125
Standardized genetic differentiation between the species was also higher for nSSRs
(G’ ST = 0.335) with respect to cpDNA (G’ ST = 0.061) data.
A latitudinal trend was revealed in both species, which is probably related to the
impact of past glaciations. Higher allelic richness and gene diversity values were
found in N. pumilio populations located around 37°–42° S (populations 2, 3, 4, 6,
and 8), whereas at about 40°–43° S for N. antarctica (V, 00, VIII, and IX). A
decrease in A R and H E toward southern populations (southward 42° S) was more
evident in N. antarctica than in N. pumilio, in agreement with previous results based
on cpDNA data (Soliani et al. 2012). Genetic differentiation was significant in both
taxa and slightly higher in N. pumilio (F ST = 0.094, p = 0.001) than in N. antarctica
(F ST = 0.083, p = 0.001). The standardized differentiation (G’ ST ) was higher but
similar in both species (G’ ST = 0.296 and G’ ST = 0.303, respectively).
A clear separation between both species was found with a Bayesian clustering
analysis (STRUCTURE; Pritchard et al. 2000) of the 41 populations (ΔK = 2 indicated the optimal number of groups) (Fig. 5.3). Some populations presented a high
level of admixture, suggesting hybridization and even introgression through backcrosses, e.g., populations 8 (42°59′ S, 71°11′ W) and 9 (43°04′ S, 71°35′ W) in
N. pumilio and X (43°51′ S, 71°33′ W), IX (43°04′ S, 71°34′ W), XII (43°50′ S,
70°45′ W), and XIII (44°51′ S, 71°38′ W) in N. antarctica. Evidence of interspecific
gene flow was also observed when cluster partitioning increased (e.g., at K = 4
and K = 5).
Through a coalescent model, the putative origin of population genetic variation
of each species at intermediate latitudes was inferred (approximate Bayesian computation (ABC) in DIYABC v1.0.4.39; Cornuet et al. 2008, 2010). Species divergence was estimated around 302,500 years BP. Then, a recent species admixture
could have occurred ~18,950 years BP (50 years/generation time) across individuals from the 8 (42° S) and 9 (43° S) populations (ABC1, scenario 1; Fig. 5.3).
Although hybridization could have determined N. pumilio population variation, it
seems not to be the most probable explanation of the N. antarctica sympatric populations. In this species, divergence from an ancestral population to around 26,700
BP could have been the origin of the current variation (ABC2, scenario 4; Fig. 5.3).
The settlement of a hybrid zone at intermediate latitudes (42° ~ 43° S, 8–9 and
VIII–IX populations) could have been facilitated by niche overlapping and a
Table 5.2 Average values of within-population gene diversity (h s ), total genetic diversity (h t ),
and gene differentiation in all populations for unordered alleles (G st ) and for ordered alleles (N ST ),
for the analyzed populations of each species
Genetic parameters
Nothofagus pumilio
Nothofagus antarctica
(h s ) (s.d.)
0.424 (0.0460)
0.488 (0.0506)
(h t ) (s.d.)
0.645 (0.0865)
0.761 (0.0536)
G ST (s.d.)
0.344 (0.0589)
0.359 (0.0786)
N ST (s.d.)
0.885 (0.0217)*
0.841 (0.0318)*
From Soliani et al. (2012)
s.d. standard deviation, * significant test, evidence of phylogeographic structure
5 Nothofagus pumilio and N. antarctica: The Most Widely Distributed…
Standardized genetic differentiation between the species was also higher for nSSRs
(G’ ST = 0.335) with respect to cpDNA (G’ ST = 0.061) data.
A latitudinal trend was revealed in both species, which is probably related to the
impact of past glaciations. Higher allelic richness and gene diversity values were
found in N. pumilio populations located around 37°–42° S (populations 2, 3, 4, 6,
and 8), whereas at about 40°–43° S for N. antarctica (V, 00, VIII, and IX). A
decrease in A R and H E toward southern populations (southward 42° S) was more
evident in N. antarctica than in N. pumilio, in agreement with previous results based
on cpDNA data (Soliani et al. 2012). Genetic differentiation was significant in both
taxa and slightly higher in N. pumilio (F ST = 0.094, p = 0.001) than in N. antarctica
(F ST = 0.083, p = 0.001). The standardized differentiation (G’ ST ) was higher but
similar in both species (G’ ST = 0.296 and G’ ST = 0.303, respectively).
A clear separation between both species was found with a Bayesian clustering
analysis (STRUCTURE; Pritchard et al. 2000) of the 41 populations (ΔK = 2 indicated the optimal number of groups) (Fig. 5.3). Some populations presented a high
level of admixture, suggesting hybridization and even introgression through backcrosses, e.g., populations 8 (42°59′ S, 71°11′ W) and 9 (43°04′ S, 71°35′ W) in
N. pumilio and X (43°51′ S, 71°33′ W), IX (43°04′ S, 71°34′ W), XII (43°50′ S,
70°45′ W), and XIII (44°51′ S, 71°38′ W) in N. antarctica. Evidence of interspecific
gene flow was also observed when cluster partitioning increased (e.g., at K = 4
and K = 5).
Through a coalescent model, the putative origin of population genetic variation
of each species at intermediate latitudes was inferred (approximate Bayesian computation (ABC) in DIYABC v1.0.4.39; Cornuet et al. 2008, 2010). Species divergence was estimated around 302,500 years BP. Then, a recent species admixture
could have occurred ~18,950 years BP (50 years/generation time) across individuals from the 8 (42° S) and 9 (43° S) populations (ABC1, scenario 1; Fig. 5.3).
Although hybridization could have determined N. pumilio population variation, it
seems not to be the most probable explanation of the N. antarctica sympatric populations. In this species, divergence from an ancestral population to around 26,700
BP could have been the origin of the current variation (ABC2, scenario 4; Fig. 5.3).
The settlement of a hybrid zone at intermediate latitudes (42° ~ 43° S, 8–9 and
VIII–IX populations) could have been facilitated by niche overlapping and a
Table 5.2 Average values of within-population gene diversity (h s ), total genetic diversity (h t ),
and gene differentiation in all populations for unordered alleles (G st ) and for ordered alleles (N ST ),
for the analyzed populations of each species
Genetic parameters
Nothofagus pumilio
Nothofagus antarctica
(h s ) (s.d.)
0.424 (0.0460)
0.488 (0.0506)
(h t ) (s.d.)
0.645 (0.0865)
0.761 (0.0536)
G ST (s.d.)
0.344 (0.0589)
0.359 (0.0786)
N ST (s.d.)
0.885 (0.0217)*
0.841 (0.0318)*
From Soliani et al. (2012)
s.d. standard deviation, * significant test, evidence of phylogeographic structure
5 Nothofagus pumilio and N. antarctica: The Most Widely Distributed…