Increased maize chromosome number by engineered chromosome fission

Activation of synthetic centromeres on chromosome 4 in maize leads to its breakage and formation of trisomic fragments called neochromosomes. A limitation of neochromosomes is their low and unpredictable transmission rates due to trisomy. Here we report that selecting for dicentric recombinants through male crosses uncovers stabilized chromosome 4 fission events which split it into 4a-4b complementary chromosome pairs, where 4a carries a native centromere and 4b a synthetic one. The cells rapidly stabilized chromosome ends by de novo telomere formation and the new centromeres spread among genes without altering their expression. When both 4a and 4b chromosomes were present in a homozygous state, they segregated through meiosis indistinguishably from wild-type, and gave rise to healthy plants with normal seed set. This work leverages synthetic centromeres to engineer chromosome fission, effectively raising the diploid chromosome number of maize from 20 to 22.