Genetic differentiation within Embothrium coccineum: two genetic groups separated since the early Pleistocene
Our result confirms previous studies in E. coccineum documenting the presence of two genetic groups, one in the northern and one in the southern part of the species distribution (Souto and Premoli, 2007; Vidal-Russell et al, 2011); yet, these datasets exhibit different levels of spatial resolution. Alloenzymes revealed only a weak geographic structure, with some northern and southern localities clustered together (Vidal-Russell et al, 2011). The authors explained this pattern by proposing a possible effect of glacial survival in multiple refugia followed by recolonization and / or the existence of actual gene flow blurring the trace of the historical geographic structure. Contrastingly, chloroplast markers revealed a clearly distinct southern genetic group characterized by a very low genetic variation (i.e., only a few haplotypes were observed in the South). In this last case, two hypotheses could explain this pattern: 1) the existence of both northern and southern glacial refugia and a recent colonization from the southern to the northern part of the country or 2) a unique refugium in the north and a recent recolonization to the south (Souto and Premoli, 2007). Nonetheless, due to the lack of marker resolution, no robust evidence is provided to support the conclusions of both studies. Contrastingly, given the higher resolution provided by genomic SNPs, the present study not only unambiguously supports the existence of the two proposed genetic groups, but also shows that individuals from the center part of the species distribution are more genetically similar to those from the south. A divergence time of 2,8 My was estimated between the North and Center-South genetic groups, corresponding to the Late Pliocene and Early Pleistocene, while the divergence between populations within each North and Center-South group was much more recent (i.e., some 0.35 My ago). These results emphasize the possible role of the glacial cycles and associated environmental changes as a possible motor of divergence in E. coccineum . Our study provides then a refined picture of the probable divergence time between the North and Center-South genetic groups.
Pleistocene glaciations and associated climate change have dramatically altered the landscape of southern South America (Ramos and Ghiglione, 2008; Martínez and Kutscher, 2011; Ponce et al., 2011). The numerous glacial advances and retreats have impacted the abundance and distribution of the local biota (Rabassa et al., 2011). Most phylogeographic studies have shown that populations located at southern latitudes and impacted by glaciations have experienced a recent and rapid expansion after glacial retreat, whereas northern populations have remained more stable (Koszak et al., 2006; Fontanella et al., 2008). Indeed, various studies (e.g., Markgraf, 1983; Vidal et al., 2005) propose the existence of refugia north of the area heavily impacted by ice (i.e., ranging between 36°S - 40°S and 54°S). For example, the absence of some native plants pollen records as Eucryphia codifolia and Caldcluvia paniculata during the LGM at 41°S (Moreno, 1997; Heusser et al., 1999; Moreno et al., 1999; Moreno and León, 2003) and in Chiloé (Villagrán, 1988; Abarzúa et al., 2004; Heusser and Heusser, 2006) suggests a contraction north of the ice-sheet line for these species during glacial maxima. Furthermore, the presence of rare private haplotypes and the high levels of genetic diversity observed in populations located at 39 °S – 40 °S suggest the existence of northern glacial refugia and a southern expansion after glacial retreat in E. cordifolia (Segovia et al., 2012). However, Sérsic et al., (2011) reported genetic patterns strongly supporting the existence of in situ refugia located south of 42°S in both plants and animals. Comparative phylogeographic studies in southern South America (Bruno et al., 2016; Perez 2017; Sérsic et al., 2011) propose the existence of at least six areas where both terrestrial plant and animal species survived during glacial maxima. These areas show higher genetic diversity than areas where post-glacial colonization occurred, as in some parts of the southern Patagonian Steppe (Hewitt 2001; Pinceel et al., 2005; Huck et al., 2009).
In the case of E. coccineum , information based on cpDNA haplotypes suggested that southern populations of E. coccineumhad been affected by glacial cycles, declining gradually in size since the Pleistocene (100 ky - 12 ky ago), and then rapidly increasing after the LGM (Vidal-Rusell et al., 2011). Distribution of chloroplast haplotypes also provides evidence of local expansion in the north. The existence of various microrefugia located in Nahuelbuta (37°S - 72°W), Manzanar (38°S - 71°W), Alerce Costero (40°S - 73°W), Laguen Ñadi (41°S - 73°W) and Lago Verde (42°S - 71°W) was suggested by the authors. In our study, relatively high genetic diversity and percentage of polymorphic loci were found in Nahuelbuta, Curacautín, Chiloe Norte (nearby Laguen Ñadi), Coyhaique (nearby Lago Verde) and Torres del Paine, and the number of private alleles was also high in these same locations. These results, combined with the observed pattern of genetic structure, support the hypothesis of E. coccineum survival during glacial periods of the Pleistocene glacial cycles in various refugia, including at least two refugia located south of 42°S. The existence of southern local refugia in E. coccineum is also supported by fossils records of cold-tolerant plant species characteristic of the austral forest reported south of 42°S (Villagran et al., 1986; Markgraf 1995). High levels of population divergence and phylogeographic structure attributed to isolation in multiple refugia have been detected in other cold-tolerant native trees such as Fitzroya cupressoides(Allnutt et al., 1999; Premoli et al., 2000; Premoli et al., 2003),Pilgerodendron uviferum (Premoli et al., 2001; Premoli et al., 2002; Ausin et al., 2003), Araucaria araucana (Bekessy et al., 2002), Podocarpus nubigena (Quiroga and Premoli, 2010),Nothofagus alpina (Marchelli et al., 1998; Marchelli and Gallo, 2006), and Nothofagus pumilio (Premoli, 2004; Mathiasen and Premoli, 2010; Premoli et al., 2010).