About sex determination in flowering plant papaya........

1. SEX DETERMINATION IN
PAPAYA

2. Course no. BGE-1205
Course Title: Principles of Genetics
Submitted to
 Md. Raihan Ali
 Professor
 Biotechnology &
Genetic Engineering
Discipline.
 Khulna University
 Khulna
 Submitted by
 Ashik Ahmed
 Student ID: 150701
 Biotechnology &
Genetic Engineering
Discipline.
 Khulna University
 Khulna

3. Index
 Abstract
 Introduction
 Sex ratio of crosses between different sex types
 Sex determination in papaya behaves as a single gene with three
alleles
 Genic balance between sex chromosomes and autosomes
 A group of linked genes confined to a small region on the sex
chromosome controlling sex determination
 X and Y sex chromosome system with two slightly different Y
chromosomes
 Two sex types of papaya
 Molecular genetics of sex determination in papaya
 Current understanding of sex determination in
papaya
 References

4. Abstract Papaya is a trioecious plant.
 Genetics of the papaya’s sex determination include,
 a single gene with three alleles,
 a group of closely linked genes,
 a genic balance of sex chromosome,
 classical XY chromosomes,
 regulatory elements of the flower development pathway.
 genomic technology helps to sex determination of papaya.
 sex determination is controlled by a pair of primitive sex
chromosomes with a small male-specific region (MSY) of the Y
chromosome.
 two sex determination genes control the sex determination
pathway of papaya-
 a feminizing or stamen suppressor gene,
 a masculinizing or carpel suppressor gene

5. Introduction
 Papaya (Carica papaya) belongs a small family with 35 species,
 32 are dioecious,
 two trioecious,
 one monoecious.
 Papaya is diploid with 9 pairs of chromosomes(2n=18).
 Papaya is somewhat unusual in that it is trioecious with three
basic sex forms: female, male, and hermaphrodite.
 Varieties typically are either dioecious or gynodioecious.
 Male trees are characterized by long, many-flowers,
 Female trees have short inflorescences with few flowers,have no
stamens.
 Hermaphrodite papaya trees are primarily self-pollinated.

6. Fig. 1. The flowers and fruits of male, female, and hermaphrodite papaya. (A)
Female flowers; (B) hermaphrodite flowers; (C) male flowers; (D) female fruit;
(E) hermaphrodite fruit;(F) male tree.

7. Sex ratio of crosses between different sex
types:
Sex
ratio
Sex
ratio
Sex
ratio
Sex
ratio
Crosses Female
(mm)
Male
(Mm)
Hermaphr
odite
(Mhm)
Non-
viable
(M(h)M(h)
)
Male (Mm) selfed
1 2 0 1
Female (mm)×male (Mm)
1 1 0 0
Hermaphradite (Mhm)
selfed
1 0 2 1
Female
(mm)×hermaphradite (Mhm)
1 0 1 0
Hermaphradite
(Mhm)×male (Mm)
1 1 1 1

8. Sex determination in papaya behaves as a single
gene with three alleles :
 Based on the segregation ratios from crosses among
three sex types , with the proposal of Hofmeyr and
Storey –
 sex determination in papaya is controlled by a single gene
with three alleles, named M, Mh, and m by Storey.
 Male individuals (Mm) and hermaphrodite individuals (Mhm)
are heterozygous.
 female individuals (mm) are homozygous recessive.
 The dominant combinations of MM, MhMh, and MMh are
lethal, resulting in a 2:1 segregation
 of hermaphrodite to female from self-pollinated hermaphrodite
seeds.
 a 1:1 segregation of male to female or hermaphrodite to
female from cross-pollinated female seeds.

10. Genic balance between sex chromosomes and
autosomes:
 Hofmeyr published an alternative genic balance hypothesis for sex
determination in papaya. In this hypothesis he proposed that-
 suggested that the chromosomes bearing the M, Mh and m
alleles were “sex chromosomes.” M and Mh represent an
inactivated region of the sex chromosomes where vital genes
were eliminated.
 female sex determining factors predominate the “sex
chromosomes”while the male sex determining factors are in
the autosome.
 inactivated region represented by M is longer than that
represented by Mh.
 The different sex types were the results of genic balance
between the sex chromosomes and autosome.
 where m sex chromosome is present in each genotype that is
viable.

11. A group of linked genes confined to a small region on the sex
chromosome controlling sex determination :
 Storey’s modified hypothesis is mainly based on the observation
that long peduncles are always associated with male flowers but not
with hermaphrodite or female flowers and that the lethal factor is
associated only with male and hermaphrodite homozygous
dominant genotypes.
 Based on Storey’s model, the genotypes of male,hermaphrodite,
and female were given as:
 Male =Mp l C + sg/+++sa +
 Hermaphrodite = + l C + sg/ + + + sa +
 Female=+++sa+/+++sa +
 The gene controlling the long peduncle of male flowers, Mp,is the
only one that distinguishes the genotype of male flowers from that of
hermaphrodite flowers.

12. X and Y sex chromosome system with two slightly
different Y chromosomes :
 Sex determination in papaya is of the 2 type-XX-
XY .
 The genotypes of male, female, and hermaphrodite
were XY, XX, and XY2, respectively.
 Researchers suggested that the Y chromosome has
a region containing a lethal factor.
 Y2 is a modified form of the Y chromosome but
includes the region of lethality.

13. Two sex types of papaya:
 from progenies of reciprocal crosses of long peduncle female
plants, they hypothesized there are only two sex types in
papaya.These are-
 Females
 variable type that ranges from complete male to hermaphrodite with
numerous intermediate types in between.
 Genotypes of sex in papaya,
 Male and andromonoecious: (sa) l C (SG)/(SA) + + (sg)
 Female: (SA) + + (sg)/(SA) + + (sg)
 Here,SA-converted the ancestral androecium into the current
gynoecium.
 sa-controlling normal androecium development.
 SG-controlling carpel abortion in male flowers.
 (sg) allows expression of the (SA) factors.
 l-function of zygotic lethal factor.
 C-suppression of recombination.

14. Molecular genetics of sex determination in
papaya
 Application of molecular techniques and
biotechnology has revolutionized the field of
sex determination research in papaya. These
techniques are-
 Sex-linked DNA markers ,
 Linkage mapping of sex determination locus ,
 Fine mapping of the sex determination locus ,
 Physical mapping of the sex determination locus ,
 Sample sequencing in the non-recombining region ,
 A primitive sex chromosome system .

15. Current understanding of sex determination in
papaya:
 Sex determination in papaya is controlled by a pair of sex
chromosomes.
 Y chromosome prevent the survival of the homozygous YY genotype
and to reinforce the heterozygosity of the male and hermaphrodite
genotypes.
 There are two slightly different Y chromosomes in papaya.
 Y for the male and
 Yh in hermaphrodite.
 At least two genes differentiate theYand Yh chromosomes-
 one gene controls the long peduncle on male trees.
 other masculinizing gene controls the carpel abortion in male flowers.
 Combinations of the Y chromosomes YY, YhYh, and YYh would lead to
the same fate of embryo abortion.
 two genes are involved in sex determination in papaya-
 feminizing gene for stamen abortion in female flowers.
 masculinizing gene for carpel abortion in male flowers.

16. References
 Hofmeyr JDJ. Genetical studies of Carica
papaya L. I. The inheritance and relation of
sex and certain plant characteristics. II. Sex
reversal and sex forms. S Afr Dept Agri and
Sci Bul No 187 1938:64.
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