Chromosome pairing in allohexaploid wheat var. Chinese Spring. Transformation of multivalents into bivalents, a mechanism for exclusive bivalent formation

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Meiosis in allohexaploid wheat, Triticum aestivum var. Chinese Spring has been investigated by complete three dimensional reconstructions of a late zygotene and an early pachytene nucleus. At zygotene, specific synapsis of homologous chromosomes and chromosome segments gives rise to extensive multivalent formation with shifts of pairing partners between apparently homoeologous chromosomes. These observations were confirmed by the identification of shifts of pairing partners in three serial sectioned, but only partly reconstructed zygotene nuclei. In striking contrast, the pairing at early pachytene was exclusively in bivalents. These findings, which are consistent with the pairing pattern described in tri- and tetraploid Bombyx oocytes, definitely exclude a presynaptic alignment as the primary cause for a regular bivalent formation, in wheat. Instead, multivalents form during the specific zygotene pairing but are later transformed into bivalents through dissolution and reassembly of synaptonemal, complexes. It is proposed that the regular bivalent formation in allohexaploid wheat is due to a temporal delay of crossing over until the pairing correction is completed at early pachytene. This temporal delay may be mediated by the action of the Ph gene(s) on chromosome 5B. Finally, interlockings were revealed in the completely reconstructed late zygotene nucleus but were resolved prior to early pachytene.