4 Discussion
To further understand the origin and evolution of mountain biodiversity,
we investigated the genetic diversity and distribution of a caddisfly
group in the Himalayas and the HM. For the first time in aquatic
insects, our results are derived from whole genome re-sequencing of
populations of four congeneric species and fine-grain environmental GIS
data. And, while we interpreted our results carefully, our study
provides evidence that changes in geodiversity and climate affect the
evolutionary history of closely related species occupying varying
ecological niches, thus supporting the MGH.
4.1 Genetic patterns of caddisfly in the Himalayas and the
HM
Present-day genetic diversity patterns offer important insights on the
historic interconnectivity of populations and can help inform the effect
of past environmental change on the studied species. Our results
indicate that in most of the cases, the high-elevation species of both
mountain regions (H. tibetana and H. gregoryi ) align best
with the headwater model pattern sensu Hughes et al. (2007). For
instance, populations of H. tibetana (high-elevation species,
Himalayas) in the West Gandaki (subbasin ID 4900) and Middle Gandaki (ID
4754) cluster together and share the same homogeneous ancestry (Fig. 3a,
b, c). Instead of being connected by a stream network, these populations
are connected by a series of local mountain ranges (including Tilitso
Himal, Anna Dakshin, Machhapuchhara and Lamjung Kailas), as defined in
the headwater model pattern. Similarly in H. gregoryi (HM):
populations in the Baima Snow Mountains (occupied in North Mekong (ID
2547) and West Yangtze (ID 1668 and 1685)) originated from an admixed
ancestral group and formed one haplogroup based on mtCOI (Fig. S22, Fig.
3g, h, i), in addition to a low level of inbreeding coefficient
(F = ~ 0.12), indicating a good connection within
this mountain range. Outside the Baima Snow Mountains, the H.
gregoryi populations located in the North and South Mekong River
(containing subbasin ID 2531, 2532, and 2544) were associated with a
stream hierarchy model since they show non-significant genetic variance
among each other. Nevertheless, the ancestral group of these populations
is distinct from others, suggesting limited gene flow between this group
and populations from other mountains. Therefore, the populations of
high-elevation species are generally restricted by local mountain ranges
and show a stronger “insularity” among the different mountain ranges,
or a “Sky Island” pattern. This pattern implies some degree of
overland dispersal among headwater streams. Caddisflies can disperse
overland during their adult stage, although the associated dispersal
distance seems to be restricted (Geismar et al., 2015). Nonetheless,
high-elevation species likely disperse along local mountain ranges
within or among basins, using suitable habitats (so-called “montane
archipelagos”) as stepping stones, thus leading to better connectivity
within these mountain ranges. Moreover, due to their strong association
with specific habitats, lowland areas, especially deep and large
valleys, may act as dispersal barriers for these species, thus leading
to limited gene flow among mountains (Myers et al. 2001; Pauls et al.
2006). In fact, freshwater organisms, in particular headwater
specialists, are often endemic to specific mountain ranges due to their
low tolerance to elevated temperature or other habitat factors
characterizing the surrounding lowland (Finn et al., 2007; Hughes et
al., 2009; Bálint et al., 2011; Vitecek et al., 2015, 2017).
In comparison, for the low-elevation species, a significant genetic
diversity was shared among populations across basins, thus providing
support for the stream hierarchy model pattern. It suggests gene flow
mostly occurred among populations from the same basins, indicating that
dispersal within the stream network appears to be the dominant process.
The divergence among dispersal strategies of high-elevation and
low-elevation species is evidenced by the pattern we observed where
populations of H. digitata and H. tibetana in the West and
Middle Gandaki (ID 4900, 4754): although populations of these two
species partly overlap, populations of the H. digitata appeared
as two distinct groups associated with the two subbasins, while
populations of H. tibetana grouped together (Fig. 3c, f). In
addition, the population connectivity of H. digitata reflects the
dendritic nature of the stream as described by Hughes et al. (2013):
minimal differentiation was observed for populations within the West
Gandaki subbasin (e.g., individuals from subbasin ID 4900, Fig. 3d, e,
f), higher differentiation among populations from different subbasins
(e.g., ID 4900 and 4754) and the highest among populations from
different primary basins (e.g., Gandaki and Koshi). The stream hierarchy
model pattern can also be observed in H. platon in the HM.
