High temperature stress has a profound impact on many aspects of plant development. Particularly male germline development is highly sensitive and has received extensive attention in the light of the ongoing climate change. We here uncover a major role for the cyclin TAM/CYCA1;2 in heat induced male meiotic restitution. By exploiting natural variation in Arabidopsis thaliana we identified rare allelic variants of TAM that confer hypersensitivity to heat stress. The increase of the ambient

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... ature to 32{degree sign}C for 24 hours is sufficient to stimulate the production of up to 100% unreduced meiocytes, resulting in sexual polyploidisation. Arabidopsis accessions with a heat sensitive TAM allele show elevated expression of TAM at 32oC and overexpression of TAM phenocopies the meiotic restitution phenotype of a TAM knock-out. These results are discussed in terms of the molecular mechanism and indicate a critical role for meiotic cell cycle regulation in sexual polyploidization and plant evolutionary processes.