Level Of Chromosome Pairing Research Articles

REGULARITY OF MEIOSIS IN EARLY GENERATIONS IN HYBRIDS OF BREAD WHEAT WITH ARTIFICIAL SPELT

Introduction. The study of meiosis in wheat-alien hybrids occupiess an important place in the introgressive hybridization of wheat.Purpose. The aim of the paper is to investigate regularity of meiosis in F1, F2 and BC1 hybrids of Triticum aestivum L. cv. Fantaziia odeska with wheat-alien amphiploid (T. dicoccum Schuebl. x Ae. tauschii Feld. et Kis.).Results. 42-chromosome PMC predominated in F1. The average number of bivalents in PMC in plants ranged from 16.7 to 19.2. Closed bivalents predominated. Multivalents were detected. The meiotic index was low (30.3-36.5 %). The main class of defective products of meiosis was represented by tetrads with micronuclei. Plants differed significantly in the frequency of polyade formation. No significant dependence between the level of chromosome pairing and the frequency of normal tetrads was detected. The F2 plants varied significantly in the pattern and level of chromosome pairing. The karyotype was stabilized, but multivalents awere detected. The meiotic index in plants ranged from 58.1 to 68.5 %. The pattern and level of chromosome pairing in BC1 were significantly higher than in F1 and F2. Depending on plant genotype the number of bivalents in the PMC was 19.3-20.8, and the closed bivalents 14.4-19.1. Multivalents were detected. The meiotic index of plants BC1 varied from 84.8 to 94.2 %. Among the anomalies, the tetrads with micronuclei are the most numerous, but the polyades and atypical tetrads were also observed.Conclusions. A high degree of chromosome homology of both parents is shown. The regularity of meiosis significantly improved from F1 to F2 and BC1. Normalization of chromosome pairing and meiotic index reflects the improvement of the meiosis regularity. Heterogeneity in the regularity of meiosis, depending on the genotype of the plant is observed. The level of chromosome pairing is not the only factor determining the regularity of meiosis in hybridization of bread wheat with wheat-alien amphiploid. Anomalies in the functioning of the mitotic spindle significantly contribute to the irregularity of meiosis.

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.

Meiotic recombination in sexual diploid and apomictic triploid dandelions (Taraxacum officinale L.).

Taraxacum officinale L. (dandelion) is a vigorous weed in Europe with diploid sexual populations in the southern regions and partially overlapping populations of diploid sexuals and triploid or tetraploid apomicts in the central and northern regions. Previous studies have demonstrated unexpectedly high levels of genetic variation in the apomictic populations, suggesting the occurrence of genetic segregation in the apomicts and (or) hybridization between sexual and apomictic individuals. In this study we analysed meiosis in both sexual diploid and apomictic triploid plants to find mechanisms that could account for the high levels of genetic variation in the apomicts. Microscopic study of microsporocytes in the triploid apomicts revealed that the levels of chromosome pairing and chiasma formation at meiotic prophase I were lower than in that of the sexual diploids, but still sufficient to assume recombination between the homologues. Nomarski DIC (differential interference contrast) microscopy of optically cleared megasporocytes in the apomicts demonstrated incidental formation of tetrads, which suggests that hybridization can occur in triploid apomicts.

A critical review of concepts and methods used in classical genome analysis

A short account of the development of classical genome analysis, the analysis of chromosome behaviour in metaphase I of meiosis, primarily in interspecific hybrids, is given. The application of the concept of homology to describe chromosome pairing between the respective chromosomes of a pair during meiosis is traced, and the relationship between this use of homology and the concept of homology as common ancestry is discussed at length. To equate the two concepts has led to the erroneous assumption that levels of chromosome pairing is an indication of phylogenetic relationship. Even accepting the fundamental premises, genome analysis is burdened by observational difficulties. Hence, chromosome pairing has been shown to be under genetic control and is also influenced by environmental conditions. Additionally, basic biological observations such as the distribution of meiotic configurations or the identity of the individual chromosomes are frequently neglected. Data from chromosome pairing are captured as pair-wise comparisons and are amenable only to phenetic analysis, and hence are not suited for phylogenetic inferences. As currently perceived, genome analysis may have a role to play in plant breeding but it has no place in systematics. With an increased knowledge and understanding of the mechanism behind meiosis, data useful in a systematic context may eventually be produced.

