Chromosome pairing, recombination nodules and chiasma formation in diploid Bombyx males

Abstract

Serial sectioning and three dimensional reconstruction of 82 nuclei divided into 15 well characterized substages covering the period from early zygotene to metaphase I in diploid Bombyx spermatocytes have permitted the following observations and conclusions: 1) At late zygotene, a mean of four chromosome and bivalent interlockings was found per nucleus. 2) During the transition from zygotene to pachytene, all interlockings resolve by breakage and reunion Of chromosomes or bivalents. 3) Recombination nodules first appear at early zygotene, reach a maximum of 91 per nucleus by late zygotene, decrease to 55 per nucleus at mid pachytene and increase again to 70-75 towards the end of pachytene. 4) At late z~cgotene and pachytene, nodules are essentially distributed at random among bivalents as well as along bivalents. 5) Small, but important deviations from randomness were, however, revealed: Three percent of the bivalents were without nodules at late zygotene, 8 % at early pachytene but at mid and late pachytene, only 2 % were devoid of nodules. 6) From mid pachytene, an increasing fraction of the recombination nodules becomes larger and more irregular in shape. These are termed chromatin nodules and at the pachytene-diplotene transition, nearly all nodules are of this type. 7) During early diplotene, the bulk of the chromatin decondenses leaving only about 60 major condensed domains. These domains originate from chromatin nodules and are frequently associated with a piece of synaptonemal complex which in its central region contains a dense core resembling a recombination nodule. 8) At late diplotene, the condensed domains of the bivalents consist of two dense chromatin segments, one on each of the homologues, and bridged by a complex circular structure giving the regions a characteristic tripartite appearance. 9) At mid diakinesis, the circular structures are located in chromatin bridges -chiasmata -between the homologous chromosomes. 10) The number of chiasmata per nucleus is constant from mid diplotene to mid diakinesis and is distributed among the bivalents similar to the distribution of recombination nodules at mid and late pachytene. 11) The circular chiasma components are eliminated from late diakinesis until metaphase I. 12) Finally, each metaphase I bivalent contains four distinct localized centromeres, two by two facing opposite spindle poles. 485

Chromosome pairing, recombination nodules and chiasma formation in the basidiomycete coprinus cinereus

“…The first appearance of recombination nodules concomitant with the initiation of pairing and synaptonemal complex formation, the subsequent morphological evolution of nodules, as well as the fluctuations in the number of nodules during the meiotic prophase, is consistent with the pattern observed in Homo (27) and Bombyx (15). Also, the presence of nodules associated with precursor material for the central region at the pairing fork further reinforces the contention that nodules are prerequisites for rather than the result of a recombination event.…”

“…Only one case of nonhomologous pairing and synaptonemal complex formation has been observed, the free ends of a broken lateral component being paired with a short stretch of central region. Hence, the present analysis constitutes another example in support of the contention that nonhomologous pairing and synaptonemal complex formation during zygotene occur only in rare cases and is nearly always confined to interlocked or broken chromosome regions (15).…”

“…The assessment of the distribution of recombination nodules among the bivalents of a nucleus at the different stages of the early meiotic prophase in Coprinus meiocytes was performed as previously described for human and Bombyx spermatocytes (15,27). The frequency of bivalents with 0, 1, 2 ...... 7, > 7 nodules was determined for each stage and compared with the frequency one would expect if the nodules were distributed randomly among the bivalents.…”

“…The organisms employed in these investigations include Bombyx (15,24,25,26,28), Homo (13,27), Lilium (12), Triticum (10), Zea (8), Drosophila (3,4,5), Saccharomyces (1), Schizophyllum (2), Neurospora (9) and Sordaria (31, 32) among others. Through these studies a common scheme for the basic events of the meiotic prophase at the ultrastructural level has evolved which has Abbreviations: sd = standard deviation.…”

“…The analysis of chromosome pairing in the ascomycetes Sordaria macrospora (31) and Sordaria humana (32) does, however, indicate that organisms with short chromosomes follow the same basic scheme with respect to chromosome pairing. The evolution of chiasmata from bivalent regions which at late pachytene contain recombination nodules has so far been described only in Bombyx spermatocytes (15) and it is not yet known whether this represents a genera1 feature of meiosis. Finally, extensive analyses of the distribution of recombination nodules among and along the bivalents of a nucleus have revealed rules of their positioning in Drosophila (3,4,5), Homo (27), Bombyx (15), Schizophyllum (2), Neurospora (9) and Sordaria (32,32).…”