(a) Discuss briefly the following and comment on their significance : 10+5+5=20 (i) In what way the transfer of genes in plants…

(a)(i) Gene transfer and sustainable development

How genes are transferred

• ***Agrobacterium tumefaciens*-mediated transformation:** the gene of interest is placed between the T-DNA borders of a disarmed Ti plasmid, and the bacterium inserts it into the plant genome.
• Direct methods: particle bombardment (biolistics), electroporation or PEG treatment of protoplasts.
• Selection and regeneration: transformed cells are selected with a marker (such as nptII for kanamycin resistance) and regenerated in tissue culture.
• Genome editing: tools such as CRISPR–Cas9 now also change a plant's own genes precisely.

How it helps sustainable development

Gene transfer can add traits that are absent from the crop's gene pool. This lets a crop produce more with fewer chemical inputs, less water and less land, which is the goal of sustainable intensification.

Example: Bt cotton in India (approved 2002)

• The gene: cry1Ac (Bollgard, Monsanto event MON 531), and later cry1Ac + cry2Ab (Bollgard II), from Bacillus thuringiensis. The gene makes Cry δ-endotoxins, which bind receptors in the midgut of lepidopteran larvae such as the American bollworm (Helicoverpa armigera) and kill them. They are harmless to mammals and most other insects.

Results:

• Insecticide sprays against bollworm fell sharply in the first decade.
• Yields and farm incomes rose.
• India became the world's largest cotton producer.
• Beneficial insects and farm workers were exposed to fewer broad-spectrum sprays.

Limits: these show why biotechnology must be managed sustainably.

• Pink bollworm (Pectinophora gossypiella) became resistant to Bollgard II by about 2014–15, partly because refuges were not planted.
• Sucking pests needed separate control.
• Seed costs are high.

Other examples

• Bt brinjal: grown by farmers in Bangladesh since 2014, with far fewer insecticide sprays.
• Rainbow papaya: saved Hawaii's papaya industry from ringspot virus.
• Drought-tolerant HB4 wheat and soybean: from Argentina.
• GM mustard DMH-11 (India): a hybrid made with the barnase/barstar system for easier hybrid seed. Its environmental release was approved by GEAC in 2022, but the Supreme Court gave a split verdict in 2024 and asked the government to frame a national policy.
• Genome-edited rice (India, 2025): the first two varieties, DRR Dhan 100 (Kamala) and Pusa DST Rice 1, were released for higher yield and for drought and salinity tolerance.

(a)(ii) Importance of biosafety of transgenic crops

Biosafety means assessing and managing the risks that a genetically modified organism may pose to human and animal health and to the environment.

Why it matters

Food and feed safety:

• New proteins must be checked for toxicity and allergenicity, and the crop compared with its conventional counterpart ("substantial equivalence").
• A Brazil-nut 2S albumin gene put into soybean was found to carry the nut allergen, and the product was dropped before release (1996).
• StarLink maize (Cry9C), approved in the US only for animal feed, entered the human food chain in 2000 and had to be recalled.

Environmental safety:

• Gene flow to wild relatives or weedy forms, for example from herbicide-tolerant canola.
• Effects on non-target organisms, pollinators and soil microbes.
• Evolution of resistant pests and weeds.
• Loss of landrace diversity in centres of origin.

Regulation in India

• Legal basis: the "Rules, 1989" under the Environment (Protection) Act, 1986.

Three tiers:

• The Institutional Biosafety Committee (IBSC) at each institution.
• The Review Committee on Genetic Manipulation (RCGM, Department of Biotechnology) for research and confined trials.
• The Genetic Engineering Appraisal Committee (GEAC, MoEFCC) for large-scale trials and environmental release.
• States also control field trials.
• Genome-edited plants without foreign DNA (SDN-1 and SDN-2) have been exempt from these rules since 2022.
• International: the Cartagena Protocol on Biosafety (adopted 2000, in force 2003; India is a party) requires advance informed agreement for transboundary movement.

Significance: careful biosafety review, confined trials, refuges and labelling protect health and biodiversity. They also build the public trust without which useful transgenic crops cannot be adopted.

(a)(iii) Polytene chromosomes

Polytene chromosomes are giant interphase chromosomes, first seen by Balbiani (1881) in Chironomus larvae. They are best known from the salivary glands of Drosophila larvae, studied by Heitz and Bauer and by Painter in 1933.

• Formation: repeated rounds of DNA replication without cell division (endoreduplication) produce up to about 1,000 chromatids (2¹⁰ in Drosophila). The chromatids stay aligned side by side, and the homologues remain paired (somatic pairing), so the haploid number of thick cables is seen. In Drosophila they join at a chromocentre formed by centromeric heterochromatin.
• Bands and interbands: about 5,000 bands in Drosophila, formed by aligned chromomeres of condensed chromatin, separated by lighter interbands. Each chromosome has a constant, recognisable pattern.
• Puffs and Balbiani rings: local decondensed regions where genes are actively transcribed. Puffs appear in a set sequence after the moulting hormone ecdysone (Ashburner), which showed directly that hormones switch on genes.
• Plants: polyteny also occurs in plants, for example in antipodal cells of some species and in the embryo suspensor of Phaseolus.

Significance

• Bridges' band maps allowed deletions, duplications, inversions and translocations to be seen and located, and genes to be mapped cytologically. The duplication behind Bar eye was identified this way.
• In situ hybridisation was first developed on polytene chromosomes (Pardue and Gall), placing cloned genes on specific bands.
• They are a model for studying chromatin structure and gene regulation.

(b) Mass selection in a breeding programme

Mass selection is the oldest and simplest method of selection:

1. Many plants with the desired phenotype are chosen from a variable population.
2. Their seed is bulked together, without progeny testing.
3. The bulked seed is sown to raise the next generation.
4. The cycle is repeated for as long as progress continues.

