Molecular Plant Breeding 2016, Vol.7, No.28, 1
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1.2 Segregation ratio
The three sex types of papaya are inherited in unexpected ratios because male dominant alleles linked with a lethal
factor (Table 1). These unexpected ratios have been becoming the topic of extensive studies. After the
self-pollination of hermaphrodite plants, their seeds always segregate into ratio of 2:1 of hermaphrodites and
females. If female trees were fertilized by pollen from a male tree, then seeds of female plants segregate at the
male to female ratio of 1:1. A similar ratio of 1:1 hermaphrodite to female obtained when female fertilized by
pollen from a hermaphrodite tree. When the males self-pollinated occasionally (in optimal growing conditions
some male flowers do not undergo their carpel abortion and form fruits), then male trees occur at a ratio of 2 male:
1 female. When male pollen fertilizes the female organ of hermaphrodite trees, then a ratio of 1 male: 1
hermaphrodite: 1 female is obtained. Therefore, no male trees are produced, when hermaphrodites are self
pollinated or when hermaphrodites are used as a pollen source to fertilize female trees.
Table 1 Segregation ratio of crosses between different combinations of sex types (Ming et al., 2007)
Crosses
Genotypes
Segregation ratio
Male
Female
Hermaphrodite
Non-viable genotypes
Hermaphrodite (selfed)
M
h
m ×M
h
m
0
1
2
1
Female ×Hermaphrodite
mm ×M
h
m
0
1
1
0
Hermaphrodite ×Male
M
h
m ×Mm
1
1
1
1
Male selfed (occasionally)
Mm ×Mm
2
1
0
1
Female×Male
mm ×Mm
1
1
0
0
1.3 Effect of genetics and environment on sex expression
Many reports have shown that genetic and environmental factors (including temperature, moisture etc.) influence
the sex expression in hermaphrodite and male plants of papaya (Awada, 1958; Allan et al., 1987; Silva et al.,
2007a), while the female flowers are most stable (Ming et al., 2007). Following genetic and environmental factors
influence the sex expression in papaya:
(1) Genetic influence: It is reported that genetic influence of genotype has great effect on sex expression. Silva et
al., (2007a) reported in segregating back cross 1 (BC1) papaya population where elongate type of hermaphrodite
flower changes into carpelloid (6-9 stamens) and pentandric (5 stamens) owing to their genetic inheritance of the
genotype. Ramos et al., (2011) similarly reported that the variation of hermaphrodite elongate type to carpelloid or
pentandric form is mainly due to genetic component.
(2) Temperature: During winter, cool temperature changes hermaphrodite (elongate form) to a carpelloid or the
pentandria form. Fusion of stamens to the ovary wall leads to the decrease in number of stamens. During summer,
high temperature produces sterile/barren type flowers which ultimately lead to no fruit formation (Awada, 1958).
Allan et al. (1987) reported that male trees also showed reversion of sterile staminate to elongate type
hermaphrodite flowers under night temperatures of 12
◦
C and short daylengths.
(3) Moisture and Nitrogen: In addition, both moisture and nitrogen also affect the sex expression of papaya.
Carpelloid and pentandric are developed in the presence of high nitrogen level and excess soil moisture (Awada,
1957). Hermaphrodite trees produces barren type flowers in drought condition (Chan, 2009).
2 Importance of Sex Determination Study in Papaya
The study of sex type identification is valuable in papaya because sex of the papaya plant cannot be predicted
morphologically at early seedling stages. Among the three sex types hermaphrodite plants are grown for its
pyriform-shaped fruits that are preferred for consumption and female plants are important for commercial papain
production, while male plants are non-desirous (Urasaki et al., 2002). However, female trees require presence of
small number (6-10%) of male trees in the field for fruit production (Eustice et al., 2008). Papaya seeds produce
seedlings of unidentified sex, therefore farmers have to remove the male plants from the field and leave the female
or hermaphrodite plants on the basis of floral morphology which can be performed only after three
