281
Initially, the genetic breeding strategy for P. alba was based on an adjustment of the
“multiple populations” method proposed by Namkoong (1980), where genetic
improvement and conservation of genetic resources are combined. This method
consists of creating a base population conformed by the progeny of individuals from
natural populations. Each base population is composed by subpopulations isolated
from each other and located in different environments. These subpopulations are
used as progeny trials and subsequently, after evaluation and selection, as seed
orchards. In this way, each subpopulation is improved separately and, at the same
time, passes through a process of differentiation from the others (due to the adaptation to the different environments). This allows for future recovering of genetic
variability in stands arising by the mixture of seeds of the different subpopulations
(Verga 2005).
Provenance and progeny trials of Prosopis in Argentina have been implemented
since 1990 (Cony 1996; Felker et al. 2001; López Lauenstein et al. 2016), allowing
the study of population performance in different environments and the estimation of
genetic parameters related to traits such as height, diameter, stem shape, and growing rate. Progeny trials are a powerful tool in forest breeding programs (Zobel and
Talbert 1984). Through the estimation of breeding values, family and/or individual
rankings are made in order to carry out backward or forward selection. These progeny trials are then thinned according to the rankings (i.e., forward selection), to
become seed orchards (Ruotsalainen and Lindgren 1998). Currently, P. alba breeding program in Argentina includes a network of three progeny trials established in
2008 (INTA net): Laguna Yema (24° 19′ 15.5″ S; 61° 17′ 31.7″ W; 161 m asl),
Santiago del Estero (27° 56′ 45.1″ S; 64° 13′ 12.5″ W; 172 m asl), and Plaza (26°
56′ 3.6″ S; 59° 46′ 22.3″ W; 78 m asl). These trials include 217 open-pollinated
families from ten different provenances, which cover a large part of the natural
range of the species in the Argentine Chaco. This base population come from seeds
collected during successive field campaigns between 2004 and 2007 (Verga et al.
2009) from phenotypically selected individuals (mother trees) in wild populations.
In this first phase of the program, the base population was constituted on the
basis of the specific purity without considering differences by geographical origin.
This decision was based on previous genetic studies in P. chilensis and P. flexuosa
from the Arid Chaco region, where the main source of genetic variation was shown
to come from hybridization processes (Verga 1995, 2005). In this species complex,
there are no significant differences between populations that grow in dissimilar
environments compared to the enormous variation within them as an effect of interspecific crosses and the presence of interspecific hybrids (Verga 1995). More
recently, however, and based on leaf trait analysis and variation detected with
molecular markers, three morphological groups within P. alba were determined,
which could be considered subspecific taxonomic groups: P. alba “chaqueño,”
P. alba “santiagueño,” and P. alba “salteño” (Verga et al. 2009, Verga 2014; Chap.
9). These groups show a separate geographic distribution and from their identification and evaluation through provenance trials, the breeding strategy was rethought.
The new strategy considers dividing the current base populations into each subspecific taxonomic group and provenance. The objective is to advance in the installation
10 Genetic Breeding of Prosopis Species from the “Great American Chaco”
Initially, the genetic breeding strategy for P. alba was based on an adjustment of the
“multiple populations” method proposed by Namkoong (1980), where genetic
improvement and conservation of genetic resources are combined. This method
consists of creating a base population conformed by the progeny of individuals from
natural populations. Each base population is composed by subpopulations isolated
from each other and located in different environments. These subpopulations are
used as progeny trials and subsequently, after evaluation and selection, as seed
orchards. In this way, each subpopulation is improved separately and, at the same
time, passes through a process of differentiation from the others (due to the adaptation to the different environments). This allows for future recovering of genetic
variability in stands arising by the mixture of seeds of the different subpopulations
(Verga 2005).
Provenance and progeny trials of Prosopis in Argentina have been implemented
since 1990 (Cony 1996; Felker et al. 2001; López Lauenstein et al. 2016), allowing
the study of population performance in different environments and the estimation of
genetic parameters related to traits such as height, diameter, stem shape, and growing rate. Progeny trials are a powerful tool in forest breeding programs (Zobel and
Talbert 1984). Through the estimation of breeding values, family and/or individual
rankings are made in order to carry out backward or forward selection. These progeny trials are then thinned according to the rankings (i.e., forward selection), to
become seed orchards (Ruotsalainen and Lindgren 1998). Currently, P. alba breeding program in Argentina includes a network of three progeny trials established in
2008 (INTA net): Laguna Yema (24° 19′ 15.5″ S; 61° 17′ 31.7″ W; 161 m asl),
Santiago del Estero (27° 56′ 45.1″ S; 64° 13′ 12.5″ W; 172 m asl), and Plaza (26°
56′ 3.6″ S; 59° 46′ 22.3″ W; 78 m asl). These trials include 217 open-pollinated
families from ten different provenances, which cover a large part of the natural
range of the species in the Argentine Chaco. This base population come from seeds
collected during successive field campaigns between 2004 and 2007 (Verga et al.
2009) from phenotypically selected individuals (mother trees) in wild populations.
In this first phase of the program, the base population was constituted on the
basis of the specific purity without considering differences by geographical origin.
This decision was based on previous genetic studies in P. chilensis and P. flexuosa
from the Arid Chaco region, where the main source of genetic variation was shown
to come from hybridization processes (Verga 1995, 2005). In this species complex,
there are no significant differences between populations that grow in dissimilar
environments compared to the enormous variation within them as an effect of interspecific crosses and the presence of interspecific hybrids (Verga 1995). More
recently, however, and based on leaf trait analysis and variation detected with
molecular markers, three morphological groups within P. alba were determined,
which could be considered subspecific taxonomic groups: P. alba “chaqueño,”
P. alba “santiagueño,” and P. alba “salteño” (Verga et al. 2009, Verga 2014; Chap.
9). These groups show a separate geographic distribution and from their identification and evaluation through provenance trials, the breeding strategy was rethought.
The new strategy considers dividing the current base populations into each subspecific taxonomic group and provenance. The objective is to advance in the installation
10 Genetic Breeding of Prosopis Species from the “Great American Chaco”