Molecular Analysis of Sex Chromosome Evolution in Papaya By

Papaya is a semi-woody tree that produces fruit rich in vitamins and minerals. It is trioecious with male, female, and hermaphrodite plants. Though many theories have been suggested in the past, papaya sex is determined by a pair of nascent sex chromosomes; females have two X chromosomes, males have an X and a Y, and hermaphrodites have an X and a Y, which varies slightly from the male Y. Any combination of the Y and Y genotype is lethal. The X and Y chromosomes have a small nonrecombining region in the centromeric and pericentromeric region. The hermaphrodite-specific region of the Y chromosome (HSY) is gene poor and has an increased amount of retroelements and chromosomal rearrangements compared to its X counterpart and the genome wide average. The X and Y chromosomes were estimated to have diverged relatively recently about 2-3 million years ago (MYA), and even more recently in the case of the Y and Y (73,000 years ago). Physical maps of the HSY and the corresponding X region have been produced and sequenced, showing the HSY sequence has expanded. Genes in these regions are being mined with a special focus on identifying the two sex determination genes, one promoting maleness and one suppressing femaleness. The impact of identifying the sex determination genes would be substantial for both the commercial production of papaya and the field of sex chromosome evolution. Introduction Sexual reproduction is a common phenomenon in eukaryotes. Across sexually reproducing flowering plants, a variety of breeding systems have arisen. Flowering plant species can be hermaphrodite, containing both male and female parts within the same flower, monoecious, having separate male and female flowers on the same individual, dioecious, having male and female flowers on different individuals, or a combination of the three. Hermaphroditism is the most common sexual type seen in angiosperms, whereas dioecy is only seen in about 6% of species across about 38% of flowering plants families (Renner and Ricklefs 1995). Dioecious angiosperm individuals were thought to have evolved independently in different families from hermaphrodite ancestors (Takhtajan 1969). There are many possible reasons for this shift from hermaphrodite to monoecious or dioecious sex types. 1 This chapter appeared in its entirety in Gschwend AR and Ming R (2011) Papaya Sex Chromosomes in New insights on plant sex chromosomes. ed Navajas-Perez R. Nova Science Publishers, Inc. (In Press). This chapter is reprinted with permissions from the publisher.