Classic Genetics and Breeding - Genetics, Genomics and Breeding of Crop Plants - page 60

Recurrent selection for yield is more complex, since it is necessary to
make hybrids in order to determine the combining ability of inbred lines.
Miller and Hammond (1985) used a system of reciprocal full-sib selection
and concluded that the population could be improved at the same time as
identifying interesting plants which could be selfed. However, the necessary
effort was large, compared with the amount of improvement obtained. The
use of tester genotypes simplifies operations and both general and specific
combining abilities can be estimated. The system used at INRA, Clermont-
Ferrand from 1980 to 2000 is given in Figure 2-3.

Population cycle n
x

tester cross

S 0

hybrid
trials S 1 self-pollination

Choice

Fixation inbred lines

Interpollination

BC1(CMS)

S2

x

Population
cycle n+1

S3

x

BC2

S4

x

BC3 tester

x

hybrid
trials

S5 BC4

x

Final Coice

Figure 2-3 Recurrent selection program (female population).

2.3.3 Pedigree Selection

This method is used to obtain fixed, homozygous lines from recurrent
selection programs, and to combine interesting characteristics from
complementary lines. In the latter case, crosses between two male fertile
lines are made either using emasculation by gibberellic acid (possible on a
small-scale only) or by crossing two male fertile plants and distinguishing
the F 1 hybrids from the selfed inbreds by their vigor. Plants are self-pollinated
at each generation until they show complete fixation, by covering capitula
with paper or cloth bags. Each progeny is followed separately and selection
can be practised at each generation. An example of this method is given in
Figure 2-4.
For the potential female lines, CMS must be introduced by backcrossing.
This takes six to seven generations. Since it involves considerable crossing,
it is usually started only after a test for combining ability has been completed.
It was suggested that this could be accelerated by in vitro culture of immature
embryos but generally additional generations are grown in the greenhouse
or out-of-season nurseries.