Paula Marchelli

The effects of Quaternary glaciations in Patagonia as evidenced by chloroplast DNA phylogeography of Southern beech Nothofagus obliqua

Climatic oscillations during the Quaternary strongly affected the distribution of warm-temperate tree species, which experienced local restrictions into ice-free areas and posterior expansions. To evaluate the impact of these range movements on the genetic structure of populations, we performed a phylogeographical analysis of the species Nothofagus obliqua with chloroplast DNA markers. A total of 27 populations covering the whole natural distribution range were analyzed using polymerase chain reaction-restriction fragment length polymorphism. Diversity (hT, hS), allelic richness (rg), and differentiation among populations for unordered (GST) and ordered alleles (NST) were calculated. The relationships among haplotypes were evaluated by the construction of a minimum spanning network. The spatial distribution of the genetic variation was analyzed through a Mantel test and with a nested analysis of molecular variance to differentiate between geographic regions. The screening of 11 non-coding regions allowed the identification of 14 haplotypes. A high genetic differentiation was detected (NST = 0.875 and GST = 0.824) with the existence of phylogeographic structure (p < 0.05). The distribution of the genetic variation was partially explained by the topography of the region when the populations were divided longitudinally into Coastal Mountains, Central Valley, and Andes Mountains (ϕRT = 0.093, p = 0.001). In agreement with pollen records, our results support the hypothesis of Coastal refuges since the region harbors high diversity together with older and private haplotypes. Long-lasting persistence of some Coastal populations without contribution to re-colonization is proposed. Additional refuges are also postulated along the Andes and Longitudinal Valley. Survival in multiple glacial refuges is discussed together with the possible migratory routes.

High genetic variation in marginal fragmented populations at extreme climatic conditions of the Patagonian Cypress Austrocedrus chilensis

Knowledge about current patterns of genetic structure of populations together with the evolutionary history of a species helps to understand and predict the adaptation of populations to future climate change. We assayed variation at nuclear microsatellite markers among peripheral vs. continuous populations of the temperate South American species Austrocedrus chilensis, to investigate the role of historical vs. demographical forces in shaping population genetic structure. This species occurs in continuous populations in the west and central distribution range, but becomes highly fragmented at the eastern limit, which comprised ice-free areas during Quaternary glaciations and has extreme climatic conditions at present times. Bayesian analysis methods identified two contrasting patterns of genetic structure; (I) populations from humid, mesic and peri-glacial regions formed a single deme with relatively low genetic differentiation and high admixture levels whereas (II) a highly heterogeneous genetic structure with low level of admixture was found in the steppe, towards the east and northeast limit of the distribution range. In the steppe, population fragmentation, restricted gene flow and isolation-by-distance were also inferred. In addition, several small steppe populations showed high genetic diversity and divergent gene pools, suggesting that they constitute ancient refuges from pre-Holocene glaciations with just a subgroup of them contributing significantly to post-glacial spread. These results are discussed in relation to patterns of genetic variation found for other temperate species and the contribution of the particular southern Andes topography and climate to post-glacial spread.

The effect of different glaciation patterns over the current genetic structure of the southern beech Nothofagus antarctica.

Different regional patterns of glaciation are expected to have brought about a differential effect on the present genetic structure of natural tree populations in the temperate regions. The aim of the present study is to test this hypothesis in Nothofagus antarctica, a key tree species of the temperate forests of southern South America. An almost continuous ice layer characterized the region of the Andes south of 41°S, while towards northern latitudes the pattern was more fragmented. Therefore, a higher chance for the location of larger or more numerous glacial refuges in the north of the Argentinean range, leads us to predict a higher genetic diversity in this region. Twelve natural populations of N. antarctica were sampled along the northern half of its Argentinean range, including six above 41°S and six below that latitude. Sampled populations were genetically characterized through cpDNA and isozyme gene markers. Both groups of populations were compared by means of several diversity and differentiation parameters. A genetic structure analysis was conducted with isozyme data through clustering and Bayesian approaches. Based on three polymorphic chloroplast regions, only two haplotypes were distinguished, one corresponding to the nine northernmost sampled populations and the other to the two southernmost ones. Only the population located between those two groups resulted polymorphic. AMOVA analyses also revealed a latitudinal genetic structure for the populations surveyed, and higher levels of genetic variation were recognized in the northern populations.

The effect of volcanism on postglacial migration and seed dispersal. A case study in southern South America

During the Quaternary, southern South American temperate forests were confined to small and isolated refugia. Recolonization could be related not only with location of refugia but also with postglacial phenomena like volcanism, which could have interrupted the expansion of the forests. The aim of this study was to analyze the local effect of volcanism during the postglacial migration of Nothofagus nervosa in a particular region of Argentina were convergence of two migratory routes was suggested. The main question is whether admixture occurred or not and if the current populations are connected by pollen or seed gene flow. Two populations separated by a 3-km-width lava flow were sampled. Buds from 30 individuals of each of the two populations and from a total of 142 juveniles were analyzed. Genetic variation was detected through maternally inherited chloroplast deoxyribonucleic acid (cpDNA; polymerase chain reaction restriction fragment length polymorphisms of two fragments) and nuclear markers like isozymes (six loci) and simple sequence repeats (three loci). Population genetic parameters were estimated and the existence of a genetic structure was tested with an analysis of molecular variance. Historical gene flow was estimated through the indirect method of the genetic differentiation (FST). Chloroplast DNA revealed a total genetic differentiation between the two populations indicating completely isolation respecting seed gene flow. On the contrary, the degree of genetic differentiation for the nuclear markers was significantly lower, and moderate levels of historical gene flow through pollen were inferred. The results suggest that in this area, volcanism has played an important local role during the expansion of N. nervosa maintaining these two populations separated.

El uso de marcadores genéticos en el género Nothofagus con especial referencia a raulí y roble

En trabajos de investigacion en genetica de poblaciones y evolutiva asi como en el desarrollo tecnologico de los programas de mejora, la evaluacion correcta de la variacion genetica es el punto crucial para la conclusion acertada y la toma de decisiones. El uso de marcadores geneticos bioquimicos y moleculares en especies forestales permitio un salto cualitativo en la confiabilidad de la informacion sobre la variacion genetica y la resolucion en forma contundente de problemas de importancia practica tanto en genetica poblacional como en programas de mejora. En especies del genero Nothofagus este tipo de marcadores comenzo a utilizarse hace aproximadamente 20 anos, aunque en especies nativas de los bosques templados de Sudamerica la primera referencia data de diez anos atras. El presente trabajo describe los aportes que la genetica molecular ha hecho hasta la fecha en los estudios de variacion genetica del genero Nothofagus en general y de Nothofagus nervosa (rauli) y N. obliqua (roble, roble pellin) en particular.