Define pureline. Discuss the relevance of pureline selection in self- and cross-pollinated crops.

Defining Pureline

A pureline, as defined by Johannsen, is the progeny of a single, self-fertilized homozygous individual. It represents a genetically uniform population, meaning all individuals within the line share the same genotype. Johannsen’s experiments with beans demonstrated that even within a visually uniform population, there existed genetic variation. He observed that repeated selfing led to the isolation of distinct, genetically pure lines, each with its own characteristic performance. The key principle is that selection within a pureline does *not* alter the mean performance, as there is no genetic variation for selection to act upon. Any observed changes are due to environmental influences.

Pureline Selection in Self-Pollinated Crops

Self-pollinated crops, such as wheat, rice, barley, and peas, are naturally inclined towards homozygosity due to their reproductive biology. This makes them ideal candidates for pureline selection. The process involves:

• Selection of Superior Plants: Identifying plants with desirable traits within a mixed population.
• Selfing: Repeatedly self-pollinating the selected plants over several generations (typically 5-7) to achieve homozygosity.
• Evaluation: Assessing the performance of the resulting purelines in replicated field trials.
• Release: Selecting the best-performing pureline for release as a new variety.

The advantage of pureline selection in self-pollinated crops is its simplicity and effectiveness in establishing genetically uniform varieties. However, it’s limited by the lack of genetic variability within the initial population. Once a pureline is established, further selection will not lead to improvement. For example, the development of high-yielding wheat varieties during the Green Revolution heavily relied on pureline selection techniques.

Pureline Selection in Cross-Pollinated Crops

Cross-pollinated crops, like maize, sorghum, and sunflower, exhibit high levels of genetic diversity due to their reliance on outcrossing. Applying pureline selection to these crops is more challenging and less effective than in self-pollinated crops. This is because:

• Maintaining Homozygosity: Achieving and maintaining homozygosity is difficult due to the constant influx of pollen from other plants.
• Limited Response to Selection: The high genetic diversity means that even after several generations of selfing, the resulting lines may still exhibit considerable variation.
• Inbreeding Depression: Repeated selfing can lead to inbreeding depression, reducing vigor and yield.

In cross-pollinated crops, pureline selection is often used as a preliminary step in developing inbred lines. These inbred lines are then crossed to create hybrid varieties, which benefit from heterosis (hybrid vigor). The process involves:

• Creating Inbred Lines: Repeated selfing of selected plants to create inbred lines.
• Evaluation of Inbred Lines: Assessing the performance of the inbred lines.
• Hybridization: Crossing two superior inbred lines to produce a hybrid.
• Evaluation of Hybrids: Evaluating the performance of the hybrid in field trials.

For instance, in maize breeding, pureline selection is used to develop inbred lines, which are then crossed to produce high-yielding hybrid varieties. The success of hybrid maize relies on the complementary traits of the inbred lines and the resulting heterosis effect.

Comparison of Pureline Selection in Self- and Cross-Pollinated Crops

Feature	Self-Pollinated Crops	Cross-Pollinated Crops
Genetic Diversity	Low	High
Ease of Achieving Homozygosity	Easy	Difficult
Effectiveness of Selection	High	Low (used primarily for inbred line development)
Inbreeding Depression	Minimal	Significant
End Product	Pureline Variety	Inbred Lines (for hybrid production)