(a) Elucidate the molecular basis of sex determination in plants. Explain the role of homomorphic and heteromorphic sex…

(a) Molecular Basis of Sex Determination in Plants; Homomorphic and Heteromorphic Sex Chromosomes

Only about 6% of flowering plant species are dioecious (separate male and female plants), and dioecy has arisen independently many times, mostly from hermaphrodite ancestors. Sex may be determined by sex chromosomes, by the balance of X chromosomes to autosomes, or, in monoecious plants, by genes and hormones acting in each flower.

The two-gene model: according to Charlesworth and Charlesworth (1978), dioecy evolves when two mutations arise in linked genes on a proto-sex chromosome: a recessive male-sterility mutation, which produces females, and a dominant female-suppressor, which produces males. Recombination between them must then be suppressed, and this non-recombining region grows into a sex-determining region (SDR). Molecular studies support both one-gene and two-gene systems:

1. Asparagus officinalis (XY): the Y carries two genes: SOFF (Suppressor of Female Function, a DUF247 gene), which suppresses the pistil, and aspTDF1, a transcription factor needed for anther (tapetum) development. This matches the two-gene model.
2. **Kiwifruit (Actinidia, XY): the Y carries *SyGI*** (Shy Girl), a cytokinin response regulator that suppresses the gynoecium, and FrBy, which maintains the tapetum and male fertility.
3. **Persimmon (Diospyros lotus, XY): a single Y-linked gene, *OGI*, produces a small RNA that silences the autosomal feminising gene *MeGI*** (a homeodomain gene) by RNA-directed DNA methylation, so XY plants are male. This is an epigenetic one-gene switch.
4. ***Silene latifolia* (XY): deletion mapping of the Y (Westergaard; later workers) identified three regions: a gynoecium-suppressing factor (GSF), a stamen-promoting factor (SPF) and a male-fertility factor**.
5. Monoecious cucurbits: sex of individual flowers is controlled by ethylene genes. In melon, CmACS-7 acts in carpel-bearing flowers to suppress stamens (andromonoecy), and the transcription factor CmWIP1 suppresses carpels; transposon-induced DNA methylation of CmWIP1 gives all-female (gynoecious) plants. In cucumber, the F (CsACS1G) and M (CsACS2) loci promote femaleness.
6. Maize (monoecious): the tassel and ear become unisexual through organ abortion. The tasselseed genes (e.g., ts1, which encodes a lipoxygenase of the jasmonate pathway, and ts2) cause pistil abortion in the tassel, and gibberellin promotes femaleness.

Heteromorphic sex chromosomes are X and Y (or Z and W) that differ visibly in size or shape. They are old, with a large non-recombining region. Loss of recombination lets transposable elements and repeats accumulate, genes decay, and the Y may enlarge or shrink.

1. In Silene latifolia the Y is the largest chromosome of the complement and actively determines maleness: XX plants are female, and XY plants are male.
2. In Rumex acetosa (XX female, XY₁Y₂ male) the two Y chromosomes are largely heterochromatic and do not determine sex. Instead, sex follows the X-to-autosome ratio: a ratio of 1.0 gives females and 0.5 gives males. This resembles Drosophila.
3. Coccinia grandis (ivy gourd) also has a large, active Y. Their role is to hold the sex-determining genes in a stable, non-recombining block, which ensures a 1:1 sex ratio, and in some species to carry sexually antagonistic genes.

Homomorphic sex chromosomes look identical under the microscope. They are young, with only a small SDR, and sex is decided by a few genes within it.

1. In papaya (Carica papaya), a small male-specific region of the Y (about 8 Mb) distinguishes XX females, XY males and XYʰ hermaphrodites; YY is lethal because essential genes have been lost from the Y.
2. In asparagus, the SDR is only about 1 Mb, and YY "supermales" are viable, which is used to produce all-male hybrids.
3. Persimmon and kiwifruit also have homomorphic sex chromosomes. Homomorphic systems show the early stages of sex-chromosome evolution and allow molecular sexing of seedlings, which is valuable in papaya, date palm and asparagus. ZW/ZZ systems, with heterogametic females, occur in Fragaria virginiana and Salix; Populus is XY in some species and ZW in others.

(b) Structure of the Nucleosome and Its Role in DNA Packaging

The nucleosome, proposed by Kornberg (1974) and seen as "beads on a string" by Olins and Olins, is the basic repeating unit of chromatin.

