Origin of the ZmCDA1 , ZmCDA2 , and ZmCDA3 genes. The progenitors of rice and maize split about 50 mya and the two progenitors of maize about 11.9 mya. Each of the progenitors of maize contain a copy of the CDA gene shared with the progenitor of rice (orthologs). When the two progenitors of maize hybridized at about 4.8 mya (allotetraplodization), the maize genome maintained two diverged orthologous CDA gene copies, ZmCDA1 and ZmCDA2. In addition, 4.5 mya the ZmCDA1 copy on chromosome 7 got copied by the action of a helicase and inserted into chromosome 4S close to the z1C1 locus, resulting in the ZmCDA3 gene copy, which is therefore a paralogous gene copy.