(a) Mention the role of embryo rescue in interspecific hybridization. (b) Explain 'cybridization'. Write one example of…

(a) Role of Embryo Rescue in Interspecific Hybridization

In wide crosses between distantly related species, post-zygotic barriers often lead to embryo abortion due to genetic incompatibility or failure of endosperm development. Embryo rescue overcomes this by excising the immature embryo from the ovule and culturing it on a nutrient-rich medium. This bypasses the maternal tissue's inability to support the hybrid, allowing the embryo to develop into a plant. A classic example is the cross between Lycopersicon esculentum (tomato) and Solanum lycopersicoides, where rescue enabled the production of hybrids that would otherwise abort, facilitating the introgression of desirable traits like disease resistance. In rice, embryo rescue of Oryza sativa × O. officinalis hybrids transferred brown planthopper resistance.

(b) Cybridization: Mechanism, Example, and Methods

Cybridization, or cytoplasmic hybridization, involves the transfer of cytoplasmic components, such as mitochondria or chloroplasts, from one species to another while retaining the nuclear genome of the recipient. This is crucial for introducing cytoplasmic male sterility (CMS), a trait essential for hybrid seed production. An economically useful example is the Brassica napus cybrid combining Ogura (radish-derived) cytoplasmic male sterility with chloroplast-encoded atrazine resistance, produced by Pelletier and colleagues and used in rapeseed hybrid seed production.

The primary methods to produce cybrids are protoplast fusion (chemical with PEG or by electrofusion), fusion with enucleated cytoplasts, and microinjection. Before fusion, donor protoplasts are irradiated with X- or gamma-rays to inactivate their nuclei, while recipient protoplasts are treated with iodoacetate to inactivate their cytoplasm, so that only fused cells, which combine a functional recipient nucleus with donor organelles, can divide. Microinjection involves directly injecting donor cytoplasts or organelles into an intact recipient cell that retains its nucleus, allowing for the incorporation of cytoplasmic traits without nuclear exchange.

(c) Plants Used as Natural Dyes

Plants have been used for centuries to produce natural dyes for textiles, cosmetics, and food. These dyes are derived from pigments such as anthocyanins, carotenoids, and flavonoids. Three dye-yielding plants used as food additives are:

1. Turmeric (Curcuma longa, Family: Zingiberaceae): The rhizomes yield curcumin, a yellow pigment used to color rice, butter, and cheeses.
2. Saffron (Crocus sativus, Family: Iridaceae): The stigmas provide crocin, a yellow-orange dye used in rice dishes and confectionery.
3. Annatto (Bixa orellana, Family: Bixaceae): The seed coat yields bixin and norbixin, orange pigments used to colour butter, cheese and snacks.

Other dye plants used mainly for textiles include indigo (Indigofera tinctoria, Fabaceae), madder (Rubia cordifolia, Rubiaceae) and henna (Lawsonia inermis, Lythraceae).

(d) Anther Culture vs. Microspore Culture

Anther culture involves the in vitro cultivation of whole anthers to induce androgenesis, where microspores develop into haploid plants. However, this method often results in contamination by diploid somatic cells from the anther wall, leading to a mixture of haploid and diploid regenerants. In contrast, microspore culture involves the isolation of individual microspores from the anther and their direct culture on a medium. This technique is more advantageous because it ensures a pure haploid origin, eliminating diploid contamination. It allows for direct embryogenesis from the microspore, resulting in a population of independently derived haploid lines. Upon chromosome doubling, these lines become homozygous doubled haploids, which are genetically stable and uniform within each line, accelerating breeding programs for crop improvement.

(e) Role of Palynology in Plant Systematics

Palynology, the study of pollen and spores, plays a significant role in plant systematics due to the chemical stability of sporopollenin, the chief constituent of the exine (outer wall layer) of pollen grains. This stability allows for the preservation of pollen morphology over geological time, making it valuable for phylogenetic reconstruction and fossil calibration. Exine ornamentation patterns, such as reticulate, echinate, or smooth surfaces, serve as taxonomic markers that help distinguish between genera and species. While aperture types like tricolpate or monosulcate provide supportive evidence for grouping angiosperms, they are variable and not absolute separators. G. Erdtman's pollen-morphology surveys showed, for example, how pollen types help delimit tribes in Acanthaceae. Palynological characters are thus used alongside molecular data to refine evolutionary relationships and understand the biogeographical history of plant groups.