1. Core particle: a histone octamer made of an (H3–H4)₂ tetramer flanked by two H2A–H2B dimers, around which 147 bp of DNA is wrapped in about 1.65 left-handed superhelical turns. Histones are rich in lysine and arginine; their positive charges bind the DNA phosphates, and their histone-fold domains form the core. The flexible N-terminal tails project outwards and carry modifications (acetylation, methylation, phosphorylation).
2. Linker DNA and H1: successive cores are joined by linker DNA of about 10–80 bp, so the nucleosome repeat length is about 160–240 bp. The linker histone H1 binds at the dyad where DNA enters and exits the core, sealing about 20 bp more, to form the chromatosome (about 167 bp).
3. Histone variants: H2A.Z at promoters, H3.3 in active genes, CENH3 at centromeres and H2A.X in DNA repair give nucleosomes special properties.

Role in DNA packaging:

1. The string of nucleosomes forms the 11-nm fibre ("beads on a string"), compacting DNA about 6–7-fold.
2. H1 and interactions between histone tails fold this into a more compact fibre, classically the 30-nm fibre (solenoid or zig-zag models), although in living nuclei chromatin is now thought to exist largely as irregular, flexible fibres.
3. The fibres form loops of tens of kilobases attached to a protein scaffold or matrix (with cohesin and condensin), and these condense further into the metaphase chromatid about 700 nm wide. Overall, about 2 m of DNA in a human nucleus, and far more in large plant genomes, fits into a nucleus a few micrometres across; the packing ratio reaches about 10,000 in mitotic chromosomes.
4. Regulation: the nucleosome controls access to DNA. Chromatin-remodelling complexes (SWI/SNF, ISWI) slide or evict nucleosomes to expose promoters, and histone acetylation loosens chromatin (euchromatin). H3K9 and H3K27 methylation with DNA methylation form compact heterochromatin and silence genes and transposons. In Arabidopsis, H3K27me3 deposited by Polycomb complexes silences FLC during vernalization.

(c) Methods of Plant Hybridization

Hybridization is the mating of genetically different parents to combine desirable genes or to exploit heterosis.

By relationship of the parents, hybridization is:

1. Intravarietal: crossing plants within a variety.
2. Intervarietal: crossing two varieties of the same species, the most common type (e.g., most wheat and rice varieties).
3. Interspecific (distant): crossing two species of a genus, e.g., Gossypium hirsutum × G. barbadense (the interspecific cotton hybrids Varalaxmi and DCH-32), or Saccharum officinarum × S. spontaneum (nobilization) for Indian sugarcane varieties.
4. Intergeneric: crossing two genera, e.g., wheat × rye (triticale), radish × cabbage (Raphanobrassica). Wide crosses often need embryo rescue, bridge species, chromosome doubling or backcrossing to overcome incompatibility and sterility.

Procedure of artificial crossing:

1. Choice of parents: parents with complementary desirable characters and good general combining ability are selected.
2. Emasculation of the female parent: removal of the anthers, or killing of the pollen, before the anthers dehisce, in bisexual flowers. The methods are: hand emasculation with forceps (wheat, cotton, tomato); hot water (rice panicles at about 40–44°C for 5–10 minutes); cold treatment and alcohol treatment; suction of anthers (small flowers); chemical hybridizing agents (gametocides such as maleic hydrazide and ethrel); and genetic or cytoplasmic male sterility, which avoids emasculation altogether. In monoecious maize the tassel is removed (detasselling).
3. Bagging: the emasculated flower and the male flowers used as the pollen source are covered with paper or butter-paper bags to prevent unwanted pollination.
4. Tagging: each crossed flower is labelled with the date and the names of the parents.
5. Pollination: fresh pollen from the male parent is dusted on the receptive stigma, usually the next morning, and the bag is replaced.
6. Harvesting and handling of F₁: the crossed seed is harvested, and the F₁ is raised for selection, or used directly as a hybrid variety.

Crossing schemes used in breeding include the single cross (A × B), three-way cross ((A × B) × C), double cross ((A × B) × (C × D)), top cross (with a tester), backcross (to transfer a single gene into an adapted variety) and multiple or composite crosses. Commercial hybrid seed is produced in isolated plots by planting female (male-sterile or detasselled) and male rows in ratios such as 4:1 or 6:2, using CMS–restorer (A, B and R line) systems in rice, pearl millet, sorghum and sunflower.

Somatic hybridization, the fusion of protoplasts with PEG or electrofusion, is a non-sexual method for combining sexually incompatible species (e.g., potato + Solanum bulbocastanum). It often gives sterile or unstable hybrids.