CoPIs: Paul H. Moore (Hawaii Agriculture Research Center), Richard C. Moore (Miami University) and Qingyi Yu (Texas A&M University)
Sex expression in papaya (Carica papaya L.; Family Caricaceae) is controlled by loci found in the male specific region of the Y chromosome (MSY). Unlike the sex chromosomes of other plants or animals, the MSY of papaya is quite small and is restricted to a region about 8 Mb in size. In addition, sex reversal mutants in papaya indicate that the two sex determination genes are confined to a smaller region of about 1 Mb that is currently being sequenced. Thus the small size of the sex determination region in papaya offers the opportunity to dissect the early events in sex chromosome evolution and the identification of sex determination genes feasible. The long-term goal of this project is to understand the evolutionary mechanisms governing the formation and divergence of sex chromosomes. To accomplish this objective, 300 bacterial artificial chromosomes (BACs) from the sex specific regions of six species in three genera of Caricaceae will be sequenced and subjected to detailed functional and comparative genome analyses. The specific objectives are to: 1) identify the sex determination genes in C. papaya. The focus will be on candidate genes in the 1 Mb deleted region of the male to female sex reversal mutants to identify the two sex determination genes; 2) elucidate the origin of sex chromosomes in Caricaceae. A comparative genomic approach using chromosome sequences of six species from three genera with diverse breeding systems to determine whether the sex chromosomes of papaya share a similar evolutionary history with the homologous regions in these sister species; 3) determine the forces that have shaped genome evolution of papaya sex chromosomes. Competing evolutionary hypotheses concerning the deterioration of the MSY will be distinguished by conducting population genomics analyses across the entire region and the corresponding region on the X chromosome; and 4) engineer true breeding hermaphrodite papaya varieties to validate the function of the sex determination genes. Candidate gene(s) promoting stamen development will be transformed into female plants of leading papaya cultivars to produce hermaphrodite varieties without the Yh chromosome.
This project will significantly advance genomic tools and knowledge for tropical trees, an under-explored node of the angiosperms that is of large and growing importance to US agriculture. The knowledge gained may enable the engineering of true breeding hermaphrodite papaya varieties and improve papaya fruit productivity while lowering the cost. With respect to training and outreach, the four institutions participating on this project, University of Illinois at Urbana-Champaign (UIUC), Hawaii Agriculture Research Center (HARC), Texas A&M University (TAMU) and Miami University (MU) each has strong teaching, training, and outreach programs with a successful history of engaging high school, undergraduate, and graduate students, including at all levels and minority groups that are under-represented in the sciences. College and high school students will be trained through summer intern programs at UIUC, HARC, TAMU, and MU. Graduate students and postdoctoral scientists will be exposed to new genomic tools and techniques. Knowledge of plant genomics will be communicated to the general public as well through participation in outreach programs such as the Annual Hawaii State Farm Fair.
