Human meiosis VII. Chiasma formation in human spermatocytes

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133

An ultrastructural analysis of spread, silver stained meiotic nuclei of wheat, Triticum aestivum, has been performed. Partial tracings of lateral components and synaptonemal complexes of a number ofleptotene and early zygotene nuclei, and complete tracings of eleven mid zygotene, seven late zygotene, twenty pachytene, twelve early diplotene, twenty-one mid diplotene and sixteen late diplotene nuclei have permitted the following observations and conclusions: 1) The chromosomes appear to be randomly distributed within the spread leptotene and early zygotene nuclei. 2) The complement length, i.e., the lateral component length of the three genomes, decreases from a mean of 2063 Ixm at mid zygotene to 1806 ~tm at late zygotene and 1474 ~tm at pachytene. 3) Chromosome pairing is initiated preferentially from the telomeres. These are aggregated during zygotene whereby a chromosome bouquet is established. Synaptonemal complex formation is thereafter also initiated interstitially, the mean number of synaptonemal complex segments being 156 per nucleus in the eleven mid zygotene nuclei where on the average 61% of the complement is paired. 4) Chromosome pairing and synaptonemal complex formation remain frequently incomplete. In most cases, however, more than 95% of the complement has synapsed. 5) Interlocking of lateral components and synaptonemal complexes is frequent at zygotene. Detailed analysis of one mid zygotene nucleus revealed more than 20 intedockings, most of them being bivalent interlockings. At late zygotene there was a mean of 5 bivalent intedockings per nucleus and a few chromosome intedockings. Some interlockings persist up to pachytene and a few may remain into diplotene. 6) Multiple associations of chromosomes, probably signifying pairing between homoeologues, are seen in eight of the eleven mid zygotene nuclei, in four of the seven late zygotene nuclei, in three of the 20 pachytene nuclei and possibly in one of the early diplotene nuclei. 7) Synaptonemal complex degradation is initiated at a limited number of sites at the pachytene to diplotene transition, the number of retained synaptonemal complex segments amounting to 89, 119 and 136 at early, mid and late diplotene. This exceeds thb number of chiasmata observable at metaphase I in the light microscope by a factor of two to three. 8) It is proposed that the diploid behaviour ofallohexaploid wheat results from: I) A high stringency of chromosome pairing at the beginning of zygotene whereby pairing preferentially is initiated between homologues. II) Transformation of multiple association into bivalents at the end of zygotene and during pachytene before crossing over occurs and/or III) a suppression of crossing over between paired segments of homoeologous chromosomes. The possible effect of the Ph gene(s) on the long arm of chromosome 5B is discussed.

Section: Chiasma Formationsupporting

confidence: 86%

“…It has thus been possible to estimate the number of crossovers in e.g., the human male and female by counting the number of retained synaptonemal complex segments at diplotene (10,32). In both cases the number of segments correlated well with the number of crossovers inferred from the study of recombination nodules.…”

Section: Chiasma Formationmentioning

confidence: 83%

Chromosome pairing and chiasma formation in allohexaploid wheat, Triticum aestivum analyzed by spreading of meiotic nuclei

Holm

1986

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133

“…It is, however, unlikely that the number and distribution of chiasmata at metaphase I is identical to the number and distribution of crossovers initially formed at pachytene. At metaphase I most chiasmata are terminal and a substantial reduction in chiasma frequency and a change in their distribution might have occurred before metaphase I as has indeed been reported for human spermatocytes (15). Hence it is conceivable that all retained synaptonemal complex segments in the present material are associated with crossovers.…”

Section: Crossing Over and Chiasma Formationsupporting

confidence: 57%

“…At mid-late diplotene the number and distribution of distinct synaptonemal complex segments was the same as at early diplotene, showing that degradation at this stage only occurred from the ends of the segments present at early diplotene. Partially degraded segments were often seen flanking the well defined synaptonemal complex segments (15).…”

Section: Crossing Over and Chiasma Formationmentioning

confidence: 99%

Chromosome pairing and chiasma formation in wheat plants triisosomic for the long arm of chromosome 5B

Wischmann

1986

Chromosome pairing and chiasma formation have been analyzed at the light and electron microscopic levels in wheat plants triisosomic for the long arm of chromosome 5B. Complete reconstruction of the chromosome complement from one zygotene nucleus, as well as analyses of several hundred spread and silver stained nuclei, showed that pairing was arrested at the beginning of zygotene, the mean degree of pairing in five nuclei being 25 percent (range 8-37). At this stage, multiple associations of chromosomes were present, signifying pairing beth between partly-homologous and nonhomologous chromosome segments. A few multivalents were present at metaphase I together with bivalents and univalents. The chiasma frequency (a mean of 18 per cell) was only half of that seen in normal wheat. In spread nuclei at late diplotene, a mean of 41 segments ofsynaptonemal complex was counted, 20 of which had thickened lateral components. The results are discussed in relation to the possible dosage effect of the Ph gene located on the long arm of chromosome 5B.

“…Using the approach of serial sectioning and three dimensional reconstruction, the processes of chromosome pairing and synaptonemal complex formation, chiasma formation and disjunction have been characterized in a number of organisms (e.g. 7,11,12,15,17,26,27,35) including the human male (13,18,19,20,28). The available corresponding information on human female meiosis is neither adequate for a comparison with spermatogenesis nor as a basis for speculations concerning the factors responsible for the high level of segregation failure.…”

Section: Introducrlonmentioning

confidence: 99%

Human meiosis VIII. Chromosome pairing and formation of the synaptonemal complex in oocytes

Bojko

1983

Human oocytes from three foetal ovaries have been investigated by serial sectioning and three dimensional reconstruction of nuclei from electron micrographs. Analysis of three leptotene-early zygotene, five mid zygotene and ten late zygotene oocytes including complete reconstruction of five zygotene nuclei have led to the following observations and conclusions. 1) Formation of the synaptonemal complex involves, as in other organisms, the organization of a lateral component along each chromosome, attachment of the telomere regions to the inner membrane of the nuclear envelope at leptotene, redistribution of the attached telomeres during the formation of a chromosome bouquet at early zygotene and thereafter progressive chromosome pairing with the synaptonemal complex. 2) Chromosome and bivalent interlockings as well as breaks were common at mid zygotene, but absent when pairing and synaptonemal complex formation were almost complete. Most of the breaks had occurred in the vicinity of presumptive interlockings and appear to be instrumental in the resolution of interlockings. 3) Nonhomologous pairing involving short regions of the complement were observed in two of the mid zygotene nuclei, thus resulting in associations of three or more chromosomes. 4) Twelve of the 23 bivalents can be identified by relative lengths, centromere indices and the structural markers on chromosomes 1, 6, 9, 13-15 and 21-22. 5) The length of the lateral component complement exceeds that of the spermatocytes by a factor of two. 6) On an average 58 recombination nodules was observed per nucleus at mid zygotene (61% of the chromosome complement paired) and 70 at late zygotene (98% of the chromosome complement paired). Thus in spite of the twofold longer synaptonemal complexes in the oocytes, the number of nodules is the same in both sexes.