Meiosis in trisomic female mice with Robertsonian translocations. II. Chromosome behaviour at first and second meiotic metaphases

Meiotic Crossing Over in Maize Knob Heterochromatin

There is ample evidence that crossing over is suppressed in heterochromatin associated with centromeres and nucleolus organizers (NORs). This characteristic has been attributed to all heterochromatin, but the generalization may not be justified. To investigate the relationship of crossing over to heterochromatin that is not associated with centromeres or NORs, we used a combination of fluorescence in situ hybridization of the maize 180-bp knob repeat to show the locations of knob heterochromatin and fluorescent immunolocalization of MLH1 protein and AFD1 protein to show the locations of MLH1 foci on maize synaptonemal complexes (SCs, pachytene chromosomes). MLH1 foci correspond to the location of recombination nodules (RNs) that mark sites of crossing over. We found that MLH1 foci occur at similar frequencies per unit length of SC in interstitial knobs and in the 1 µm segments of SC in euchromatin immediately to either side of interstitial knobs. These results indicate not only that crossing over occurs within knob heterochromatin, but also that crossing over is not suppressed in the context of SC length in maize knobs. However, because there is more DNA per unit length of SC in knobs compared to euchromatin, crossing over is suppressed (but not eliminated) in knobs in the context of DNA length compared to adjacent euchromatin.

Pachytene chromosomes in trisomy 19 male mice with Robertsonian translocations

Three male mice with trisomy 19 induced by a Robertsonian translocation system were used for the study of meiotic prophase cells and germ cell differentiation. Present in these males were two Robertsonian chromosomes each with a chromosome 19 arm in common, two acrocentric chromosomes corresponding to the second arms of the two Rbs and one acrocentric chromosome 19. These five chromosomes showed a wide range of meiotic pairing configurations. One particular observation was the formation of a true double synaptonemal complex (SC) with three lateral axes and two central elements, which joined the three chromosomes 19 together. Integration of the acrocentric chromosome 19 in a complex pentavalent configuration was seen in 45% of the pachytene nuclei. The proportion of spermatocytes showing association between a quadrivalent and the acrocentric no. 19 was 26%. In 29% of the nuclei, the acrocentric no. 19 was free, integrated or associated with the XY complex, paired with the X chromosome or associated with a bivalent. Finally, in 57% of pachytene cells, the meiotic multivalents or the free univalent 19 were associated with the proximal part of the X chromosome or integrated in the sex chromatin. Therefore, the question arises with regard to the fate of these spermatocytes. The testicular histology shows an arrest of germ cell development at the spermatocyte stage. Several mechanisms seem to be the cause of germ cell depletion in a sequence of different, impaired developmental processes.