2. Majority of the
flowering plants are
monoecious
Only about 6% of
the plants are
dioecious.
Eg. Melandrium
album of family
Caryophyllaceae
Diploid
chromosome
number of
Melandrium is
24(n=12)
3. Mechanism of sex determination in
Melandrium album:
Melandrium album shows sex
chromosomal sex determination.
They have X and Y sex chromosomal
constitution similar to Drosophila and
Human.
In Melandrium album,
the XX individuals will be females and
XY individuals will be males.
4. Y chromosome plays a crucial role in the
sex determination of Melandrium
Y chromosome contains two main genes
◦ Male determining genes
◦ Female suppresor genes
Both are necessary for the production of
fertile anthers in male plants
Since the females are homogametic, the
segregation of X and Y chromosome
occurs only in the male individuals during
meiosis.
5. Usually plants with XY chromosomal
constitution are males.
Sometimes XY individuals will be
hermaphrodite (bisexual) or
androhermaphrodites (forming both bisexual
and male flowers on the same individuals).
The formation of bisexual individuals in XY
individuals may be due to
◦ the mutation or deletion of the female
suppressing genes on the Y chromosome
Or
◦ due to the change in DNA methylation pattern.
6. The plants may be diploids, triploids or
tetraploids in their chromosomal
constitution.
Irrespective of the ploidy of the
chromosomes, the presences of a single Y
chromosome produce male plants.
Thus XY, XXY and XXXY individuals will be
males.
However XXXXY individuals will be
hermaphrodite (bisexual).
7. Structure of X and Y sex
chromosomes in Melandrium
album
• In Melandrium album,
the Y chromosome is
longer than X
chromosome.
• The X and Y
chromosome are divided
into five segments
• These segments control
the development of sex
organs in the plant.
8. Y chromosome segments and its
functions:
Segment – I: Suppress femaleness: it
inhibit the development of female
structure in the flower
Segment – II: Initiate anther
development
Segment–III: control the later stages of
anther development (segment II and III
are essential for the formation of
functional- fertile anthers
Segment-IV: Homologous with region
IV on X chromosome, used for pairing
with X-chromosome during meiotic
prophase
X chromosome segments and its
functions:
Segment-IV: Homologous with region
IV on Y chromosome, used for pairing
with Y-chromosome during meiotic
prophase
Segment-V: X chromosome specific
region, it promote femaleness in the
absence of segment I on Y
chromosome
9. A plant with XY constitution that
lacking segment I of Y chromosome
due to mutation or deletion, will be
bisexual. This is because, in the
absence of segment I on the Y
chromosome (whose function is to
suppress femaleness) the segment V
on X chromosome promote the
development of female structures.
This flower will NOT be a female
flower because, segment II and III on Y
chromosome is still there in the
chromosome which produce the fertile
anthers.