the ovules. Regular meiotic behavior was confirmed in tetraploid (2n = 4x = 40) P. ionanthum (= P. guaraniticum), which has 20II at meiosis and is cross-pollinated. Hybrids were produced between P. equitans and diploid P. cromyorrhizon, a species with the basic N genome. A high degree of meiotic chromosome pairing in the hybrids demonstrated that P. equitans also has the N genome. However, these species are isolated because the F1 hybrids are sterile. Meiotic chromosome pairing in hybrids of P. ionanthum x diploid and tetraploid P. cromyorrhizon indicated that at least one genome of P. ionanthum is a form of the N genome. Moreover, 1-7III were observed in ca. 75% of pollen mother cells of P. ionanthum x diploid P. cromyorrhizon triploid hybrids, and quadrivalents were observed in P. ionanthu...