Metaphase-I analysis of a Triticum aestivum x T. monococcum hybrid by the C-banding technique

The meiotic pairing behaviour at metaphase I of a Triticum aestivum×Triticum monococcum hybrid has been studied by means of the C-banding technique to ascertain the homology between the chromosomes in the A genome of the two species. The technique allowed the A and B genome chromosomes and the 2D, 3D and 5D chromosomes to be identified. Differences in the level of chromosome pairing in the A genome were noted. The T. monococcum 4A chromosome did not pair with any of the T. aestivum chromosomes in any of the metaphase I cells analysed. Two reciprocal translocations between the 2B and 2D chromosomes on one side and the 2A and 3D on the other side have been identified. The usefulness of the C-banding technique in the study of chromosome homology among species related to wheat is discussed.

Developments in the meiotic analysis of hybrids. III. Amended models for pentaploids

Amended models of meiotic behaviour in pentaploid hybrids are presented, which better represent the theory and assumptions about chromosome pairing that such models necessarily incorporate. They correct problems concerning the distribution of chiasmata among and within chromosome configurations inherent in earlier models. In general these analyses give similar interpretations to earlier models but in some cases suggest differing conclusions. The extent of pairing possibilities at the pentaploid level can make it difficult to choose with confidence among the proposed models when the level of chromosome pairing is low.

A New Intergeneric Hybrid between Triticum aestivum L. and Agropyron fragile (Roth) Candargy: Variation in A. fragile for Suppression of the Wheat Ph-Locus Activity

New intergeneric hybrids were obtained between Triticum aestivum L. cv. Tukuho’ (2n = 6x = 42, AABBDD) and Agropyron fragile (Roth) Candargy PGR 8097 (2n = 4x = 28, PPPP) at a frequency of 1.06 %, through the use of direct embryo culture and in ovulo embryo culture. Such hybrids could be used to transfer barley yellow dwarf virus (BYDV) resistance and winterhardiness into bread wheat. The somatic chromosome number in all the hybrid plants was 2n = 5x = 35, as expected. Considerable variation in chromosome pairing was observed among the different hybrid plants. Average meiotic chromosome configuration at metaphase I was 17.29 Is + 6.57 rod Us + 1.97 ring Us + 0.18 III + 0.03 IV + 0.002 VI. The high level of chromosome pairing in some F1 hybrids was attributed to Ph-suppressor gene(s) present in A. fragile. The hybrids could not be backcrossed to wheat, but amphiploid seeds have been obtained by colchicine treatment.

The inheritance of genetic variation in Triticum speltoides affecting heterogenetic chromosome pairing in hybrids with Triticum aestivum

Triticum speltoides (Tausch) Gren. ex Richter plants which differed in the ability to promote heterogenetic chromosome pairing in hybrids T. aestivum L. × T. speltoides were crossed and a single F1 plant from each cross was hybridized with T. aestivum. The segregation among the hybrids for mean number of chiasmata per cell showed that two gene systems in T. speltoides genotypes were involved in the promotion of heterogenetic pairing. One system was composed of two duplicate gene loci segregating independently of each other. Variation at these loci determined two basic levels of heterogenetic pairing. The other system was composed of several minor genes extensively modifying the effects of the major genes. The minor genes interacted mostly in an additive fashion. Triticum speltoides inbred plants were then crossed with diploid species. T. tauschii (Coss.) Schmal and T. dichasians (Zhuk.) Bowden. Consistent differences in the levels of chromosome pairing were found in these hybrids. However, this variation in chromosome pairing did not coincide with the variation at the major gene loci. This indicated that the major genes were ineffective in the diploid hybrids.Key words: Triticum, pairing regulation, homeologous pairing, heterogenetic pairing.

The effect of homoeologous group 3 chromosomes on chromosome pairing and crossability in Triticum aestivum

Differences in the level of chromosome pairing in hybrids between 'Chinese Spring' wheat homoeologous group 3 aneuploids and rye and in homoeologous group 3 aneuhaploids were studied. Factors affecting chromosome pairing were detected or confirmed on both arms of the chromosomes of homoeologous group 3 in wheat. Effects were also identified on chromosome 3R of rye and a chromosome of Hordeum bulbosum. Factors affecting crossability between 'Chinese Spring' wheat and H. bulbosum were also found on chromosomes 3A, 3B, and 3D. A correlation was shown between increased pairing and reduced crossability.