Several populations located in the Yangtze River (subbasin ID 1625,
1634, 1280, 1205) formed distinct groups, suggesting higher connectivity
among populations within subbasins (Fig. 3j, k, l). In addition, these
populations exhibit low genomic heterozygosity (π < 0.001) and
an extreme inbreeding coefficient (F = ~0.75 –
~0.1, Fig. 2d, h), indicating isolation and future
allopatric speciation. The rest of the populations of H. platonthat extend across all four primary basins (Irrawaddy, Salween, Mekong,
and Yangtze) demonstrate panmixia, accompanied by low genetic diversity
and low inbreeding coefficient (Fig. 2d, h). This pattern suggests that
all of these populations are highly connected and gene flow is strong
within all four primary basins until recent times, which may be due to
their very well-developed dispersal abilities as adults or continuous
habitats (Finn et al., 2007; Baggiano et al., 2011).
Our results highlight that identifying the prevalent model of genetic
structure for aquatic organisms may not always be clear-cut, especially
for taxa with a broad distribution (Hughes et al., 2013; Sproul et al.,
2014). In our case some species even appear to follow multiple patterns:
Stream hierarchy model patterns observed in the low-elevation species
could also be interpreted as isolation by distance since the genetic
diversity also increased with geographic distance (or “stream
distance”, the same as geographic distance in this case).
In line with previous studies that suggested that a general pattern of
population structure is challenging to identify for caddisflies (Lehrian
et al., 2009; Pauls et al., 2009; Engelhardt et al., 2011; Previšić et
al., 2014), our study also reveals that the patterns inHimalopsyche are likely to be species-specific: high-elevation
species are associated with a headwater model pattern, whereas the
low-elevation species tend to be more similar to stream hierarchy model
or panmixia patterns, indicating that habitat specialization is a
crucial factor for shaping the population structure. Further, species in
the HM show a better-connected gene flow within the mountain range than
species distributed in the Himalayas (panmixia appears in the HM),
implying that the effects of topography are strong for the dispersal of
species with an aquatic life history in this region. Overall, these
complex patterns may result from either the complexity of the dendritic
stream network embedded in a high-gradient landscape or simply the
geographic scale of the studies examined (Hughes et al., 2013).
4.2 Key abiotic factors in shaping the montane biodiversity
of
caddisflies
Ecological preferences and dispersal abilities cannot explain
present-day distribution patterns without interpreting the data in the
context of a species’ history (Avise 2000). In particular, topography
and historical climate changes may have a strong impact on the current
genetic diversity pattern of caddisflies. Our study, combined with the
current literature (e.g., Favre et al., 2015; Antonelli et al., 2018;
Mosbrugger et al., 2018; Muellner‐Riehl et al., 2019; Muellner-Riehl,
2019), suggests that the evolution of climate, mountain topography, as
well as historical drainage rearrangement, are key factors for the
evolution of aquatic biodiversity in the Tibeto-Himalayan Region. These
factors have interacted with each other through time and vastly
contributed to shaping current diversity patterns in these aquatic
insects.
