Chromosomal Rearrangements and Evolution of Recombination: Comparison of Chiasma Distribution Patterns in Standard and Robertsonian Populations of the House Mouse

The effects of chromosomal rearrangements on recombination rates were tested by the analysis of chiasma distribution patterns in wild house mice. Males and females of two chromosomal races from Tunisia differing by nine pairs of Robertsonian (Rb) fusions (standard all-acrocentric, 2N = 40 and 2N = 22) were studied. A significant decrease in chiasma number (CN) was observed in Rb mice compared to standard ones for both sexes. The difference in CN was due to a reduction in the number of proximal chiasmata and was associated with an overall more distal redistribution. These features were related to distance of chiasmata to the centromere, suggesting that the centromere effect was more pronounced in Rb fusions than in acrocentric chromosomes. These modifications were interpreted in terms of structural meiotic constraints, although genic factors were likely involved in patterning the observed differences between sexes within races. Thus, the change in chromosomal structure in Rb mice was associated with a generalized decrease in recombination due to a reduction in diploid number, a lower CN, and a decrease in the efficiency of recombination. The effects of such modifications on patterns of genic diversity are discussed in the light of models of evolution of recombination.

Recombination nodule mapping and chiasma distribution in spermatocytes of the pigeon, Columba livia

Pigeon spermatocytes were processed with a drying-down technique and their synaptonemal complex (SC) complements were analyzed by electron microscopy. The synaptonemal complex karyotype of the macrobivalents shows an excellent correspondence with the mitotic karyotype. The number and distribution of recombination nodules (RNs) were scored in complete nuclei stained with phosphotungstic acid. The average number of RNs per nucleus is 64.7. The number of nodules per bivalent shows a clear linear relationship with SC length in the 10 longest synaptonemal complexes, while the microbivalents usually bear a single RN. The location of RNs has a non-random distribution along the largest synaptonemal complexes, with lower frequencies near kinetochores and higher frequencies toward the telomeres. The ZZ bivalent is the fourth in size and shows free recombination, having on average 3.8 RNs. The mean number of nodules per cell and the mean number of nodules in the largest bivalents show very good agreement with the corresponding number of chiasmata scored in metaphase-I spermatocytes. It is concluded that the recombination nodules provide a good check for reciprocal exchanges in this and other species of birds. Additionally, a new morphology for the recombination nodules is presented, consisting of groups of electron-dense particles measuring 43 nm in diameter.Key words: meiosis, chiasmata, recombination nodules, pigeon spermatogenesis.

Chiasma redistribution in the presence of different sized supernumerary segments in a grasshopper: dependence on nonhomologous synapsis

Eight different sized supernumerary segments located at distal ends of the long arms of chromosomes M4, M5, M6, and S8 of the grasshopper Stenobothrus festivus were studied in males with regard to the synaptic process and chiasma distribution in the bivalents that carry them. The M4, M5, and M6 bivalents heterozygous for extra segments were always monochiasmate, in contrast to their bichiasmate condition observed in basic homozygotes. Furthermore, the presence of any of these extra segments led to chiasma redistribution in the carrier bivalents, so that such chiasmata were formed preferentially further away from the extra segment. The intensity of this effect is dependent on the size of the segment. Not all heteromorphic bivalents exhibited synaptonemal complexes with equalized axes at pachytene, but there was always a variable proportion of heterosynapsis around the distal ends of the long arms that was dependent on both the size of the segment and the size of the carrier chromosome. It is proposed that the absence of chiasmata in nonhomologous synapsed regions is responsible for the results obtained. Length measurements of the different extra segments and their carrier chromosomes between pachytene and diplotene indicated that synaptonemal complex is underrepresented in supernumerary heterochromatin.Key words: chiasma distribution, grasshopper, heterosynapsis, supernumerary segment, synaptonemal complex.

Synapsis in grasshopper bivalents heterozygous for centric shifts

Analysis of surface-spread synaptonemal complexes of zygotene and pachytene spermatocytes was carried out on centric-shift heterozygotes of grasshoppers. These rearrangements affected the M7 chromosome in Chorthippus vagans and the M6 and S8 chromosomes in Chorthippus apricarius. The shifts in the latter two chromosomes were also associated with C-heterochromatin variations between homologous chromosomes. Rearranged chromosomes proceeded directly to heterosynapsis without an apparent intervening homosynaptic phase in M7 bivalents of Ch. vagans and M6 bivalents of Ch. apricarius. In the latter case, axial equalization of the heterochromatin polymorphism was also achieved. On the other hand, asynapsis of the intercentromeric regions throughout pachytene was the rule in the centric shift involving the S8 chromosome of Ch. apricarius. In the three cases analysed, the production of unbalanced gametes in the heterozygotes is precluded either by the lack of chiasma formation in heterosynapsed rearranged segments or by the lack of pairing between such segments. Chiasmata were limited to the homologous regions of the heteromorphic bivalents.Key words: synapsis, surface spreading, centric shift, chiasma distribution, meiosis.

Colchicine-induced heritable modification of chiasma frequency and distribution in perennial ryegrass (Lolium perenne)

One-week-old seedlings of inbred lines of perennial ryegrass (Lolium perenne L.) were given a 3-h period of treatment with 0.2% colchicine, and this treatment was found to have heritable effects on chromosome behaviour at meiosis. The mean pollen mother cell chiasma frequency was reduced, and there was a corresponding increase in the number of univalents per cell and also changes in cell and bivalent variances. Patterns of chiasma distribution within bivalents were also modified. The effects were seen in the generation in which the treatment was given and they were also observed in the following sexual generation.Key words: chiasmata, colchicine, Lolium perenne, meiosis, perennial ryegrass.

Sex differences in chiasma distribution along two marked mouse chromosomes: differences in chiasma distribution as a reason for sex differences in recombination frequency

SummaryChiasma distributions along bivalents 1 and 14 in female and male mice were studied. It was shown that the average chiasma number in both chromosomes show no sex difference. There are however, significant sex differences in chiasma distribution along 1 and 14 chromosomes. In males there are two terminal chiasma peaks in chromosome 1 and one subtelomeric peak of chiasmata in chromosome 14. In females chiasma distributions are more even. According to genetic data, females produce more recombinants between loci of chromosome 1 than males do. By means of a computer simulation it was demonstrated that the differences in the average recombination frequency result from differences in chiasma distribution.