How is inbreeding depression measured experimentally in plants?

Inbreeding depression in plants is measured experimentally by comparing the fitness of selfed and outcrossed progeny across multiple life stages, using both experimental crosses and quantitative fitness indices.

1. Experimental Design

Researchers generate two groups of offspring from the same parent plants:

• Selfed progeny (obtained through manual self-pollination)
• Outcrossed progeny (produced via controlled cross-pollination)

Both groups are then grown under identical conditions—either in controlled environments (greenhouse) or in the field—to measure and compare key life-history traits.

2. Key Traits Measured

Inbreeding depression is expressed throughout the plant’s life cycle and is quantified for traits such as:

• Seed set and seed viability
• Germination percentage
• Seedling survival and growth rate
• Flower number or inflorescence mass
• Biomass and reproductive output (e.g., fruit or seed yield).

Each of these traits indicates how selfing (inbreeding) affects plant performance compared with outcrossing.

3. Quantitative Formula

The magnitude of inbreeding depression (δ) is calculated using the formula:

δ=1−Wo/Ws

where:

Ws = mean fitness of selfed progeny

Wo = mean fitness of outcrossed progeny

A higher δ value (closer to 1) indicates stronger inbreeding depression, while a value near 0 shows little or no depression.

4. Cumulative and Stage-Specific Estimation

Inbreeding depression can be estimated:

• Stage-wise (e.g., for germination, survival, or reproduction individually)
• Cumulatively (over the entire life cycle), often using multiplicative or family-based models that combine all life-stage data into an overall δ value.

For example, a field experiment on Lythrum salicaria found cumulative inbreeding depression values ranging from δ = 0.48 to 0.68 across multiple years and environmental conditions.

5. Statistical and Genomic Methods

Modern studies also incorporate:

• Resampling and bootstrapping methods to measure confidence intervals around δ estimates
• Quantitative trait loci (QTL) mapping and molecular markers to identify genomic regions contributing to inbreeding depression.