Extensive synaptonemal complex formation was found at prophase I in whole mount spread preparations of a spontaneous haploid rye, Secale cereale, with values of up to 87.8 per cent of the chromosome complement synapsed. Pairing-partner switches were frequent, giving rise to multiple associations in which all or most of the chromosomes were involved. However, the distribution of synaptonemal complex stretches suggests that synapsis does not occur at random. The frequency of multivalents and the mean frequency of bonded arms at metaphase I were 0.03 and 0.39, respectively. Associations between chromosome arms without heterochromatin were more frequent than between the remaining arms. The observation of recombinant chromosomes for telomeric C-bands at anaphase I indicates that metaphase I bonds are true chiasmata. The correspondence between the location of pairing initiation sites and chiasmata indicates that early synapsis could be confined to homologous regions.
Four Korean rye populations with B chromosomes at frequencies ranging from 15 to 55 per cent have been analyzed in two successive generations in a field experiment. B chromosome frequencies were maintained in spite of the environmental change from Korea to Madrid. Values of variables with effect on viability and fertility have been estimated in plants with and without Bs, in both generations. B chromosomes did not affect viability, while fertility decreased when the number of Bs increased. Our data are against an adaptive nature of B chromosomes.First pollen mitosis was observed in 2B plants. The frequency of metaphases with lB and the rate of nondisjunction at anaphase were high and similar in all populations. Since the decrease of fertility due to Bs and the cytological behaviour of Bs at gametogenesis were similar in all populations, the balance between these two factors cannot account for the establishment of equilibria at different frequencies.
The chromosome structure of four different wild populations and a cultivated line of Triticum timopheevii (2n = 28, AtAtGG) relative to Triticum turgidum (2n = 28, AABB) was studied, using genomic in situ hybridisation (GISH) and C-banding analysis of meiotic configurations in interspecific hybrids. Two wild accessions and the cultivated line showed the standard C-banding karyotype. The other two accessions are homozygous for translocation 5At/3G and translocations 1G/2G and 5G/6G. GISH analysis revealed that all the T. timopheevii accessions carry intergenome translocations 6At/1G and 1G/4G and identified the position of the breakpoint in translocation 5At/3G. C-banding analysis of pairing at metaphase I in the hybrids with T. turgidum provides evidence that four species-specific translocations (6AtS/1GS, 1GS/4GS, 4GS/4AtL, and 4AtL/3AtL) exist in T. timopheevii, and that T. timopheevii and T. turgidum differ in the pericentric inversion of chromosome 4A. Bridge plus acentric fragment configurations involving 4AL and 4AtL were identified in cells at anaphase I. This result suggests that the paracentric inversion of 4AL from T. turgidum does not exist in T. timopheevii. Both tetraploid species have undergone independent and distinct evolutionary chromosomal rearrangements. The position, intercalary or subdistal, of the breakpoints in species-specific translocations and inversions contrasts with the position, at or close to the centromere, of intraspecific translocations. Different mechanisms for intraspecific and species-specific chromosome rearrangements are suggested.
Chromosome pairing of standard B chromosomes (Bs) in rye has been studied by synaptonemal complex surface spreading of pollen mother cells containing from one to eight Bs. The main characteristics exhibited by the Bs are: (i) They are often located peripherally in surface spread nuclei and do not show presynaptic alignment. (ii) The period of pairing is delayed relative to that of the A set. (iii) Bs undergo extensive non-homol-ogous pairing at pachytene. (iv) The frequency of trivalents and quadrivalents in plants with 3Bs and 4Bs is about 66% following the expectations on the basis of random pairing among the short and the long arms with a single distal pairing initiation site per arm. Some quadrivalents containing two partner switches were observed, but on the same basis of randomness there was a deficiency of pachytene multivalents in plants with 5-8 Bs. The possible causes of the discrepancy between these observations are discussed. (v) There is a reduction in the frequency of multivalents between pachytene and metaphase I.
Chromosome pairing at metaphase I was studied in different interspecific hybrids involving Aegilops speltoides (SS) and polyploid wheats Triticum timopheevii (AtAtGG), T. turgidum (AABB), and T. aestivum (AABBDD) to study the relationships between the S, G, and B genomes. Individual chromosomes and their arms were identified by means of C-banding. Pairing between chromosomes of the G and S genomes in T. timopheevii x Ae. speltoides (AtGS) hybrids reached a frequency much higher than pairing between chromosomes of the B and S genomes in T. turgidum x Ae. speltoides (ABS) hybrids and T. aestivum x Ae. speltoides (ABDS) hybrids, and pairing between B- and G-genome chromosomes in T. turgidum x T. timopheevii (AAtBG) hybrids or T. aestivum x T. timopheevii (AAtBGD) hybrids. These results support a higher degree of closeness of the G and S genomes to each other than to the B genome. Such relationships are consistent with independent origins of tetraploid wheats T. turgidum and T. timopheevii and with a more recent formation of the timopheevi lineage.
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