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Synapsis and chiasma formation have been analyzed at the ultrastructural level in trihaploid wheat with and without chromosome 5B by spreading of microsporocytes. The euhaploids were obtained by anther culture of euhexaploid or monosomic 5B Chinese Spring wheat or from the hexaploid Chinese winter wheat variety Kedong. All three types of euhaploid wheat were virtually achiasmatic. Analysis of 31 spread zygotene nuclei, covering the entire zygotene stage, revealed that synaptonemal complex formation primarily was initiated interstitially whereafter the formation proceeded in the telomeric regions. Pairing partner exchanges were frequent, most of the lateral components within the individual nucleus being combined into a single multiple association. As synapsis proceeded the number of pairing partner exchanges decreased and at late zygotene many associations involved only two or three lateral components. The mean degree of synapsis achieved in haploids of Chinese Spring amounted to 35–40% while in haploids from Kedong up to 90% of the complement was combined with synaptonemal complexes. Analysis of a nullisomic 5B haploid plant derived from Chinese Spring wheat revealed a 45-fold increase in the number of chiasmata per cell at metaphase I compared to euhaploids. The differences in number of chiasmata were correlated with a nearly twofold increase in the number of pairing partner exchanges per lateral component compared to that found in euhaploid wheat.  The elimination of the synaptonemal complex during diplotene was monitored in 37 euhaploid nuclei ranging in stage from early diplotene to diakinesis. As in hexaploid wheat the degradation of the synaptonemal complexes started at a low number of sites, generating a number of segments. These were thereafter degraded from their ends until by diakinesis elimination was complete or almost so.

Chromosome pairing analysis in haploid wheat by spreading of meiotic nuclei