4.2.1 Genetic patterns versus climate and topographical
features
The influence of climate on population diversity of caddisflies is
significant, especially in the Himalayan region. In line with other
organisms in previous studies (e.g., Wikramanayake et al., 2002; Price
et al., 2011; Yan et al., 2013; Rana et al., 2019), we find an increased
genetic diversity of our target caddisfly species from west to east in
the Himalayan region at the population level, especially for the
high-elevation species. There may be a link between this climatic
gradient and the genetic diversity of caddisflies across the whole
mountain region, leading to patterns associated with topography and
longitude (Bookhagen & Burbank, 2006, 2010): species richness increased
approximately threefold from the northwest to the east of the Himalayas,
especially at higher elevations, which is in accordance with the fact
that the lowland receives about three times more precipitation in the
subtropical East than the Northwest (Anders et al., 2006; Kreft & Jetz,
2007; Bookhagen & Burbank, 2010; Rana et al. 2019, 2022). This pattern
is strongly supported by studies on regional plant diversity (Yan et
al., 2013; Muellner-Riehl, 2019; Rana et al., 2022). Precipitation may
be the direct or indirect factor resulting in the current distribution
of caddisfly species (Collier & Smith, 1997). Furthermore, since both
larval and adult stages of caddisflies are reliant on flowing waters,
surface water availability from rainfall and snowmelt in the east of the
Himalayas may significantly benefit them (Bookhagen & Burbank, 2010).
Compared to the straightforward genetic diversity pattern of caddisflies
in the Himalayas, a common distribution pattern of genetic diversity is
difficult to identify for caddisflies in the HM. The complex terrain
characterizing the HM region offers highly heterogeneous microclimates,
making it difficult to evaluate the impacts of climate change in general
(Yin et al., 2020). Yet, a variety of genetic diversity patterns of
caddisflies were observed in the HM, which is strongly correlated with
topographic complexity, in particular those associated with elevation
and hydromorphology. Generally, species in the HM show better
connectivity all over the region, but there are plenty of allopatric
populations observed for both species. For the high-elevation species,
the main distribution pattern is the “Sky Island”, thus the geographic
barrier made by certain lower elevations. However, for the low-elevation
species, the mechanism of formation of the allopatric populations is
different. Unlike the alpine species, the low-elevation species
generally show better connectivity within basins, especially in H.
platon which shows near-panmixia across four primary basins.
Furthermore, the fact that both strongly connected (the upper Yangtze:
Jinsha River) and allopatric populations (the middle Yangtze: Yalong
River and Dadu River) occurred in the Yangtze Basin, may derive from
ancient drainage re-organizations (e.g., via a so-called “river
capture”, Fig. 3m). Firstly, the upper Yangtze River used to flow
southward as a tributary of the paleo-Red River until the late Eocene
(Clark et al., 2004; Zheng, 2015; He et al., 2021; Zheng et al., 2021),
which was not the case of its middle part (see Zhang et al., 2017).
Fuelled by regional orogeny and erosion (Zheng, 2015; Wang et al.,
2018), a so-called river capture event occurred, changing the course of
the upper Yangtze away from the paleo-Red River and into its current
course (Clark et al., 2004; Zheng, 2015; Deng et al., 2021a). Our
results reveal patterns consistent with an ancient disconnection:
populations located in the Yalong (ID 1634) and Dadu River (ID 1280 and
1205) showed a distinct genetic ancestry with any other populations with
any K values, including those located at the upper Yangtze River (ID
1625 and 1659). Secondly, although the populations located in the
Yanglong and Dadu River represented the same ancestral, they are
distinct clusters in population structure, indicating a recent
divergence. It is in line with the interpretation that the paleo-Yalong
and paleo-Dadu Rivers drained together with the middle Yangtze to the
paleo Xigeda lake before ca. 1.3 Ma, but segregate after ca. 1.3 Ma
since the paleo-Dadu River was integrated into the lower Yangtze
directly (Deng et al., 2021a). Until ca. 1.3 Ma, the middle Yangtze
captured the upper Yangtze and drained into the lower Yangtze drainage
basin (Kong et al., 2009; Deng et al., 2021a). Thus, populations ofH. platon potentially experience some gene flow along the
connected river networks, as shown in our TreeMix analysis: gene flow
has occurred among populations from the upper Yangtze (ID 1625) and
middle Yangtze (ID 1634). However, the genetic connection seems to be
very limited, since the populations located in the middle Yangtze showed
extremely high inbreeding coefficient.
