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Sex Determination in Melandrium
- 2. ï‚— Majority of the flowering plants are monoecious ï‚— Only about 6% of the plants are dioecious. ï‚— Eg. Melandrium album of family Caryophyllaceae ï‚— Diploid chromosome number of Melandrium is 24(n=12)
- 3. Mechanism of sex determination in Melandrium album: ï‚— Melandrium album shows sex chromosomal sex determination. ï‚— They have X and Y sex chromosomal constitution similar to Drosophila and Human. ï‚— In Melandrium album, the XX individuals will be females and XY individuals will be males.
- 4. ï‚— Y chromosome plays a crucial role in the sex determination of Melandrium ï‚— Y chromosome contains two main genes â—¦ Male determining genes â—¦ Female suppresor genes ï‚— Both are necessary for the production of fertile anthers in male plants ï‚— Since the females are homogametic, the segregation of X and Y chromosome occurs only in the male individuals during meiosis.
- 5. ï‚— Usually plants with XY chromosomal constitution are males. ï‚— Sometimes XY individuals will be hermaphrodite (bisexual) or androhermaphrodites (forming both bisexual and male flowers on the same individuals). ï‚— The formation of bisexual individuals in XY individuals may be due to â—¦ the mutation or deletion of the female suppressing genes on the Y chromosome Or â—¦ due to the change in DNA methylation pattern.
- 6. ï‚— The plants may be diploids, triploids or tetraploids in their chromosomal constitution. ï‚— Irrespective of the ploidy of the chromosomes, the presences of a single Y chromosome produce male plants. ï‚— Thus XY, XXY and XXXY individuals will be males. ï‚— 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. ï‚— 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. ï‚— 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.