How to collect only homozygous seeds of cultivated plants?
What methods are used when you want to collect only seeds that are purely homozygous in genotype, such as from cultivated plants? Do you repeatedly cross strains with the same phenotype for many generations (referencing Mendel's laws)? Or do you analyze the genes using equipment like PCR before crossing?
AI Response:
In traditional plant breeding, the following methods are primarily used to collect seeds that are purely homozygous in genotype. These are based on Mendel's laws (the Law of Segregation and the Law of Independent Assortment) and are processes for segregating and fixing homozygosity from heterozygosity. The main methods are explained below.
1. Methods using Inbreeding
- Overview: Repeatedly cross strains with similar phenotypes (or within the same line) (self-pollination or sibling mating) to increase homozygosity over many generations. This is a classic approach applying Mendel's laws.
- Example of procedure:
- First, select a heterozygous parent strain with the desired trait.
- Self-pollinate this (pollinate the same plant with its own pollen) or cross it with a closely related strain, and grow the progeny (F1, F2...).
- Based on Mendelian segregation ratios (e.g., 3:1 or 1:2:1), select individuals with uniform phenotypes and repeat the crossing.
- Usually, after continuing inbreeding for 6 to 10 generations or more, homozygosity is fixed at most loci, resulting in an inbred line.
- Advantages: No equipment required and low cost.
- Disadvantages: Time-consuming (takes years). Inbreeding depression is likely to occur, which may reduce vigor.
- Application examples: Widely used for crops such as corn and wheat. Like Mendel's pea experiments, selection is based on phenotypic observation.
2. Methods utilizing molecular markers and genetic analysis
- Overview: Use modern biotechnology to directly analyze the genotype at the DNA level to select and cross homozygous individuals. Efficiency is improved using equipment (PCR machines, sequencers, etc.).
- Main techniques:
- Marker-Assisted Selection (MAS): Use molecular markers (SSR, SNP, etc.) linked to the target gene to confirm homozygosity at the seedling stage. Eliminate heterozygous individuals and cross only homozygous ones.
- Genomic Selection: Analyze the entire genome using next-generation sequencing (NGS) and predict/select for homozygosity using statistical models.
- Gene editing such as CRISPR: Directly edit genes to be homozygous, though this is more of a modification technology than a seed collection method.
- Advantages: Fast and accurate (shortens generations). Can handle hidden traits that are difficult to distinguish by phenotype alone.
- Disadvantages: Requires expensive equipment and specialized knowledge. Markers may not be established for all crops.
- Application examples: Used in rice and tomato breeding, with SNP analysis performed using genetic analysis equipment (e.g., Illumina sequencers).
3. Doubled Haploid (DH) technology
- Overview: Induce haploid plants from pollen or ovules and double them (e.g., colchicine treatment) to immediately obtain homozygous plants. An innovative method that shortens inbreeding.
- Example of procedure:
- Generate haploids through pollen culture or gynogenesis.
- Double the chromosomes to convert them into homozygous diploids.
- Seeds collected from these will all be homozygous.
- Advantages: Pure lines can be obtained in 1 to 2 generations. Fixes random homozygosity.
- Disadvantages: Success rate is low depending on the crop, and tissue culture technology is required.
- Application examples: Commercialized in corn and tobacco.
These methods are combined depending on the target crop species (self-pollinating or cross-pollinating) and available resources. While traditional inbreeding is the foundation, genetic analysis has become mainstream in recent years, and homozygous lines are important as parent lines for creating hybrid varieties (F1). Details vary by crop, so more accurate advice can be provided if you specify the plant name.