Interestingly, the population of H. platon located in subbasin ID
1625 shares the same ancestral source with the near-panmictic
populations when K = 3. However, when increasing the value of K, it is
assigned to a unique ancestry and the near-panmictic populations are
inferred to have a certain level of ancestry from this unique population
with relatively high inbreeding (Fig. 3, Fig. 2). Notably, this
population is located at the middle of the First Bend of the Yangtze
River (as shown in Fig. 3m). The First Bend is the place where upper
Yangtze makes a dramatic turn to the North and starts flowing eastward
to the East China Sea (Zhang et al., 2017). It is regarded as the
capture point of the upper Yangtze away from the Red River, thus
resulting in a key period for geo(hydro)morphology in this area (Clark
et al., 2004; Li et al., 2022). The processes leading to the present-day
structure and diversity patterns remain unclear, however. Very limited
information is available on the genetic diversity patterns of plants or
animals here. It would be worthwhile to investigate species of this
specific region in future studies.
4.2.2 Distributional and demographic history of the four
caddisfly
species
Repeated climatic fluctuations in the Pleistocene promoted range
contractions and expansions of numerous plant and animal species, which
is believed to result in high diversification rates in the
Tibeto-Himalayan Region (Xing & Ree, 2017; Mosbrugger et al., 2018;
Ding et al., 2020). However, the effects of Pleistocene glacial cycles
on the distribution and demography of species may vary across regions
(Hewitt, 2000). For example, in addition to ecological preferences,
life-history traits, and dispersal ability (Tonzo & Ortego, 2021),
local topography and latitude likely have strong effects on rates of
dispersal, colonization, and extinction of a species during climate
fluctuations.
Our ensemble model results show that in comparison to the present,
suitable habitats for all four target species were more extensive during
the LGM but shifted from their current core distribution (Fig. 4).
Nevertheless, SDMs depict similar suitable habitats for species from the
same mountain region, but divergent patterns for species from the
Himalayas and the HM. In the Himalayas, the core potential habitat of
both species (at intermediate elevations), seems to have only shifted a
little between the LGM and today, indicating minor in-situvertical displacement along the elevational gradient (Fig. 4). Dispersal
between the East and the West of the range seems also to have been
possible, as indicated by the results of the ancestral components.
However, suitable habitats at the eastern and westernmost part of the
Himalayan range were likely only available during the LGM, indicating
potential local extinction or long-distance migration if species
occurred there before the LGM. In comparison, for both species in the
HM, a latitudinal displacement (from South at warm times to North at
colder times) and a vertical displacement (move up to a higher
elevation) occurred after the LGM, which was not the case in the
Himalayas. Latitudinal displacement was aided by the mountain ranges
long North-South extent (Fig. 5b), whereas the Himalayas are
latitudinally strongly constrained by the high plateau to the North and
the tropical lowland to the South (Fig. 5a).
In line with the SDM results, evidence of the population size history
based on genomic data showed that all species experienced an abrupt
demographic expansion during the LGM, followed by a population size
decline from the end of the LGM until recent times (Fig. 4). Previous
studies combining genetic data and ecological niche modelling have also
detected such demographic expansions during the LGM, particularly in
cold-tolerant/adapted species, for example in grasshoppers (Tonzo &
Ortego, 2021), Vipers (Yousefi et al., 2015), caribou (Taylor et al.,
2021), birds (Garg et al., 2020), and plants (Manthey et al., 2017). TheHimalopsyche species we investigated are cold-tolerant, and their
ability to disperse overland (to some extent) may have allowed them to
persist in situ regionally throughout cold snaps. In fact, this
was the case for many mountain caddisfly species in Central Europe
(Pauls et al., 2006; Kubow et al., 2010; Lehrian et al., 2010; Previšić
et al., 2014), despite a much stronger impact of glaciation on this
continent (Višnjević et al., 2020) than in the Tibeto-Himalayan Region
(Mao et al., 2021). The limited formation of inland ice during the LGM
in the Tibeto-Himalayan Region may have represented an opportunity for
cold-adapted species to expand their range rather than being moved far
away by massive ice sheets.
