Pacific Fruit Genetic Resource Management and Sustainable Production Systems
Location: Tropical Plant Genetic Resources and Disease Research
Title: The origin of the non-recombining region of sex chromosomes in Carica and Vasconcellea
Submitted to: Plant Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 11, 2010
Publication Date: September 1, 2010
Citation: Wu, X., Wang, J., Na, J., Yu, Q., Moore, R.C., Zee, F.T., Huber, S.C., Ming, R. 2010. The origin of the non-recombining region of sex chromosomes in Carica and Vasconcellea. Plant Journal. 63(5):801-810.
Interpretive Summary: Papaya (Carica papaya) is an important tropic fruit crop and was the first fruit tree to have its genome sequences. Interestingly, papaya has the most primitive Y (sex) chromosome found to date in any organism. It has been estimated that the papaya sex chromosomes originated about 2 to 3 million years ago. Papaya is the only species in the genus Carica but the closely related sister genus Vasconcellea (also in the Caricaceae family) contains 21 species, which are thought to be the closest evolutionary relatives of papaya. We wanted to determine whether sex chromosomes in papaya evolved before or after the speciation event of Carica and Vasconcellea. To do this, we compared six sex-linked gene pairs between papaya and five species in Vasconcellea that vary in flowering behavior. The cloning and sequencing results showed that sex chromosome had also evolved in Vasconcellea and suggested that the sex chromosomes had evolved from the autosomes, or chromosomes that are the same in males and females of the organism. Cluster analysis of the X and Y sex-link gene pairs resulted in a distinctive separation of papaya X/Y gene pairs from those of Vasconcellea species, suggesting that the sex chromosomes in Carica and Vasconcellea evolved independently after the speciation event that now separates the two genera. These studies have potential application to agriculture, because gender affects quality and productivity of the crop but can only be determined when the plants flower. However, discovery of markers linked to the sex chromosomes may provide approaches to identify gender at an early seedling stage.
Carica and Vasconcellea are two closely related sister genera in the family Caricaceae, and were once classified as two sections under Carica. After the section Vasconcellea with 21 species was reinstated as a separate genus based on molecular marker data, papaya became the sole species in Carica. Sex chromosomes were found in papaya and originated about 2 to 3 million years ago (MYA). The objective of this study was to investigate the origin of sex chromosomes in Carica and Vasconcellea and determine whether sex chromosomes have evolved in Vasconcellea. Six X/Y gene pairs were cloned, sequenced, and analyzed from three dioecious, one trioecious, and one monoecious species of Vasconcellea, including the X/Y gene pair of Gene 1 that has not been reported before. The isolation of distinctive X and Y alleles in dioecious and trioecious species of Vasconcellea demonstrated that sex chromosomes have evolved in this genus. Cluster analyses resulted in a distinctive separation of papaya X/Y gene pairs from those of Vasconcellea species. Distinctive clusters of X/Y gene pairs were also documented in V. parviflora and V. pulchra. Our results indicated that Carica and Vasconcellea sex chromosomes originated from the same autosomes bearing the X allelic form that still exist in the monoecious species. The sex chromosomes in Carica and Vasconcellea have evolved independently after the speciation event that separated Carica from Vasconcellea. Within Vasconcellea, sex chromosomes evolved at the species level at least for some species.