Botany and Improvement 25
and flower extracts of vegetative male and female plants in papaya. As the flowers
developed some new bands were observed and some of the earlier ones disappeared
and these changes were specific. In a chemical analysis study of papaya parts, it was
concluded that it may be possible to predict the sex of papaya plants by a study of their
physiological metabolism. Even the sex expression may be controlled by physiological
adjustments. Papaya exhibits wide morphological and biological diversity of its types
with prominent sex specific characters. The papaya plants can be either dioecious or
gynodioecious with male and female parts befalling in the same plant. Monoecious,
presence of male and female flowers on the same plant is also found in some related
species of papaya. In papaya, the change of sex occurs in some plants at high tempera-
ture, where short stalked male flowers are produced instead of usual perfect flowers.
Male or bisexual plants changing completely in female plants after being beheaded and
some ‘all male’ plants occasionally producing small flowers with perfect pistils leading
to abnormally slender fruits are also instances of change of sex in papaya. If the sex of
dioecious papaya is identified at the seedling stage, prior to their transplantation in the
field, cultivation of male and female plants in a desired ratio would be achieved and
resources like planting space, fertilisers and water could be devoted to female plants. A
single female papaya plant normally produces as many as 100 fruits in its life cycle and
about 250 g of crude papain a year. Thus, an increase in the number of fruit-bearing
plants per hectare of land would directly lead to increase in the yield of fruits and
papain, making the cultivation more profitable. Therefore, it is of immense agricultural
importance to identify the sex of papaya plants at the juvenile stage.
Rouging unwanted male plants from female and dioecious papaya plantings is a
cumbersome procedure usually followed in papaya cultivation. Dioecious nature of
papaya is one of the major constraints for papaya for large-scale cultivation. Over the
past seven decades, various hypotheses, based on the knowledge and information
available at the time, have been proposed to explain the genetics of the papaya’s sex
determination. A high-density genetic map of papaya was constructed using 54 F 2
plants derived from cultivars, Kapoho and SunUp with 1501 markers, including 1498
amplified fragment length polymorphism (AFLP) markers, the papaya ringspot virus
coat protein marker, morphological sex type and fruit flesh colour. These markers
were mapped into 12 linkage groups at a LOD score of 5.0 and recombination fre-
quency of 0.25. The 12 major linkage groups covered a total length of 3294.2 cM,
with an average distance of 2.2 cM between adjacent markers. This map revealed
severe suppression of recombination around the sex determination locus with a total
of 225 markers co-segregating with sex types. The cytosine bases were highly meth-
ylated in this region on the basis of the distribution of methylation-sensitive and
insensitive markers. This high-density genetic map is essential for cloning of specific
genes of interest such as the sex determination gene and for the integration of genetic
and physical maps of papaya (Hao et al. 2004).
2.5.1 Sex Determination
Sex chromosomes are of great interest due to their role in sexual reproduction. Sex
chromosomes evolve through the suppression of recombination in the sex determin-
ing region between the X and Y chromosomes which allows for each region to evolve