32 Papaya
Different hypotheses have been put forward on the genetics of sex determina-
tion in papaya. Hofmeyr’s (Hofmeyr 1967) hypothesis involves genic balance. The
symbols M 1 and M 2 represent inert or inactivated regions of slightly different lengths
on sex chromosomes from which vital genes are missing. This accounts for zygotic
lethality of M 1 M 1 , M 2 M 2 and M 1 M 2 genotypes (Table 2 .1).
The greater concentration of genes for femaleness is on the sex chromosomes,
whereas that for maleness on the autosomes. Thus, the genotype mm is pistillate and
its homozygosity confers phenotypic stability. Since M 1 is longer of the inert regions,
it is expressed phenotypically as staminate because of the greater influence of the
autosomal factors. The shorter M 2 region is less influenced by autosomal genes, so,
the M 2 m genotype is expressed phenotypically as hermaphrodite or andromonoe-
cious. The heterozygosity of M 1 m and M 2 m renders them susceptible to alternation in
phenotypic expression by external influences (Ray 2002).
The hypothesis of Horovitz and Jimenez (1967) proposes a reversionary process
for sex determination and expression. Its basic assumption is that dioecism is the
primitive state in Caricaceae and sex determination is of the classical XX–XY type
with heterogametic male and YY lethal to the zygote. At some point in time, a sexual
ambivalent form occurred in the genus from which three present day exceptions to
dioecism (C. monoica, C. pubescens and C. papaya) arose. In C. papaya, the ambi-
sexual mechanism built up on the Y chromosomes, giving rise to a modified homo-
logue, the Y 2 chromosome which (in the heterogametic genotype XY 2 ) is expressed
as the sexually ambivalent andromonoecious form. This occurred without alteration
of the × chromosome, which explains the stability of the pistillate form. This hypoth-
esis holds, therefore, that andromonoecism and polygamy followed the evolution of
XX–YY system and are of fairly recent origin.
Storey (1967, 1969) hypothesised progressive evolution of dioecism in the family.
Sexual differentiation in the form of dioecism followed the derivation of unisexual
flowers. Since dioecism seems to be the evolutionary norm in Caricaceae, it is pos-
sible that ambisexual forms owe their continued existence to human selection. In
Storey’s (Storey 1976) revised hypothesis, the symbol (SA) represents the sum of the
factors involved in transmuting the ancestral androecism into the present day gynoe-
cium; (SA) represents normal androecium development; (SG) represents the factor
or factors responsible for suppression of the gynoecium in the staminate flower; (SG)
permits the (SA) factors to function ontogenetically in developing the replacement
gynoecium. The symbol ‘l’ represents the recessive sex-linked zygotic lethal factor
that enforces heterozygosity on the staminate and andromonoecious plants; C repre-
sents the factor that prevents crossing-over between the sex-determining factors and
the lethal factor, accounting for the non-existence of pistillate plants carrying the ‘l’
factor. As in other two hypotheses, heterozygosity permits ambisexuality. The sex
determining genotypes are expressed as follows:
i. Staminate and andromonoecious: (sa)l C (SG)(SA) ++ (sg)
ii. Pistillate: (SA) ++ (sg) (SA) ++ (sg)
For convenience, the sex homologues may also be represented as: Mh – andro-
monoecious or hermaphrodite; Ms – androecious, that is staminate or male; m – gynoecious,