New hybrids between Agropyron and wheat

Intergeneric hybrids of Triticum aestivum (2n=42,AABBDD) with Agropyron ciliare (2n= 28,SSYY), A. trachycaulum (2n=28,SSHH), A. yezoense (2n=28,SSYY) and A. scirpeum (2n=28) are reported for the first time. F1 hybrids of T. aestivum were also produced with A. intermedium (2n=42,E1E1E2E2Z1Z1) and A. junceum (2n=14,JuJu). All wheat-Agropyron hybrids were obtained by embryo rescue technique. Cultivars and reciprocal crosses differed for seed set, seed development and F1 plant production. The F1 hybrids were sterile. Attempts to obtain amphiploids were unsuccessful. However, backcross derivatives were obtained with wheat as the recurrent parent.The level of chromosome pairing in A. trachycaulum x wheat, A. yezoense x wheat and wheat x A. junceum hybrids provided no evidence of homologous or homoeologous pairing. Mean pairing frequencies in A. ciliare x wheat, wheat x A. scirpeum and wheat x A. intermedium hybrids indicated homoeologous or autosyndetic pairing. Ph gene was more effective in regulating homoeologous pairing in A. yezoense x wheat hybrids than in A. ciliare x wheat hybrid. Chromosome pairing data of BC1 derivatives indicated that either some of the wheat chromosomes were eliminated or Agropyron chromosomes caused reduced pairing of wheat homologues.

Regulation of homoeologous pairing in pentaploid wheat hybrids

Comparisons were made of levels of chromosome pairing in pentaploid hybrids between normal and ph1b-and ph2-mutant forms of Triticum aestivum ssp. vulgare cv. Chinese Spring each crossed with Triticum kotschyi and T. turgidum var. dicoccoides. Higher levels of multivalent formation in the ph1b-kotschyi, compared with the ph1b-dicoccoides pentaploid was attributed mainly to the absence of allelic buffering, by the Ph1 allele, in the Kotschyi pentaploid and its presence in the dicoccoides pentaploid. The higher level of homoeologous pairing in the ph2-dicoccoides pentaploid compared with that of kotschyi was believed to be due to differential levels of homoe-and non-homoallelic buffering in the two pentaploids. The high level of homoeologous pairing caused by the ph2-mutant in the dicoccoides pentaploid indicates a potential use of pairing promoters, free of homoallelic buffering, as a means of increasing thelevel of homoeologous pairing in wheat and in certain of its hybrids with alien species.

Meiotic duration in wheat genotypes with or without homoeologous meiotic chromosome pairing.

In order to investigate the possible relation between meiotic time and meiotic chromosome pairing behaviour, meiosis was timed in various forms of wheat and wheat hybrids. First, meiosis was timed in tenTriticum aestivum(var. Chinese Spring) genotypes with different chromosome constitutions which differed widely in the meiotic pairing behaviour. Secondly, in order to escape from the disadvantage of aneuploid material, meiosis was also timed in plants which differed in the extent of homoeologous pairing because of the activities of different alleles at one or two loci. For this experiment use was made of F1-hybrids from the crossT. aestivumxAegilops muticawhich, although they all have 28 chromosomes, differ widely in the amount of homoeologous pairing. Thirdly, meiosis was also timed in 28-chromosome and 29-chromosome plants derived from the cross between rye(Secale cereale)x 43-chromosomeT. aestivumcontaining a singleAe. muticaaddition chromosome known to carry genes which greatly affect the level of homoeologous pairing in wheat. Although the 28-chromosome plants display very little pairing (chiasma frequency per cell (c. f.) = 0.5) while 29-chromosome plants display a much higher amount of pairing (c. f. = 7.8) no difference in meiotic time was detected between them. Similarly, the duration of meiosis was not significantly different between the three types of F1-hybrids betweenT. aestivumxAe. muticawhich had chiasma frequencies of 14.3, 7.4 and 0.9. Thus, these results agree in showing that there was no correlation between the duration of meiosis and the amount of homoeologous chromosome pairing. The results obtained for genotypes of Chinese Spring also provided no evidence to support the notion that there is a relation between the level of chromosome pairing and the duration of the pairing process. Consequently some doubt must be cast upon the idea that the time available for pairing is limiting to the pairing process. It was shown that individual wheat chromosomes in Chinese Spring differed in their effects on meiotic duration. For instance, the absence of chromosome 7B has no detectable effect on meiotic duration. The absence of chromosome 5B in two genotypes resulted in an increase in meiotic time from that found in euploid plants (24 h) to that found in tetraploid wheat species (about 30 h). By using plants ditelosomic for chromosome 5BLit was shown that most, if not all, of the genetic effects of chromosome 5B on meiotic time are determined by the short arm.

