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

Abstract

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.

The effect of chromosome 5B on synapsis and chiasma formation in wheat, triticum aestivum cv. Chinese Spring

“…The materials, the growth conditions and the procedures for light and electron microscopy are described in the individual reports in this series (18,20,21,22,23,30,79,80,84).…”

“…Ultrastructural studies ofeuploid wheat using serial sectioning and three dimensional reconstruction (18,30) or spreading and silver staining of meiotic nuclei (20) revealed that chromosome pairing and synaptonemal complex formation at zygotene primarily occurred between homologues. However, in most nuclei (8 of 11 mid zygotene and 4 of 7 late zygotene nuclei (20)) one or more multiple associations had formed due to pairing partner exchanges.…”

“…However, in most nuclei (8 of 11 mid zygotene and 4 of 7 late zygotene nuclei (20)) one or more multiple associations had formed due to pairing partner exchanges. Nearly all multiple associations were corrected before pachytene, but a few persisted through this stage where crossing over occurs.…”

“…Nearly all multiple associations were corrected before pachytene, but a few persisted through this stage where crossing over occurs. These observations led to the general proposal that the disomic inheritance in allohexaploid wheat can be attributed to the following (20): l) A high stringency of synapsis during zygotene, whereby synaptonemal complex formation primarily occurs between homologues.…”

“…The ultrastructural investigations now comprises studies of chromosome pairing in trihaploid wheat with and without chromosome 5B (79), wheat-rye hybrids nullisomic for 5B, euploid hybrids and hybrids where chomosome 5B has been replaced by an isochromosome for the long arm of chromosome 5B (monoisosomic 5BL) (80), hexaploid wheat nullisomic for 5B (22), monosomic for 5B (21), euploid wheat (18,20,31), and wheat monoisosomic and diisosomic for 5BL (23) and triisosomic for 5BL (84). These studies were all performed on the cultivar Chinese Spring.…”