Relationships between frequency, localization and errors in chiasma formation in desynaptic rye

Connection between rod bivalents and incomplete meiotic association at NORs in Hordeum marinum Huds

The hypothesis of a connection between rod bivalent formation and incomplete meiotic association at NORs of SAT-chromosomes of H. marinum is supported. PMCs of H. marinum ssp. gussoneanum (2x), two diploid ssp. marinum × ssp. gussoneanum (2x) hybrids and two ssp. gussoneanum (4x) × Secale cereale hybrids at metaphase I (M-I) were analyzed by in situ hybridization. The probe pTa71 labelled rDNA sites at NORs of a single pair of homologous or near-homologous SAT-chromosomes of H. marinum in each material. In the three diploids, M-I was regular with ring bivalents and one or a few rods (av.13.52 bound arms cell(-1)). More rod bivalents than the expected one out of seven, i.e. 30, 67 and 89% included rDNA-carrying chromosomes. Corresponding bound short arm frequencies were 0.89, 0.72 and 0.52, while long arms and arms of other chromosomes presented complete or near- complete association. The two heterogenomic hybrids had a less regular M-I (av. 8.04 bound arms cell(-1)) including 20% rDNA-carrying rods with bound arm frequencies of 0.29 in short and 0.87 in long arms. Positions of chromosome associations were established in all 150 rDNA-carrying bivalents. In 77 bivalents with short arm associations, 4% of these occurred proximally to, none at, and 96% distally to rDNA sites, i.e. in satellites. In 143 bivalents with long arm associations, 83% occurred at interstitial and 17% at terminal positions. The observations combine increased frequency of rDNA-carrying rods with decreased frequency of association at NOR regions of SAT-chromosomes. The basis for the relationship is discussed.

Analysis of chromosome pairing and breakage in pearl millet

The relationship between chromosome pairing and chromosome fragmentation has been studied in a gene controlled mutant of pearl millet (2n= 14). Premeiotic mitosis, premeiotic cell development and early prophase I are normal without any fragments, which first appear at pachytene. The extent of fragmentation varies from zero to very extreme with two discrete classes of plants, namely those with partial fragmentation and those with multiple fragmentation. A quantitative analysis of bivalent distribution and the distribution of AI bridges in desynaptic and fragmented cells show all of them to be nonrandom events. We suggest that in cells showing partial fragmentation the bridges and fragments result from U-type exchanges at pachytene. The reduced frequency of AII bridges indicates relatively low sister chromatid reunion at pachytene. In multiple fragmented plants numerous minute fragments were seen from pachytene. Despite these anomalies most PMCs complete meiosis but subsequently abort at the pollen grain stage. The mutant gene also causes disturbances in the sequence of meiotic development in the ear and in the synchronous development of PMCs within an anther. It has no effect on the tapetum or on the physiological development of the anther.

The dicentric nature of the metacentric B chromosome of Metaleptea brevicornis adspersa (Acridinae, acrididae)

The meiotic behaviour of a large B isochromosome of Metaleptea brevicornis adspersa (Acridinae, Acrididae) was studied in both sexes using conventional preparations and in males by silver staining of surface-spread synaptonemal complexes and kinetochores. In males, both arms of the B chromosome synapse at zygotene-early pachytene suggesting its isochromosomal nature. Both arms separate precociously at late pachytene and remain joined only through the formation of a single distal chiasma in 92.5% and 91.7% of male and female meiocytes, respectively. In both sexes, the B chromosome tends to migrate precociously to one of the spindle poles, auto-orientating at the equator in 19.2% and 16.6% of metaphase I cells of males and females, respectively. In males, this produces a high frequency of anaphase I/telophase I lagging with the consequent inhibition of cytokinesis and the formation of second division restitution nuclei. For a better understanding of the synaptic and segregation behaviour of the B chromosome in males, pachytene synaptonemal complex (SC) formation was studied. In 0B males, pachytene nuclei invariably showed 11 fully paired SCs, each one with a densely stained pair of terminal kinetochores, and a single axis with a single kinetochore corresponding to the X univalent. In 1B males, both arms of the B chromosome appeared fully paired in 93. 8% of the nuclei, showing complete asynapsis in the rest. This frequency is almost exactly the same as distal chiasma formation in diplotene. The centric region of the SC of the B chromosome had two kinetochores connected by a single axial filament of variable length. The dicentric nature of the B was confirmed by silver staining of kinetochores at metaphase I and anaphase II stages. This is the first time that a dicentric B isochromosome has been described. Based on these results, two alternative models for the origin of isochromosomes are postulated.

A cytogenetic map on the entire length of rye chromosome 1R, including one translocation breakpoint, three isozyme loci and four C-bands

A cytogenetic map of the whole 1 R chromosome of rye has been made, with distances between adjacent markers shorter than 50% recombination. Included in the map are isozyme loci Gpi-R1, Mdh-R1 and Pgd2, the telomere C-bands of the short arm (ts1) and the long arm (tl1), two interstitial C-bands in the short arm proximal to the nuclear organizing region (NOR) (is1) and in the middle of the long arm (il1), respectively, and translocation T273W (Wageningen tester set). By means of electron microscope analysis of spread pachytene synaptonemal complexes, the breakpoint of this translocation was physically mapped in the short arm of 1R, proximal to NOR, and in the long arm of 5R (contrary to previous assumptions). The data indicated the marker order: ts1 - Gpi-R1 - is1 - T273W/Mdh-R1 - il1 - Pgd2 - tl1. A comparison between genetic and physical maps revealed that recombination is mainly restricted to the distal regions of both arms. For the translocation T273W, in heterozygotes no recombinants were observed between the translocation breakpoint and its two adjacently located markers (is1 and Mdh-R1), but recombination was not reduced in the distal regions of the chromosome. The segregations of several other isozyme and C-band markers also analyzed in the investigation presented here were consistent with observations of earlier authors concerning chromosome asignment and linkage relationships.

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

In five genetically different inbred lines of rye and in the seven 'Chinese Spring'/'Imperial' wheatrye addition lines, chiasma distribution in rye chromosomes was studied with respect to the amount and position of constitutive heterochromatin (Giemsa C-bands). In all inbred lines, rye chromosomes with one primary terminal band were more frequently found as univalents than those with primary bands on both telomeres. These chromosomes were most probably 5R and/or 6R. In the addition lines a highly significant reduction in the number of arms bound by chiasmata was found for rye chromosomes 5R and 6R. Because of the similar chiasma distribution in the inbred lines and in the rye chromosomes of the addition lines, no effect of the wheat genome on the number of chiasmata in the rye chromosomes can be ascertained. However, a relationship between chiasma frequency and chromosome arm length seems to exist, since under reduced chiasma conditions the two shortest arms of the rye complement, those of chromosomes 5R and 6R, frequently fail to form a chiasma. No effect of the large blocks of constitutive heterochromatin in the telomeres of the rye chromosomes on the position of chiasmata within a bivalent could be established.

Cytogenetics of Lolium perenne : Part 3. Correlation between chiasmata and U-type exchange

In two contrasting types of S5 inbred Lolium perenne a correlation has been found between the distribution pattern of chiasmata within bivalents and that of meiotic U-type exchange. The relationship between the two processes is discussed, in terms of inbreeding effects upon chiasma redistribution, and mention is also made of some other types of chromosome breakages which arise following enforced self-pollination.