181
span of A. araucana with specimens that could reach more than 1000 years, relic
populations could be composed of ancient genotypes.
The analysis of chloroplast and mitochondrial DNA revealed a particular result:
a very low transferability of universal primers for amplifying both organelle intergenic spacer regions and chloroplast SSRs, since only 15 out of 49 markers gave a
reliable amplification product. The most likely explanation for the low transferability of universal primers is the occurrence of sequence divergence between the
younger taxa from which the primers were designed (mostly Pinus spp.) and the
evolutionary old A. araucana. Moreover, of these 15 markers, only one showed
polymorphism between populations. These low levels of genetic diversity were also
reported in other members of Araucariaceae. The extreme case was that of Wollemia
nobilis, which could represent the only living clone of an “extinct” species (Peakall
et al. 2003), but also low diversity was reported for Araucaria cunninghamii (Scott
et al. 2005), Agathis robusta and Agathis borneensis (Peakall et al. 2003), suggesting an evolutionary trend in the family. However, considerable levels of genetic
diversity were detected throughout A. araucana range when using nuclear markers
like SSRs, RAPDs or isozymes (Bekessy et al. 2002; Gallo et al. 2004a; Ruiz et al.
2007; Martín et al. 2014; see below), which could be related to a higher mutation
rate of the nuclear genome compared to the chloroplast (Wolfe et al. 1987).
In spite of the low levels of polymorphism, the variation detected in the chloroplast DNA allowed the identification of five haplotypes (Fig. 7.3a). As expected for
a paternally inherited plastid that moves with pollen grains, genetic differentiation
was very low (G″ ST = 0.267), as in other conifer species (e.g. Vendramin et al. 1998;
Schlögl et al. 2007). The low level of differentiation implies that gene flow via pollen in A. araucana might be extensive and therefore counterbalancing the divergence among populations (Gallo et al. 2004a).
Many of the sampled populations were located beyond the limits of the ice cap
(Hollin and Schilling 1981, Fig. 7.3a) and could therefore be considered as remnants
of pre-Holocene origin (i.e. before the Last Glacial Maximum that occurred c.
20,000 years BP). The genetic structure observed at the easternmost isolated and
marginal populations (reduced allelic richness and increased genetic differentiation;
Marchelli et al. 2010) is compatible with a long-lasting isolation and the effects of
stochastic processes on small populations. On the other hand, fragmented populations, meaning populations currently affected by fragmentation but still large and
connected with the continuous forests, hold the higher allelic richness (Marchelli
et al. 2010). Multiple refugia for the species were suggested by Bekessy et al. (2002)
based on variation detected with RAPDs among populations from Chile and
Argentina and by Ruiz et al. (2007) among Chilean populations using isozyme markers. In addition, Martín et al. (2014) using microsatellite markers reported four gene
pools among Chilean populations that clearly separated coastal from Andean populations. Thus, genetic drift due to isolation could have been counteracted by gene flow
via pollen and “frozen” genetic structures due to clonal persistence. Unfortunately, a
poor pollen representation of A. araucana which left hardly a trace (Kershaw et al.
1995) excludes genuine comparison of molecular and palaeobotanic data.
7 Araucaria araucana and Salix humboldtiana…
span of A. araucana with specimens that could reach more than 1000 years, relic
populations could be composed of ancient genotypes.
The analysis of chloroplast and mitochondrial DNA revealed a particular result:
a very low transferability of universal primers for amplifying both organelle intergenic spacer regions and chloroplast SSRs, since only 15 out of 49 markers gave a
reliable amplification product. The most likely explanation for the low transferability of universal primers is the occurrence of sequence divergence between the
younger taxa from which the primers were designed (mostly Pinus spp.) and the
evolutionary old A. araucana. Moreover, of these 15 markers, only one showed
polymorphism between populations. These low levels of genetic diversity were also
reported in other members of Araucariaceae. The extreme case was that of Wollemia
nobilis, which could represent the only living clone of an “extinct” species (Peakall
et al. 2003), but also low diversity was reported for Araucaria cunninghamii (Scott
et al. 2005), Agathis robusta and Agathis borneensis (Peakall et al. 2003), suggesting an evolutionary trend in the family. However, considerable levels of genetic
diversity were detected throughout A. araucana range when using nuclear markers
like SSRs, RAPDs or isozymes (Bekessy et al. 2002; Gallo et al. 2004a; Ruiz et al.
2007; Martín et al. 2014; see below), which could be related to a higher mutation
rate of the nuclear genome compared to the chloroplast (Wolfe et al. 1987).
In spite of the low levels of polymorphism, the variation detected in the chloroplast DNA allowed the identification of five haplotypes (Fig. 7.3a). As expected for
a paternally inherited plastid that moves with pollen grains, genetic differentiation
was very low (G″ ST = 0.267), as in other conifer species (e.g. Vendramin et al. 1998;
Schlögl et al. 2007). The low level of differentiation implies that gene flow via pollen in A. araucana might be extensive and therefore counterbalancing the divergence among populations (Gallo et al. 2004a).
Many of the sampled populations were located beyond the limits of the ice cap
(Hollin and Schilling 1981, Fig. 7.3a) and could therefore be considered as remnants
of pre-Holocene origin (i.e. before the Last Glacial Maximum that occurred c.
20,000 years BP). The genetic structure observed at the easternmost isolated and
marginal populations (reduced allelic richness and increased genetic differentiation;
Marchelli et al. 2010) is compatible with a long-lasting isolation and the effects of
stochastic processes on small populations. On the other hand, fragmented populations, meaning populations currently affected by fragmentation but still large and
connected with the continuous forests, hold the higher allelic richness (Marchelli
et al. 2010). Multiple refugia for the species were suggested by Bekessy et al. (2002)
based on variation detected with RAPDs among populations from Chile and
Argentina and by Ruiz et al. (2007) among Chilean populations using isozyme markers. In addition, Martín et al. (2014) using microsatellite markers reported four gene
pools among Chilean populations that clearly separated coastal from Andean populations. Thus, genetic drift due to isolation could have been counteracted by gene flow
via pollen and “frozen” genetic structures due to clonal persistence. Unfortunately, a
poor pollen representation of A. araucana which left hardly a trace (Kershaw et al.
1995) excludes genuine comparison of molecular and palaeobotanic data.
7 Araucaria araucana and Salix humboldtiana…