4 Discussion
4.1The genetically distinct clusters from Western to Eastern Siberia
correspond well to the main larch species
The GBS resulted in a total of 14,565 variable SNPs among the 148
individuals, which is a high number in comparison to other studies with
Pinaceae (e.g. Johnson et al. 2017; Eckert et al. 2010; Chen et al.
2013). With GBS the complexity of the large genome of Larix was
efficiently reduced for downstream analyses (Dong et al. 2019; Deschamps
et al. 2012).
The cluster analysis based on all SNPs identified four distinct genetic
clusters. The boundaries of the genetically inferred main clusters
correspond well to the boundaries of the main Siberian larch species
described by Abaimov (2010). As expected, the Western Siberian cluster
(cluster 1) can be assigned to L. sibirica and corresponds to its
distribution across the territory between the Ob and Yenisey Rivers
(Abaimov 2010). The easternmost site belonging to this cluster is
located at Lake Khamra (59.97° N, 112.96° E). The Western Yakutian
cluster (cluster 2) shows the distribution area of L. gmelinii ,
which extends from the basin of the Pyasina River in the west to the
Lena River delta in the east. The five tree individuals from the Taymyr
Peninsula region are the westernmost trees in this L. gmeliniicluster in our study. According to Abaimov (2010), the northernmostL. gmelinii forests are located on the Taymyr Peninsula. The
Eastern Yakutian cluster (cluster 3) is attributable to L.
cajanderi , whose western border habitat goes along the Lena River.
However, in the discussion about the taxonomic status of L.
cajanderi , there are different views on where to place the boundary
between L. gmelinii and L. cajanderi . Some authors
classify L. cajanderi Mayr as a synonym of L. gmelinii(Rupr.) Kuzen (Borsch et al. 2020). Dylis (1961), who divided L.
dahurica into a western and an eastern race, drew a dividing line
between them at 120–123° E longitude, which Bobrov (1972) later
described as two species L. gmelinii and L. cajanderi .
This also coincides with the transition zone between our separate
Western Yakutian and Eastern Yakutian clusters and supports the
differentiation of 4 rather than 3 clusters in our study. Further
evidence for a separation has been found by molecular studies using both
mitochondrial and chloroplast markers, which also reveal weak but
visible genetic differentiation between L. gmelinii and L.
cajanderi (Polezhaeva et al. 2010). In summary, we argue that the
genetic differences between L. gmelinii and L. cajanderiare very small but existent.
Particularly striking are the topographical conditions such as mountain
ranges or river valleys along the cluster and species boundaries,
especially as the positions of the samples were not considered in the
cluster inference. The border between the West Siberian cluster (cluster
1) and the Western Yakutian cluster (cluster 2) corresponds almost
exactly with the course of the Yenisey River. Nevertheless, it can be
assumed that at this point the Central Siberian Plateau serves more as a
geographical barrier which is responsible for the genetic
differentiation between L. sibirica and L. gmelinii .
Similarly, the Verkhoyansk Range likely serves as a geographical barrier
between the Western Yakutian cluster (cluster 2) and the Eastern
Yakutian cluster (cluster 3) rather than the Lena River and is thus
responsible for the separation between L. gmelinii and L.
cajanderi . For a lowland species like larch, the mountains present a
persistent geographic barrier to expansion. In addition to long-term
geographical isolation, mountains can increase climatic gradients and
thus further facilitate population differentiation through environmental
isolation (Li et al. 2019). There may be gene flow between adjacent
watersheds, mountain ranges, or valleys, but less than between
populations that are not separated by landscape components (Priyadarshan
and Schnell 2012). The separation could be responsible for keeping two
populations genetically isolated so that they slowly diverge genetically
from each other (Rowe et al. 2017). A study on the effects of the last
glacial period on genetic diversity and genetic differentiation inCryptomeria japonica in East Asia (Tsumura et al. 2020) concludes
that geographical barriers between populations are one of the strongest
drivers for genetic variety.
The individuals in the core area of the clusters have an affiliation to
exclusively one cluster, but at the periphery, individuals have a
fragmented affiliation to different clusters. In the contact zone of
their geographic ranges, we found hybrids from the L. gmeliniiand L . cajanderi clusters forming a stripe of transitional
populations on the left bank of the Lena River (Abaimov 2010; Isaev et
al. 2010). These findings meet expectations, since cross-pollination of
two different plant varieties is taking place in these transitional
areas (Baltunis et al.1998). Unfortunately, no tree genetic samples from
the mountainous region of Krasnoyarsk Krai between cluster 1 and cluster
2 were available in this study. However, it can be assumed that the
species in the contact zone between the eastern limit of L.
sibirica and the western limit of L. gmelinii are involved in
natural hybridization (Abaimov 2010; Isaev et al. 2010). Szafer (1913)
described this stripe of hybrid populations as L. czekanowskii .
From the observation of genetic clusters and hybrid zones, the modern
and historical genetic influences can be deduced. Populations that have
evolved independently due to isolation by geographical barriers or
because they retreated into refugia during glacial periods met again
when physical barriers disappeared or when the populations expanded
again during warm periods (Abaimov 2010; Isaev et al. 2010).
Noticeable is the significant geographical distinction of the Chukotka
cluster (cluster 4), which can be considered as a variant of L.
cajanderi . The intraspecific genetic deviations between cluster 3
(Western Yakutia) and cluster 4 (Chukotka) could stem from separation by
the Chersky Range and the Kolyma Mountains, where varied microclimates
could be present at different elevations, which could have led to
differentiation due to regional climatic differences. Genetic
differentiation can also increase with geographical distance even when
there are no apparent barriers to gene flow. Since very little gene flow
occurs between the L. cajanderi population in Chukotka and the
main distribution area of the species, then during the glacial and
inter-glacial cycles, these Chukotka populations could have diverged
genetically due to gene drift, a process called isolation by distance
(Rousset 1999). The genetic variability of L. cajanderi is also
supported by observations showing that L. cajanderi is involved
in hybridization with L. kamschatica and other hybrid forms ofLarix species in the far northeast (Abaimov 2010).
4.2 Refugia likely assisted the invasion of northern areas of the
forest-tundra ecotone