Excess endopolyploidy associated with cellular defects in neo‐autotetraploid Arabidopsis arenosa is in part corrected by a naturally evolved allele of CYCD3;2

Summary Endopolyploidy (within‐organism polyploidy) is an often‐beneficial aspect of development. But polyploids, which have a higher baseline genome content, have been reported to have less endoreduplication than diploid relatives. Why has remained unclear; we explored this in Arabidopsis arenosa. We measured endopolyploidy levels and phenotyped cellular traits, comparing diploids, neo‐autotetraploids, and established autotetraploids of A. arenosa . We used a genetic approach to ask whether CYCD3;2 , a polyploidy regulator with evidence of selection in tetraploids, might be responsible for endopolyploidy‐related phenotypic differences between neo‐ and established polyploids, and whether there are associated costs. Neo‐tetraploids had the most 16C and 32C cells, whereas established tetraploids have proportions of 16C and 32C cells comparable to diploids. Tetraploid plants homozygous for a novel allele of CYCD3;2 have reduced endopolyploidy compared to homozygotes for the diploid allele and, surprisingly, also show cellular defects seen in neo‐tetraploids. Suggesting this link is specific, we find that A. thaliana cycd3;2 mutants have not only increased endopolyploidy but also root hair defects like neo‐tetraploids. Our results suggest reverting endopolyploidy in polyploids to diploid‐like levels could be advantageous, and that one driver might be cellular defects that arise with excess high‐ploidy cells.