Biology Seminar

The Evolution of Sex Chromosomes in Papaya

Dr. Qingyi Yu Assistant Professor

AgriLife Research Center at Dallas Texas A&M University

Unlike animal species, most flowering plants are hermaphrodite, producing flowers that contain both male and female organs. Papaya is one of the limited number of plant species that produce male, female, and hermaphrodite flowers on separate individuals, offering us the opportunity of investigating sex determination in plants. Sex determination in papaya is controlled by a pair of nascent sex chromosomes that contain a male-specific region of the Y chromosome (MSY). Dioecious and gynodioecious papayas are controlled by two slightly different Y chromosomes, Y for males and Yh for hermaphrodites. The genotypes of male, female, and hermaphrodite are XY, XX, and XYh, respectively. All combinations of the Y and/or Yh chromosomes are lethal. We have mapped and sequenced the MSY and its corresponding region of the X chromosome to further our understanding of the evolution of sex chromosomes in papaya. Our results demonstrated that papaya sex chromosomes originated from a pair of autosomes. Recombination suppression in the sex-determining region and accumulation of deleterious mutations lead to degeneration of the Y chromosomes. High frequencies of sequence duplications and transposable element insertions contributed to the degeneration of the MSY resulting in low gene density. Gene synteny analysis between MSY and X revealed that the MSY had undergone two large scale inversions, which likely caused the recombination suppression between the X and Y chromosomes. Analysis of sequence divergence between X and Yh gene pairs indicated that sex chromosomes did not evolve at the family level in Caricaceae, and reinforce the theory that sex chromosomes evolve at the species level in some lineages. When:        2:00-­‐  2:50  PM,  Jan  17,  2014  Where:    ENV  110  (EESAT  building)