विस्तृत व्याख्या:

Evolution of Wheat (गेहूँ की उत्पत्ति एवं उद्विकास):

• Statement (A) is CORRECT: Tetraploid wheat (Triticum turgidum ssp. durum, $2n = 4x = 28$, genome AABB) is an allopolyploid formed by natural hybridization between T. urartu (AA donor) and an ancestor related to Aegilops speltoides (BB donor).
• Statement (B) is CORRECT: Bread wheat (Triticum aestivum, $2n = 6x = 42$, genome AABBDD) is an allohexaploid.
• Statement (C) is INCORRECT: T. turgidum (AABB) did NOT arise from T. timopheevii (AAGG); they are distinct tetraploid lineages.
• Statement (D) is CORRECT: Hexaploid bread wheat (T. aestivum, AABBDD) arose from spontaneous hybridization between cultivated tetraploid wheat (T. turgidum, AABB) and the wild goat grass Aegilops tauschii (Ae. squarrosa, DD donor), followed by spontaneous chromosome doubling.

विस्तृत व्याख्या:

Climatic Adaptation in Wheat (गेहूँ में जलवायु अनुकूलन कार्यिकी):

• Physiological traits governing wide climatic adaptation in wheat cultivars worldwide are controlled by major gene systems:

• Vernalization requirement ($Vrn$ genes): Cold temperature requirement to switch from vegetative to reproductive phase (distinguishes winter wheat from spring wheat).
• Winter hardiness / freezing tolerance: Ability of winter wheat crowns to survive extreme sub-zero soil and air temperatures.
• Photoperiod response ($Ppd$ genes): Day-length sensitivity governing heading date and flowering time across different latitudes.
  • In contrast, Fertilizer requirement is an agronomic/nutritional management factor, not an intrinsic physiological climatic adaptation trait.

विस्तृत व्याख्या:

Origins of Cytoplasmic Male Sterility (CMS की उत्पत्ति):

• CMS is controlled by mitochondrial genes (mtDNA) causing failure of viable pollen production.
• CMS originates via three primary pathways:

• Inter-specific / wide hybridization (A): Combining nuclear genome of one species with foreign cytoplasm of a related wild species (alloplasmic male sterility, e.g., Tift 23A pearl millet, WA cytoplasm in rice from wild rice Oryza rufipogon).
• Spontaneous mutation (C): Spontaneous mutations or rearrangements in the mitochondrial genome (e.g., Texas cytoplasm in maize, CMS-S, CMS-C).
• Induced mutation using mitochondrial mutagens (D): Chemical mutagens like Ethidium bromide and streptomycin specifically target and mutagenize extranuclear/mitochondrial DNA to induce CMS.
  • Self-incompatibility (B) is a genetic pre-fertilization barrier preventing self-pollen fertilization, governed by nuclear $S$-alleles, and has no causal relation to CMS cytoplasm.

विस्तृत व्याख्या:

Analysis of Statements (कथनों का विश्लेषण):

• is INCORRECT: Unlike $F_1$ hybrids, farmers do not need to replace seeds of synthetic varieties every year; the open-pollinated synthetic population maintains heterozygosity and can be grown for 3–4 seasons without significant yield decline.
• is CORRECT: Hybrid crops require optimal agronomic conditions, higher fertilization, irrigation, and careful management to realize their full heterotic yield potential.
• is INCORRECT: In maize and pearl millet breeding, evaluating parental lines for general combining ability is done via topcross or polycross tests, not open-pollinated progeny tests (which are used mainly in perennial tree crops/forestry).
• is INCORRECT: Composite varieties are developed by mixing open-pollinated genotypes evaluated for General Combining Ability (GCA), primarily exploiting additive gene action and GCA rather than Specific Combining Ability (SCA).