Bar- and thorn-like abnormalities in synaptonemal complexes of a mutant rye,Secale cereale

Abstract

A novel type of aberration in synaptonemal complex (SC) assembly was found in electron micrographs of surface spread midprophase I nuclei from pollen mother cell preparations of rye. The plants with the mutant phenotype were selected from the fifth- to seventh-generation progenies of an inbred line (Ms6) characterised by partially reduced fertility and known to segregate for the desynaptic mutation sy6. The SC abnormality designated mei6 was at first detected in three sibling plants of the fifth generation of the inbred line. The most striking features were (i) the thorn-like protrusions of the lateral elements (LEs), which were often folded over towards the opposite LE, and (ii) peculiar bars inside the central region of the SC, which were either obliquely or perpendicularly positioned between the LEs. The bars and thorns often coincide with discontinuities in the adjacent LEs, giving rise, in a few cases, to minor branched structures. We analysed 424 surface spread pachytene nuclei obtained from 26 plants selected from the three inbred generations. Bars represented 92% of all abnormalities: their number per cell increased significantly with the progress of meiotic prophase, varying from an average of 1.96 ± 0.39 at midzygotene to 5.16 ± 0.52 at late pachytene and 5.14 ± 0.91 at early diplotene. Our observations suggest that bars are caused by modification and (or) erroneous self-assembly of meiosis-specific proteins intercalating with LE proteins at or near the sites of crossover events. It is also probable that the gene(s) involved is inherited independently from the gene for desynapsis.Key words: Secale cereale, synaptic mutant, meiosis, synaptonemal complex.