Sex Determination in Plants Melandrium album (Silene latifolia)
In Melandrium album (or Silene
latifolia) sex determination is governed by a highly specialized XX-XY
system. Melandrium relies on an active Y-chromosome mechanism,
similar to humans.
Watch our simplified Video here: Sex Determination in Plants Melandrium album
- The plant is Dioecious, meaning male and female
reproductive organs are on separate plants.
- The diploid number is 2n = 24
- Females: Homogametic (XX).
- Males: Heterogametic (XY).
- Chromosomes are heteromorphic: The Y chromosome is significantly larger (about 1.4 times) than the X chromosome and is the primary determinant of sex.
Functional Regions of the Y Chromosome
Segment I (Female-Suppressing
Region): This
region actively inhibits the development of female organs (carpels/gynoecium).
If this segment is lost or mutated, the plant develops bisexual flowers.
Segment II (Male-Initiating
Region): This
region initiates the development of the male organs (stamens).
Segment III (Male-Maturing
Region): This
region controls the later stages of stamen development, specifically anther
maturation and pollen fertility.
Segment IV (Homologous Region): This is the "pairing region"
that is homologous to a segment on the X chromosome, allowing for pairing and
segregation during meiosis.
Functional Regions of the X
Chromosome
- Segment V (X-Specific Region): This region contains genes that
promote "femaleness." However, its effects are suppressed whenever
Segment I of the Y chromosome is present.
Sex Expression and Mutations
The interaction between these
segments determines the final phenotype of the plant:
- Normal Male (XY): All Y-segments are present.
Femaleness is suppressed; stamens develop and mature.
- Normal Female (XX): No Y-segments are present. The
X-specific genes promote carpel development.
- Bisexual (Hermaphrodite): Occurs if an XY plant has a
mutation or deletion in Segment I. The lack of suppression allows the
gynoecium to grow alongside the stamens.
- Asexual (Neuter): Occurs if an XY plant has a
deletion in Segment II. Femaleness is still suppressed by Segment I, but
maleness cannot be initiated.
What is the role of Ploidy in sex
determination?
Studies by Warmke and Westergaard
showed that the Y chromosome is so dominant that even in polyploid individuals,
a single Y is usually enough to induce maleness. Th following are some
genotypes and phenotypes indicating the dominance of Y chromosomes in sex
determination in polyploids.
- AAXX: Female
- AAXY: Male
- AAXXY: Male
- AAAXXY: Male
AAAAXXXXY: Hermaphrodite (In this extreme
case, the "dosage" of four X chromosomes finally overcomes the single
Y).