PB Ch 24. Inbreeding Depression

Definition

Heterosis and inbreeding depression are closely related — they are the opposite sides of the same coin. Genetic theories explaining heterosis also explain inbreeding depression 

Effects of Inbreeding

• When a cross-pollinated plant is forced to self-pollinate, its genetic diversity plummets. Instead of carrying a healthy mix of dominant and recessive genes (heterozygosity), the plant's genes become identical pairs (homozygosity). This uncovers a host of biological problems:
• Unmasking Lethal Alleles: In a diverse population, harmful recessive genes are hidden by healthy dominant ones. Selfing pairs these bad recessive genes together. This results in mutant plants with missing chlorophyll, missing roots, or deformed flowers, which quickly die off.
• Loss of Vigor: The surviving plants shrink. They suffer a general reduction in leaf area, stem thickness, and overall size, making them weak and prone to falling over (lodging).
• Reproductive Collapse: Fertility drops dramatically. Some plants produce so little seed after a few generations of inbreeding that the breeder can no longer maintain the line.
• Separation into Distinct Lines: As heterozygosity vanishes, the original varied population splinters into highly distinct, uniform families. The plants within a specific family look completely identical, but the differences between the families become extreme.
• Crashing Yields: Even the strongest surviving inbred lines are shadows of their ancestors. In maize, a fully inbred line typically yields only 20% to 50% of the original open-pollinated variety.

Degrees of Inbreeding Depression

Development of Inbred Lines

• Despite the devastating effects, breeders must create pure, inbred lines to eventually make profitable hybrids. The goal is to reach about 99% homozygosity so the plant breeds perfectly true every time.
• The original selfed plant is designated S0; first selfed progeny = S1; sec and so on.
• After each generation of selfing, desirable plants are selected and self-pollinated. Inbreeding must be done carefully — all inflorescences must be bagged to prevent natural crossing.
• After 6-7 generations (S6-S7), lines reach approximately 99% homozygosity.
• Inbred lines are identified by numbers, letters or combinations.
• Indian maize inbreds from the All India Coordinated Maize Improvement Scheme: CM-100 to 199 (yellow flint), CM-200 to 299 (yellow dent), CM-300 to 399 (white flint), CM-400 to 499 (white dent).

Evaluation of Inbred Lines 

• Stage 1 — Phenotypic evaluation: Based on phenotypic performance of inbreds themselves. Effective for highly heritable characters. Poorly performing inbreds are rejected based on replicated yield trials — eliminates the worst performers before investing in expensive cross evaluations.
• Stage 2 — Top cross test: Inbreds selected in Stage 1 are crossed with a tester having a broad genetic base (an OPV, synthetic variety, or double cross). In maize, top crosses produced by planting alternate rows — inbred rows detasselled; seed from inbreds = top cross seed. Evaluated in replicated yield trials at multiple locations. Top cross test provides a reliable estimate of GCA. About 50% of inbreds eliminated at this stage.
• Stage 3 — Single cross evaluation: Remaining inbreds crossed in diallel or line x tester design to identify the best specific cross combinations (SCA). A superior single cross is fully heterozygous and homogeneous — it regains the vigour and productivity lost during inbreeding and can be more vigorous and productive than the original OPV.

Number of single crosses possible with n inbred lines:

3.5  Doubled Haploid (DH) Technology — Fastest Inbred Development

• Historically, creating a 99% pure inbred line took 6 to 8 long years. Today, breeders use Doubled Haploid (DH) Technology to achieve 100% purity in a single generation.
• How it works: Scientists take pollen grains (microspores) or anthers—which naturally contain only half of the plant's genetics (haploid)—and grow them in a lab. They then treat these plantlets with a chemical called colchicine, which forces the chromosomes to duplicate.
• The Result: The plant goes from having half a set of genes to a full, perfectly identical pair of genes. It is instantly 100% homozygous.
• Impact: What used to take up to 8 years now takes 1 to 2 years. First used commercially in China with Hua Han 1 rice in the 1970s, DH technology is now the global standard in wheat, rice, maize, and canola breeding programs worldwide.