Agriculture Reference
In-Depth Information
III. ADVANCES IN POPULATION IMPROVEMENT IN LATIN
AMERICA AND THE CARIBBEAN
Since the late 1990s, various reports have mentioned an alarming reduc-
tion in genetic diversity among the germplasm used for rice breeding in
LAC (Cuevas-Pérez et al. 1995; Guimarães et al. 1995; Acevedo et al. 2007;
Ghneim-H et al. 2008). These reports raised interest among the rice
breeders
community in developing methodological approaches allowing
breeders to diversify the germplasm used for developing improved
cultivars. Prebreeding activities based on the use of recurrent selection
(RS) were considered to increase genetic variability in breeding programs.
This method allows for continuous genetic gain for quantitative traits
often characterized by low heritability and controlled by many genes
(Hallauer et al. 2010). The principle of population improvement through
RS relies on enhancing the frequency of favorable alleles within a
population while maintaining genetic variability, to allow further genetic
progress. To avoid losing potentially valuable alleles from the population,
it is important to properly manage the effective population size (
Ne
)to
prevent genetic drift. An additional advantage presented by the method is
the framework it offers for both decentralized and centralized breeding
efforts, the former for site-speci
'
c population improvement and the latter
being more adapted for highly heritable traits.
Initially developed for allogamous plants, RS breeding is applied in rice
following two approaches. One method consists of performing controlled
recombination through hand-crossing (emasculation by hand and prefera-
bly using a partial diallel scheme) while the secondmethod uses a recessive
nuclear male-sterility (ms) gene to allow random recombination within the
population (Châtel and Guimarães 1997). This gene, used in rice, was
identi
ed in a mutant of IR36 and transferred to elite lines designated to
constitute a synthetic population (Singh and Ikehashi 1981). The principle
of RS relies on the evaluationof S
0
plants or S
1
progenies and recombination
of plants selectedwith the best performance for the trait of interest. Through
various cycles of RS, the population mean is improved while diversity is
maintained at other loci that are not under selection and novel allele
combinations emerge due to frequent recombination.
A. Population Improvement in Latin America and the Caribbean
Pioneer researchers from the Centre de coopération international en
recherché agronomique pour le développement (CIRAD), CIAT, and
Embrapa started the rice population improvement program in LAC in
the late 1980s. Two base populations, CNA-IRAT4 for lowland and
