B-class MADS-box genes in trioecious papaya: Two paleoAP3 paralogs, CpTM6-1 and CpTM6-2, and a PI ortholog CpPI

In the ABC model of flower development, B function organ-identity genes act in the second and third whorls of the flower to control petal and stamen identity. The trioecious papaya has male, female, and hermaphrodite flowers and is an ideal system for testing the B-class gene expression patterns in trioecious plants. We cloned papaya B-class genes, CpTM6-1, CpTM6-2, and CpPI, using MADS box gene specific degenerate primers followed by cDNA library screening and sequencing of positive clones. While phylogenetic analyses show that CpPI is the ortholog of the Arabidopsis gene PI, the CpTM6-1 and CpTM6-2 loci are representatives of the paralogous TM6 lineage that contain paleoAP3 motifs unlike the euAP3 gene observed in Arabidopsis. These two paralogs appeared to have originated from a tandem duplication occurred approximately 13.4 million year ago (mya) (bootstrap range 13.36 ± 2.42). In-situ hybridization and RT-PCR showed that the papaya B-class genes were highly expressed in young flowers across all floral organ primordia. As the flower organs developed, all three B-class genes were highly expressed in petals of all three-sex types and in stamens of hermaphrodite and male flowers. CpTM6-1 expressed at low levels in sepals and carpels, whereas CpTM6-2 expressed at a low level in sepals and at a high level in leaves. Our results showed that B-class gene homologs could function as predicted by the ABC model in trioecous flowers but differential expressions of CpTM6-1, and CpTM6-2, and CpPI suggested the diversification of their functions after the duplication events.