Genomic Structures and Expression Patterns of MADS-box Gene Located on Y and Yh Chromosomes of Papaya

Papaya, Carica papaya, is trioecious plant species with male, female and hermaphrodite. These sex types are suggested to be determined by the locus, Sex1. Previous study (Liu et al., 2004) showed the severe suppression of genetic recombinations around Sex1 and presence of primitive sex chromosomes harboring a male-specific region (MSY) on Y and Y^h chromosomes controlling males and hermaphrodites, respectively. To understand the mechanism of sex determination in papaya, we conducted a transcriptome analysis of flower samples from male, female and hermaphrodite plants using high-throughput SuperSAGE for digital gene expression analysis (Urasaki et al., 2012). Among the short sequence tags obtained from the transcripts, 312 unique tags were specifically mapped to the primitive sex chromosome (X or Y^h) sequences. From an annotation analysis and PCR, a Y and Y^h-specific MADS-box gene, CpMBP, was identified. In this study, the cloning and phylogenetic analysis of CpMBP were carried out. Furthermore, we also compared the gene structures among three sex types and confirmed their expression patterns. The ORF of CpMBP was 687 bp in its size, encoding 228 amino acids. The phylogenetic analysis using putative amino acid sequence of CpMBP showed that CpMBP had a similarity to short vegetative phase MADS-box protein in Arabidopsis. The comparison of gene structure between sex chromosomes showed that CpMBP on Y showed an intact gene structure. On the other hand, the insertion of retroelement in its promoter region of CpMBP was observed on Y^h. RT-PCR analysis revealed that this gene was constitutively expressed in several organs.