Papaya (Carica papaya L.) is an important tropical fruit crop with a yielding potential of approximately 45 tons/hectare, which is normally consumed fresh and is valued as a health food because it is rich in vitamins C and A. Papaya is widely grown in Brazil, Australia, South Africa, South-East Asia, Hawaii, India and other tropical areas. Papaya is a polygamous species, and sex inheritance is controlled by a single locus with multiple alleles. There are three sex forms of papaya trees, i.e. hermaphrodite or bisexual, pistillate or female, and staminate or male. The sex types of papaya may not be identified according to the phenotype of the juvenile plant or other chemical or biochemical methods. The male, female and hermaphroditic flowers of papaya are distributed on separate papaya plants and sex types are revealed only after flowering. Methods to identify sex type at juvenile stage have been studied (Bojappa and Singh 1974, Choudhri et al. 1957, Parasnis et al. 1999, 2000; Singh et al. 1977, Sondur et al. 1996, Somsri et al. 1998). Storey proposed that the sex of papaya is determined by three homologous gene complexes on sex chromosome (W. B. Storey, J. Hered. 44, 70–78, 1953; and W. B. Storey, Crop Plants, 21–24. Wisley, N.Y.). The genes are so tightly linked that no crossing over occurs among them; thus the complexes are transmitted to offspring as if they are single gene alleles with pleiotrophic effects on phenotypic expression. The genotypes of the male, hermaphroditic and female plants are M1m, M2m, and mm, respectively. Genotypes with homozygous dominant alleles are lethal (W. B. Storey, J. Hered. 44, 70–78, 1953).
The hermaphroditic papaya, bearing perfect flowers and producing fruits shaped from long-cylindrical to ellipsoidal, is preferred by the markets in Hawaii, Japan, South-East Asia and Taiwan. In addition, the consumers and farmers prefer hermaphrodite papaya because it has small seed cavity and is easier to package. The cross between two hermaphroditic papayas will yield a ratio of 2:1 hermaphrodite to female papaya. Therefore, the papaya growers usually plant at least two seedlings in each hole in the field, and later remove the females at flowering, a practice that is time-consuming and wasteful. Therefore, there is a need to develop a technology of obtaining all hermaphrodite papaya.