Plant Breeding: Inbreeding and Heterosis

• The subject is plant improvement.

Inbreeding and Heterosis

• Inbreeding, also known as consanguinity, enhances the percentage of homozygotes in a heterozygous population through various crossing methods.
• Examples of inbreeding include self-fertilization, full-sib crossing, half-sib crossing, back-crossing, and haplo-diploidization.
• The percentage of homozygous individuals for a given locus after m generations of self-fertilization of a heterozygous individual for that locus is (2^m - 1) / 2^m.
• For n loci, this percentage is ((2^m - 1) / 2^m)^n.
• D and E are full siblings; they share two parents, A and B.
• D and F are half-siblings; they share parent B.
• E and F are half-siblings; they share parent B.

Consequences of Inbreeding

• There is an increase in the homozygosity of the population.
• The degree of uniformity among individuals from the same parent increases.
• Inbreeding results in loss of vigor, including decreased height, total weight, grain production, and disease resistance.
• The phenomenon is marked in allogamous plants, such as pumpkin, cucumber, squash, watermelon, and castor bean.
• Inbreeding has no effect on autogamous plants.

Heterosis

• Heterosis, or hybrid vigor, can be defined as either of two things:
  • The difference between the F1 hybrid and the average of the parents.
  • The superiority of the F1 generation compared to the best parent.
• Consequences of heterosis include:
  • Increased height.
  • Increased root volume.
  • Increased size of leaves and panicles.
  • Increased number and size of grains.
  • Increased disease resistance.
  • Increased precocity.

Explanations of Heterosis

• The theory of superdominance, proposed by Shull and East in 1908, states that the most vigorous individual has the greatest number of heterozygous loci.
• The theory of dominance suggests the accumulation of dominant genes in the F1 generation contributes to heterosis.
• A drawback is that no differences exist between AABBCC and AaBbCc.
• Self-fertilization leads to a loss of vigor.
• The theory of epistasis exists, but there is not a concrete example to support it.

Variability

• A plant population is variable if not all plants exhibit phenotypically identical traits.

Environmental Variability

• Environmental factors influencing variability include temperature, soil type, water, photoperiodism, wind direction and speed, light, fertilization, insects, and pathogens.
• Environmental variability is not transmitted from one generation to the next.

Genetic Variability

• Genetic variability arises in individuals with different genotypes.
• P1, P2, and F1 generations are genetically identical, while the F2 generation is genetically variable (AA, Aa, aa).
• Genetic variability can be caused by:
  • An accidental mix with other genotypes.
  • Gene recombination following crosses with other populations.
  • Mutations and chromosomal instability.

Measuring Variability

• The population mean is calculated as x = Σxi/n or x = Σnixi/n.
• Standard deviation: s= √(Σ(xi-x)²/n-1.
• Variance: V= ∑(x₁-x)²/n-1.
• Confidence interval with error sx: x ± sx.
• Coefficient of variation: CV = 100 s/x.

Heritability

• Heritability measures the degree of transmission of quantitative trait variability from ancestors to descendants.
• H = Genetic variance / Phenotypic variance (broad-sense heritability)
• P = G + E + GxE, where:
  • P is phenotypic value.
  • G is the genotypic component of the phenotype.
  • E is the environmental component's effect.
  • GxE is the genotype by environment interaction.
• If GxE is negligible, P = G + E.
• VP = V(G + E).
• If G and E are independent: VP = VG + VE and VG = VA + VD + VI.
• The range of heritability is between 0 (if VG = 0) and 1 (if VE = 0).
• VP1 = E; VP2 = E; VF1 = E (K. Mather).
• VF2 = 1/2 A + 1/4 D + E.
• VB1 = 1/4 A + 1/4 D + E.
• VB2 = 1/4 A + 1/4 D + E.
• VB1 + VB2 = 1/2 A + 1/2 D + 2E.
• Narrow-sense heritability h²: h²= VA/VP.