Melandrium album uses an XY sex determination system where XX individuals are female and XY are male. The Y chromosome contains male determining and female suppressor genes necessary for male flower and pollen production. The X and Y chromosomes each contain five segments that control sex organ development. Mutations in the Y chromosome segment that suppresses femaleness can result in XY individuals developing as bisexual hermaphrodites.

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.