4.1 Genetic Diversity and Structure of Notholirion
Biogenetic diversity is the result of the long-term evolution of species
and serves as an essential foundation for organisms to adapt to their
environment and evolve. It is also a key factor in maintaining the
stability and function of ecosystems. The higher the genetic diversity
of a species, the better it can adapt to environmental changes.
Conversely, when genetic diversity is low, a species is often vulnerable
to environmental changes and may lose ITS distribution or even
become extinct(Soltis & Soltis, 1991; Vranckx, Jacquemyn, Muys, &
Honnay, 2012). Previous studies have reported that the average genetic
diversity of angiosperms, as inferred from plastid gene data, is
0.67(Petit et al., 2005). Our analysis of both plastid and ITSdata sets revealed that the overall genetic diversity
(HT value) of the Notholirion was
significantly higher than this average (cpDNA: HT = 0.931; ITS : HT = 0.796), indicating a
remarkably high level of genetic diversity of this group (Table 1).
Through a series of analyses of the evolutionary aspects of the origin
of the Notholirion , we suggested that two factors may have
contributed the most to the higher genetic diversity ofNotholirion : (1) Notholirion is the earliest divergent
clade of the tribe Lilieae, during the late Oligocene. During ITSlong evolutionary history, it experienced numerous geological and
climatic events such as orogenic movements, monsoonal climatic events,
and drought events, which led to the accumulation of a large amount of
genetic variation to adapt to environmental changes; (2)Notholirion grows at high altitudes with an average altitude of
about 3000 meters, mainly in the Himalayan-Hengduan Mountains. The area
is influenced by frequent mountain-building movements in the Miocene,
resulting in an intricate and complex topography with interlocking rows
of high mountain valleys that form the ”sky islands”. The unique ”sky
island” environment fragments habitat, leading to alpine plant
populations that form relatively isolated and special ecosystems subject
to different environmental selection and ecological niche
differentiation. This increases the opportunity for genetic drift within
populations, which results in a wealth of genetic variation. Thus, the
plant taxa distributed in this environment often exhibit a high degree
of genetic diversity and unique genotypic assemblages, adapted to the
challenges of these extreme environments. Similar results have been
found in other plants distributed in the HHM region, such asChamaesium (cpDNA:HT = 0.794; ITS : HT = 0.718), Notopterygium incisum (cpDNA:HT = 0.939; ITS : HT= 0.725) and Allium section Sikkimensia (cpDNA:HT = 0.974; ITS : HT = 0.988)(Shahzad et al., 2017; C. Xie et al., 2019; H. Y. Zheng et
al., 2021).
In addition, AMOVA analyses were performed based on both ITS and
cpDNA datasets. The results showed (Table2) that if Notholirionis considered as a whole, the genetic variation among the 31 populations
was large (ITS : 91.45%; cpDNA: 98.85%) and within the
populations was relatively small (ITS : 8.55%; cpDNA: 1.15%),
which may be related to the following reasons: (1) field surveys
revealed that although Notholirion species are distributed in
four provinces of southwestern China, they have very specific habitat
requirements. Apart from altitude requirements, this genus also
necessitates specific heights of companion plants. For instance,N. macrophyllum and N. thomsonianum prefer to grow in
scrub or tall grasses at around 3000m above sea level, where the plants
are roughly the same height as their own height. Hence, almost all
populations collected in the field have an extremely limited local
distribution, and communication between flora and populations is rare(J.
Li et al., 2022); (2) Through our field investigations, we also have
observed that Notholirion species have very limited seed
dispersal ability, and their seed production cycle is quite lengthy. It
takes approximately 5 years from seedling to flowering, and after
flowering, the primary bulbs of the roots wither, making it impossible
for seed production to occur in the following year. Mature seed
production is only attainable through newborn seeds and asexual
reproduction of small bulbs, which take an additional 5 years to reach
the mature stage. This situation with seeds also makes interflora and
interpopulation communication very difficult; (3) The genetic
differentiation of many plant taxa in the HHM region has been observed
to be primarily influenced by the complex geological activities and
climate changes that occur in the area. For example, orogenic movements
can lead to fragmented habitats, which can reduce gene flow among
populations and consequently increase genetic variation among them.
Therefore, it is likely that the high genetic differentiation observed
among Notholirion flora is due to the significant fluctuations in
the external environment in the HHM region and ITS own
specificity. Similar results were found in Chamaesium ,Allium section Sikkimensia and Polygonatum (Xia et
al., 2022; C. Xie et al., 2019; H. Y. Zheng et al., 2021). However,
AMOVA results of N. bulbuliferum based on ITS data showed
a lower percentage of genetic variation among ITS populations
(74.86%) compared to the results based on cpDNA data (93.73). The
reason for this inconsistent genetic structure is hypothesized to be as
follows: according to the ITS data, multiple haplotypes were
detected in 27 N. bulbuliferum populations, of which N1 was a
haplotype common to 15 populations, thus reducing genetic variation
among populations.
Both cpDNA and ITS datasets showed high levels of haplotype
diversity in the haplotype analysis (cpDNA: Hd = 0.801; ITS : Hd =
0.788). There were no shared haplotypes among the threeNotholirion species, and most haplotypes were restricted to a
single population or between several geographically adjacent
populations, which is similar to the results found inChamaesium (H. Y. Zheng et al., 2021). We speculate thatNotholirion , as a high-altitude distributed taxon, has different
populations that remain isolated and lack communication with each other
during ice ages and interglacial periods. This situation may be due to
several environmental factors and the species ITS elf, including
the inability of high-altitude plants to adapt to low-altitude
environments, the fragmentation of habitats in the HHM region, and the
formation of unique habitats such as sky islands that make communication
between populations difficult, and the long seed production cycle and
difficulty of dissemination of Notholirion species themselves
that limit communication between populations.