Farmers created most landraces this way.

Uses

• Cross-pollinated crops (maize, pearl millet, rye, sugar beet, forage grasses): mass selection raises the frequency of favourable alleles in the population while keeping it heterozygous and broad-based. The result is an improved open-pollinated variety.
• Self-pollinated crops: it is used to make a variety from a landrace and to purify or maintain existing varieties, by rogueing off-types and keeping true-to-type plants.

Why it works: selection shifts the population mean.

• Response: R = h²S, where S is the selection differential and h² the narrow-sense heritability.
• Traits: progress is good for highly heritable traits (plant height, maturity, ear or panicle size, oil content, disease resistance) and slow for low-heritability traits like yield.
• Cross-pollinated species: if selection is made only on the female side after open pollination, the response is roughly halved (about ½h²S), because the pollen parents are unselected.

Improvements

• Gardner's gridded (stratified) mass selection (1961): the field is divided into small grids, and the best plants are chosen within each grid to reduce soil variation. This gave about 3–4% gain in yield per cycle in the maize variety Hays Golden.
• Pollination control: selecting before flowering, or detasselling inferior plants, controls the male parent too.

Other examples

• Sugar beet: Vilmorin's selections in France in the 19th century raised the sugar content of sugar beet greatly.
• Maize grain composition: long-term selection at Illinois (from 1896) greatly changed oil and protein content in maize grain.
• Pearl millet and maize: many open-pollinated composites and synthetics in India have been improved by recurrent mass selection before release.

Advantages

• It is simple, cheap and quick (one season per cycle), and it can handle large populations.
• It keeps genetic diversity and so gives stable, widely adapted varieties.
• It can be used by farmers themselves.

Limitations

• Selection is on phenotype, so environmental effects mislead it, and there is no progeny test.
• It works poorly for low-heritability traits.
• In cross-pollinated crops the male parent is not controlled.
• Its gains are therefore smaller than those from progeny or recurrent selection and hybrid breeding. Mass selection remains a useful first step and a way to maintain varieties.

(c) Theories of organic evolution

**1. Lamarckism (J.B. Lamarck, Philosophie Zoologique, 1809)**

• Ideas: an inner drive towards complexity, use and disuse of organs, and inheritance of acquired characters. The standard illustration is the giraffe's neck lengthening through stretching.
• Evidence against: Weismann's germ-plasm theory (1880s) showed that somatic changes do not reach the germline. Cutting the tails of mice over 22 generations did not shorten them.
• Legacy: "Neo-Lamarckism" survived for a time. Some epigenetic inheritance in plants is now known, but it is limited and usually unstable.

**2. Darwinism (Darwin and Wallace, 1858; On the Origin of Species, 1859)**

Ideas:

• Organisms overproduce, so there is a struggle for existence.
• Heritable variation exists.
• Individuals with favourable variations survive and reproduce more (natural selection, the "survival of the fittest").
• Over long periods this produces adaptation and new species by gradual change.
• Example: the Galapagos finches, whose beaks suit their food. Peter and Rosemary Grant measured beak depth rising after the 1977 drought, when only large, hard seeds were left.
• Weakness: Darwin could not explain how variation is inherited (he assumed blending).

3. Mutation theory (Hugo de Vries, 1901)

• Ideas: new species arise suddenly by large mutations ("saltation"), not by slow selection.
• Basis: de Vries worked on Oenothera lamarckiana. Most of his "mutants" were later shown to be chromosomal: the translocation-ring system and polyploidy (O. gigas is a tetraploid).
• Contribution: it brought mutation into evolutionary thinking.

4. Modern (synthetic) theory, or neo-Darwinism (1930s–1950)

• Founders: Fisher, Haldane and Wright in population genetics, with Dobzhansky, Mayr, Huxley, Simpson and, for plants, G.L. Stebbins (Variation and Evolution in Plants, 1950).

Ideas:

• Evolution is a change in allele frequencies in populations.
• It is caused by mutation, recombination, natural selection, genetic drift and gene flow.
• Speciation needs reproductive isolation.

Examples:

• **Industrial melanism in the peppered moth Biston betularia:** dark forms rose with soot pollution and declined after clean-air laws (Kettlewell's bird-predation studies).
• Resistance evolving in real time: pests to Bt cotton, weeds to glyphosate, bacteria to antibiotics.
• Instant speciation by polyploidy in plants: Primula kewensis (1909), Spartina anglica, and the new allotetraploid Tragopogon species that appeared in the USA in the 20th century. Karpechenko's Raphanobrassica (1928) was a synthetic example.

5. Later additions

• Neutral theory (Kimura, 1968): much molecular change is by drift of selectively neutral mutations. This underlies the molecular clock.
• Punctuated equilibrium (Eldredge and Gould, 1972): long stasis broken by rapid change, as seen in some fossil series.
• Extended synthesis: adds evo-devo (changes in developmental genes, such as Hox in animals and CYCLOIDEA in flower symmetry) and epigenetics.

Evidence of evolution overall

• Fossils and transitional forms: Archaeopteryx between reptiles and birds; Tiktaalik between fish and tetrapods; seed ferns (Pteridospermales) between ferns and seed plants.
• Homologous and vestigial organs: homologous organs include the pentadactyl limb, and leaf tendrils and spines. Vestigial organs include scale leaves and staminodes.
• Embryology and biogeography.
• Molecular evidence: the universal genetic code, and the similarity of cytochrome c and DNA sequences. Human and chimpanzee DNA are about 98–99% identical in aligned regions. Such sequences give phylogenies consistent with the fossil record.