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).