Sex determination in plants

Sex determination in plants

About 94% of all flowering plants have only one type of individual, which produces flowers with male organs (the stamens) and female organs (the carpels). Such plants are termed as sexually monomorphic. Some 6% of flowering plants which have two separate sexes are called dimorphic. Male plants produce flowers with stamens and female plants produce flowers with carpels only. Researchers are interested to study the mechanism of sex determination in plants.C.E. Allen (1917) discovered sex determination in plants. Sex determination is a complex process determined by genes, the environment and hormones.

Sex determination in Silene latifolia (Melandrium album) is of controlled by three distinct regions in a sex chromosome.

1.     Y chromosome determines maleness

2.     X specifies femaleness

3.     X and Y show different segments (I II III IV and V)

1.  Sex determination in papaya

Recently researchers in Hawaii discovered sex chromosomes in Papaya Papaya (Carica papaya, 2n=36). Papaya has 17 pairs of autosomes and one pair of sex chromosomes. Male Y X papaya plants have XY Figure 3.18 : Sex and female plants have chromosome of XX. Unlike human sex papaya chromosomes, papaya sex chromosomes look like autosomes and it is evolved from autosome. The sex chromosomes are functionally distinct because the Y chromosome carries the genes for male organ development and X bears the female organ developmental genes (Figure 3.18).

In papaya sex determination is controlled by three alleles. They are m, M₁ and M₂ of a single gene.

2. Sex Determination in Sphaerocarpos

Sex determination was first described in the bryophyte Sphaerocarpos donnellii which has heteromorphic chromosomes. The gametophyte is haploid and heteromorphic. The male gametophyte as well as the female gametophyte is an haploid organism

with 8 chromosome (n=8). The diploid sporophyte is always heterogametic. Seven autosomes are similar in both male and female gametophyte. But the eighth chromosome of female is X which is larger than the seven autosomes. The eighth chromosome of male is Y which is comparatively smaller than autosomes. The sporophyte containing XY combination produces two types of meiospores, that is some with X and others with Y chromosomes. The meiospores with X chromosomes produce female gametophyte and those with Y chromosome produces male gametophyte.

3. Sex determination in maize

Zea mays (maize) is an example for monoecious, which means male and female flowers are present on the same plant. There Tassel are two types of inflorescence.

The terminal inflorescence which bears staminate florets develops from Ear shoot apical  meristem called tassel. The lateral inflorescence which develop pistillate florets from axillary bud is called ear or cob. 

Unisexuality in maize occurs through the selective abortion of stamens in ear florets and pistils in tassel florets. A substitution of two single gene pairs 'ba' for barren plant and 'ts' for tassel seed makes the difference between monoecious and dioecious (rare) maize plants. The allele for barren plant (ba) when homozygous makes the stalk staminate by eliminating silk and ears. The allele for tassel seed (ts) transforms tassel into a pistillate structure that produce no pollen. The table-3.7 is the resultant sex expression based on the combination of these alleles. Most of these mutations are shown to be defects in gibberellin biosynthesis. Gibberellins play an important role in the suppression of stamens in florets on the ears.