Divergence of Recently Duplicated Mγ-Type MADS-Box Genes in Petunia

The MADS-box transcription factor family has expanded considerably in plants via gene and genome duplications and can be subdivided into type I and MIKC-type genes. The two gene classes show a different evolutionary history. Whereas the MIKC-type genes originated during ancient genome duplications, as well as during more recent events, the type I loci appear to experience high turnover with many recent duplications. This different mode of origin also suggests a different fate for the type I duplicates, which are thought to have a higher chance to become silenced or lost from the genome. To get more insight into the evolution of the type I MADS-box genes, we isolated nine type I genes from Petunia, which belong to the M gamma subclass, and investigated the divergence of their coding and regulatory regions. The isolated genes could be subdivided into two categories: two genes were highly similar to Arabidopsis M gamma-type genes, whereas the other seven genes showed less similarity to Arabidopsis genes and originated more recently. Two of the recently duplicated genes were found to contain deleterious mutations in their coding regions, and expression analysis revealed that a third paralog was silenced by mutations in its regulatory region. However, in addition to the three genes that were subjected to nonfunctionalization, we also found evidence for neofunctionalization of one of the Petunia M gamma-type genes. Our study shows a rapid divergence of recently duplicated M gamma-type MADS-box genes and suggests that redundancy among type I paralogs may be less common than expected.