Chiasma distribution in rye chromosomes of diploid rye and in wheat/rye addition lines in relation to C-heterochromatin

Variable Patterning of Chromatin Remodeling, Telomere Positioning, Synapsis, and Chiasma Formation of Individual Rye Chromosomes in Meiosis of Wheat-Rye Additions

Meiosis, the type of cell division that halves the chromosome number, shows a considerable degree of diversity among species. Unraveling molecular mechanisms of the meiotic machinery has been mainly based on meiotic mutants, where the effects of a change were assessed on chromosomes of the particular species. An alternative approach is to study the meiotic behavior of the chromosomes introgressed into different genetic backgrounds. As an allohexaploid, common wheat tolerates introgression of chromosomes from related species, such as rye. The behavior of individual pairs of rye homologues added to wheat has been monitored in meiotic prophase I and metaphase I. Chromosome 4R increased its length in early prophase I much more than other chromosomes studied, implying chromosome specific patterns of chromatin organization. Chromosome conformation affected clustering of telomeres but not their dispersion. Telomeres of the short arm of submetacentric chromosomes 4R, 5R, and 6R failed more often to be included in the telomere cluster either than the telomeres of the long arms or telomeres of metacentrics such as 2R, 3R, and 7R. The disturbed migration of the telomeres of 5RS and 6RS was associated with failure of synapsis and chiasma formation. However, despite the failed convergence of its telomere, the 4RS arm developed normal synapsis, perhaps because the strong increase of its length in early prophase I facilitated homologous encounters in intercalary regions. Surprisingly, chiasma frequencies in both arms of 4R were reduced. Similarly, the short arm of metacentric chromosome 2R often failed to form chiasmata despite normal synapsis. Chromosomes 1R, 3R, and 7R showed a regular meiotic behavior. These observations are discussed in the context of the behavior that these chromosomes show in rye itself.

The cytogenetic and molecular architecture of chromosome 1R--one of the most widely utilized sources of alien chromatin in wheat varieties

Chromosome 1R of rye (Secale cereale) is one of the most intensively used sources of alien chromatin in bread wheat. It provides a source of valuable disease resistance genes and its widespread use has stimulated the development of genetic maps incorporating protein and DNA markers. Published data as well as new mapping data were combined into a consensus map by using common markers in the different mapping studies to orient the various data sets relative to each other. The consensus map provides a tentative order, and relative distances, between the genetic markers. A summary of the dispersed repetitive sequences that are now available for the detection and characterization of chromosome 1R segments in wheat is also provided.

Effects of constitutive heterochromatin and genotype on frequency and distribution of chiasmata in the seven individual rye bivalents

Number and distribution of chiasmata were studied in the single pair of homologous rye chromosomes in 29 chromosomal F1 hybrids between the seven disomic wheat rye addition lines of 'Chinese Spring'/ 'Imperial' and five selected inbred genotypes of cultivated rye by using the differential Giemsa staining technique. The results indicate that the number and position of chiasmata is independent from the amount and position of C-heterochromatin. Genotype had an effect on chiasma number, whereas chiasma distribution within bivalents appeared to be determined by morphological features of chromosomes. Late replicating DNA in constitutive heterochromatin may delay the separation of half bivalents if chiasmata are formed between them and the centromere.

Identification of the chromosomes of the rye translocation tester set

Intercrossing the Wageningen translocation tester set of rye and the series of 'Imperial' rye additions to 'Chinese Spring' wheat of Sears yielded 29 chromosome disomic translocation hybrids. Observation of trivalents led to the identification of the chromosomes of the tester set in terms of the terminology system used in the Triticinae. The analysis was complicated by very low chiasma frequencies in some short chromosome segments in the hybrids. Nevertheless, it could be safely concluded that 1R=VII; 2R=III; 3R=II; 4R=IV; 5R=VI; 6R=V; 7R=I, which deviates slightly from previous classifications based on other methods.