Cytology of species hybrids in the Setaria sphacelata complex

Hybrids were produced between diploid strains of Setaria anceps, S. trinervia, tetraploid strains of S. anceps and S. splendida, and hexaploid strains of all three species. Very high levels of chromosome pairing at meiosis indicate close relationship of the different species and throw doubt on the validity of their separation. It is probable that characters from S. trinervia and S. splendida may be incorporated in the commercially useful S. anceps without too much difficulty.

THE ALLOCATION OF THE CHROMOSOMES OF TRITICUM AESTIVUM TO THE A AND B GENOMES AND EVIDENCE ON GENOME STRUCTURE

The common wheat of agriculture, T~iticz~nz aestivz~7gz (2n = 6x = 42), is an allohexaploid in xvhich the chromosome complements of three distinct diploid species are combined. I t has the genome structure AABBDD. The 4 genome was derived either directly from Triticum I~TOIZOCOCCZ~WZ (2 = 14) 2nd the B genome from Aegilops sljeltoides, or froill close relatives of these species, while Aegilops s q z ~ a ~ ~ o s a contributed the D genome. Sears (1954) determined which lines viere monosomic for chromosomes of the D genome among the full set of 21 monosomic lines he isolated in the ~ a r i e t y Chincse Spring. This was accomplished by pollinating every monosomic line by the AABB tetraploid species Triticz~nz dicoccoides (2n = 4s = 8 ) . Anlong the hybrids resulting from these crosses there were 35-chromosome plants, to ~v l i i c l~ the chromosoi~~e monosomic in the Chinese Spring parent had been transmitted. I n addition there were 31-chroinoson~e plants which had not received the monosomic chron~osome. A t meiosis in the 35-chrornoson~e plants there were I4 bivalents, resulting from the pairing of 11o111ologous cl~ron~osomes of the A or B genomes, and seven univalents representing the D genome cl~romosomes. T w o patterns of pairing occurred in the 31-cl1romosoi11e hybrids. Plants with 14 bivalents and sis univalents were clearly deficient for one of the I1 genome univalents present in the 35-chromosome hybrids. I t could therefore be concluded that the Chinese Spring parent was monosomic for a chromoson~e of the D wenome. s In hvbrids with the alternative behaviour. in which there were 13 bivalents and light univalents, the number of A or I3 genome bivaleilts was reduced. From this it could be coilcluded that the Chinese Spring parent was illonosoinic for a chron~osome of the A or the B genome. On the basis of this classificatio~l of the chromosome complement of T. aestivz17~r Sears assigned the numbers I-XIV to the chronlosomes of the A and B genomes and XV-XXI to those of the D genome. The identification of the cl~romosomcs of thc A and B genorncs was carried out by Oliailloto (1962) using the lines of Chinese Spring disonlic for telocentrics of chromosomes I-XIV. In these lines each pair of chron~oson~es, in turn, was structurally nlarlted having one arm represented by a pair of telocentrics \vhile the other arnl was entirely deficient. Evely telocentric line was pol1inate.d by the svnthetic AADD a l l~ t e t r a~ lo id (T . aegilopoides s Ae. sqzra~rosa) (2n = 4 s =-28). Allocation of the chron~osome, represented by a telocentric, to the A or B genome resulted from the study of meiotic pairing in the 35-chron~osoine AABDD hybrids. When a telocentric participated 111 bivalents the chromosome that it lrrarli~d was assioned to the A L7 genome. Alternatively nlllen a telocentric failed to pair the irrarlted chromosome was assigned to the B genome. A generally satisfactory classification xvas achieved in this way, but because of the rather low and variable level of chromosome pairing at ineiosis in the