Principles of Plant Genetics and Breeding

550 CHAPTER 34
Genetic gain
Corn
Soybean
Wheat
Resources
Cotton
Figure 1 A comparison of resource cost per unit genetic
gain between cotton and selected field crops.
Such changes have included: utilization of winter nurseries
(time savings); modified single-seed descent procedures
(cost and labor savings); single boll to F 3:4 progeny row
advancement (cost, labor, and time savings); F 4:5 progeny
row yield testing (higher selection pressure); and yield trial
technique changes (higher selection pressure). Additionally,
changes in harvest mechanization and data collection have
enabled better utilization of resources in the advanced
stages where development costs are much higher.
A key aspect addressed is the degree of outcrossing due
to pollinator activity. At Scott, the main breeding station,
very little outcrossing occurs and the plants are treated as
self-pollinated. In Costa Rica, a high degree of outcrossing
occurs, therefore selfing is required in the F 3 generation. A
single selfed boll is harvested from each plant.
For each population, two opportunities are given for
intensive selection: in the F 3:4 single boll-derived rows and
in the F 4:5 progeny rows. In the F 3:4 , intensive visual selection
is practiced, along with postharvest selection based on
lint percent (a component of lint yield) and fiber quality.
The F 4:5 progeny are planted in single rows that are replicated
twice. Visual preselection at harvest is followed by
harvest for lint yield, lint percent, and fiber quality. Values
for each row are compared against the average values of four surrounding check rows. Selected lines are advanced to replicated
tests and breeder increase (Table 1).
The testing advancement schedule includes breeder tests within region for 2 years and an advanced testing program that
involves uniform testing across all regions. Experimental lines advanced to the fourth year of testing (stage 4) are additionally
entered into independently conducted trials managed by the Technical Services Department.
Transgene introgression program
At the beginning of the second year of testing, conventional lines are crossed to transgene donor parents to begin backcross introgression.
Typically, three backcrosses are made prior to deriving lines homozygous for the desired transgene(s). Once F2:3 rows
Table 1 Cotton breeding program.
Year
1
1
2
2
3
4
5
6
7
Location
Scott, MS
Costa Rica
winter
Scott, MS
Costa Rica
Scott, MS
Scott, MS
Scott and
Winterville, MS
Three midsouth
locations
Advanced
Testing
Program
Generation
F0 F1 F 2
F 3
F 3:4
F 4:5
F 4:6
F 4:7
F 4:8
Procedure/nursery
Crosses made
Single row
Bulk population, F 2
yield trial
Bulk population
Progeny rows (7 ft)
Progeny rows (27 ft)
two replicates
Replicated tests
Replicated tests, begin
transgenic integration
Replicated tests
Harvest
Bulk
Modified bulk
(single lock from
300+ plants)
Single selfed boll
from 200 plants
Two plants per
selected row
Bulk row
Bulk harvest,
breeder increase
Bulk harvest,
breeder increase
Bulk harvest,
breeder increase
Selection
None
None
Light visual, some
populations discarded
Light visual
Heavy visual, fiber
quality
Heavy visual, lint yield,
fiber quality
Lint yield, fiber quality,
yield consistency
Lint yield, fiber quality,
yield consistency
Lint yield, fiber quality,
yield consistency