Rye (Secale cereale) supernumerary (B) chromosomes associated with heat tolerance during early stages of male sporogenesis

Rye supernumerary (B) chromosomes have an accumulation mechanism involving the B subtelomeric domain highly enriched in D1100- and E3900-related sequences. In this work, the effects of heat stress during the early stages of male meiosis in 0B and +B plants were studied. In-depth cytological analyses of chromatin structure and behaviour were performed on staged rye meiocytes utilizing DAPI, fluorescence in situ hybridization and 5-methylcytosine immune labelling. Quantitative real-time PCR was used to measure heat effects on the expression of the Hsp101 gene as well as the 3·9- and 2·7-kb E3900 forms in various tissues and meiotic stages. Quantitative real-time PCR established that heat induced equal up-regulation of the Hsp101 gene in 0B and 2B plants, with a marked peak in anthers with meiocytes staged at pachytene. Heat also resulted in significant up-regulation of E3900-related transcripts, especially at pachytene and for the truncated 2·7-kb form of E3900. Cytological heat-induced anomalies in prophase I, measured as the frequency of anomalous meiocytes, were significantly greater in 0B plants. Whereas telomeric sequences were widely distributed in a manner close to normal in the majority of 2B pachytene cells, most 0B meiocytes displayed abnormally clustered telomeres after chromosome pairing had occurred. Relevantly, bioinformatic analysis revealed a significantly high-density heat responsive cis regulatory sequence on E3900, clearly supporting stress-induced response of transcription for the truncated variant. Taken together, these results are the first indication that rye B chromosomes have implications on heat tolerance and may protect meiocytes against heat stress-induced damage.