4.3 Implication of MGH in different mountain
systems
In line with the MGH, we have shown that the historical evolution of
abiotic conditions such as changing topography (including paleo-drainage
reorganization) and climate are likely to have played dominant roles in
shaping the genetic diversity pattern of caddisflies in the Himalayas
and the HM. The three MGH boundary conditions (i.e. extensive
elevational zonation, response to climatic fluctuations, and a
high‐relief terrain with environmental gradients; Mosbrugger et al.,
2018) are met in both the Himalayas and the HM. Yet, we observe distinct
genetic diversity patterns of species in each mountainous region,
indicating that although the conditions of the MGH are generally
realized opportunities and constraints for species vary across different
mountain systems. To explain the formation of different genetic
diversity patterns in each mountain region, it seems crucial to compare
the geological and climatic conditions in each region and their effects
on the evolution process of species under the concept of MGH.
The uplift of the Himalayas and HM are both caused by the India-Eurasia
collision since the Paleocene (65–60 Ma). The ranges reached their
modern height before the Pliocene (as reviewed by Ding et al., 2022).
Although Muellner-Riehl (2019) indicates that the major uplift of the
Qinghai-Tibet plateau proper would not have had an impact at the level
of intraspecific divergence (as it is vastly older), the distinctive
topographic features of these two mountain systems that resulted from
this orogenic process has influenced the interconnectivity of habitats
among and within each mountainous region (Craw et al., 2016; Rahbek et
al., 2019). Compared to the Himalayas, the HM provide a more extensive
area in the North-South orientation, thus implying better geographic
connectivity for species to shift their distribution towards the south
during the cooling period (Xing & Ree, 2017). Our results have verified
that caddisfly populations in the HM show better connectivity and a
broader potential habitat during the LGM. For this particular scenario
that is associated with Pleistocene climatic fluctuations, Flantua et
al. (2018) recommended a concept called “flickering connectivity system
(FCS)” to explain the rapid diversification and high species richness
in mountain ecosystems. Unlike the MGH, which attempts to explain the
origination and evolution of mountain biodiversity throughout the whole
uplift history, the FCS emphasizes the connectivity dynamics resulting
from repetitive climatic changes during the Pleistocene. As reviewed by
(Muellner-Riehl, 2019), many previous phylogeographic studies have
indicated that the late Pleistocene is a crucial time period for
intraspecific diversification in the Tibeto-Himalayan Region. Therefore,
the FCS concept is applicable to explain the formation of intraspecific
diversification of caddisflies that occurred in a more recent time
period (the LGM).
Caddisflies of the HM and the Himalayas may have reacted to the climatic
fluctuations of the Pleistocene by shifting their geographic range
within mountain areas. Consequently, the dynamically changing
connectivity levels may lead to fragmentation, colonization (dispersal),
intermixing, and hybridization, thus promoting diversification in
mountain ecosystems (Flantua et al., 2018; Muellner-Riehl, 2019).
Moreover, since topography and climate conditions are distinct in
different mountain systems, the diversification of species shaped by
these four processes under the concept of the FCS is unique in each
mountain. The resulting interaction between topography and climate is
considered the “fingerprint” of a mountain (Flantua et al., 2018).
Although the “fingerprints” are not yet specified for all the mountain
systems of the world, our study revealed that the “fingerprints”
differ between the Himalayas and the HM. Hence, in agreement with
Muellner-Riehl (2019), we think that together with climate fluctuations
as one driver of diversification in mountain systems sensu MGH,
the FCS is relevant for explaining the montane biodiversity patterns of
caddisflies. However, given the
fact that species react differently to changes in their environment,
more studies are needed in the future to explore how the entire biota
reacts when facing climatic challenges.