Hereditas 143: 33 40 (2006)
Genetic diversity of five Kobresia species along the eastern
Qinghai-Tibet plateau in China
QING-FANG ZHAO1,2, GANG WANG1, QIAO-XIA LI2, SHI-RONG MA2, YAN CUI2 and
MICHAEL GRILLO3
1
College of Life Sciences, Lanzhou University, Lanzhou, PR China
College of Life Sciences, Northwest Normal University, Lanzhou, PR China
3
Department of Plant Biology, Michigan State University, East Lansing, MI, USA
2
Zhao, Q. F., Wang, G., Li, Q. X., Ma, S. R., Cui, Y. and Grillo, M. 2006. Genetic diversity of five Kobresia species along the
eastern Qinghai-Tibet plateau in China. * Hereditas 143: 33 40. Lund, Sweden. eISSN 1601-5223. Received
August 24, 2005. Accepted December 20, 2005
Plants of the genus Kobresia are alpine grass species of high ecological and economic importance. Vegetative growth is the
dominant means of reproduction for the Kobresia . Studies suggest that substantial vegetative growth can reduce genetic
diversity and renders populations less able to buffer changing and extreme conditions. Kobresia are dominant species in the
Qinghai-Tibet plateau in China where they face harsh conditions and frequent disturbance. The genetic diversity of five
Kobresia species (K. humilis, K. royleana, K. kansuensis , K. tibetica and K. setchwanensis ) from the Qinghai-Tibet plateau
was assessed. The results reveal high genetic diversity at the population level for all of the species and there does not appear
to be a relationship between altitude and genetic diversity. AMOVA analysis shows that most genetic variability resides
among individuals within populations, whereas only a minor portion is found among populations. Of the five species,
K. royleana and K. kansuensis have the highest levels of gene flow and the lowest genetic differentiation. While
K. setchwanensis has the lowest gene flow and the greatest genetic differentiation. The level of gene flow between populations
and the mating system play a critical role in the genetic structure of these Kobresia populations. Despite the predominance of
vegetative growth enough sexual reproduction occurs to maintain the relatively high genetic diversity in Kobresia
populations.
Qing-Fang Zhao, College of Life Sciences, Lanzhou University, Lanzhou 730000, PR China. E-mail: zhaoqingfang@nwnu.
edu.cn
The Qinghai-Tibet plateau is the highest plateau in the
world. Most of the plateau is an alpine arid and semiarid climate where drought, cold temperatures, strong
winds and high radiation are common. In alpine
environments flowering plants must overcome many
difficulties including a short growing season, low
temperatures, high radiation, low nutrient and moisture availability and relative scarcity of pollinators, all
of which reduce the possibility of flowering and seed
setting in one season (BARBARA and GLIDDON 1999;
YOUNG et al. 2002). Sexual reproduction consequently
has limited success in these environments and annual
plants are scarce. When a species occupies an altitudinal range of habitats, the alpine forms often allocate
fewer resources to sexual reproduction and produce
fewer flowers and seeds than individuals below the tree
line (BARBARA and GLIDDON 1999). Many perennials
overcome these difficulties through vegetative growth
by rhizomes, layering, vivipary etc.
The genus Kobresia is distributed mainly in the
alpine mountains of the Northern Hemisphere and
includes about 70 species, with 59 reported throughout
China. In China, Kobresia species are mainly distributed south and east of the Qinghai-Tibet plateau from
latitude 278398N and longitude 8281038E (ZHOU
2001). All Kobresia species are perennial herbs and are
dominant in alpine-meadow communities. The five
species included in this study (K. humilis, K. royleana,
K. setchwanensis, K. tibetica and K. kansuensis) are
widely distributed in the Qinghai-Tibet plateau. Kobresia plants are of high ecological and economic
importance. Kobresia contain high levels of crude
proteins and fats and are preferred by livestock over
other plants found on the plateau. In addition to their
agricultural value for grazing livestock, Kobresia serve
a vital role in ecosystem functioning throughout their
range. For example, Kobresia meadows upstream of
the Chang Jiang River and Huang He River are
important in soil and water conservation in midstream
and downstream regions (ZHOU 2001). Kobresia
meadows are also highly involved in carbon storage
and serve as important carbon sinks in China (JIAN
2002).
Currently, Kobresia pastures are seriously subject to
environmental degradation and decreasing carrying
capacity due to natural and anthropogenic impacts.
Burrowing mammals such as the pika and vole
increase habitat diversity but are substantial competitors with livestock in Kobresia dominated systems.
Human influences include damage by yak trampling
34
Q.-F. Zhao et al.
and livestock overgrazing, as well as sod removal for
construction that results in vast areas of ‘‘black soil’’.
We have selected the five most dominant species (K.
humilis, K. royleana, K. setchwanensis, K. tibetica and
K. kansuensis ) to study genetic diversity. The genus
Kobresia has been divided into sections based upon
the type of inflorescence as well as flower gender
and morphology. Figure 1 shows the phylogenetic
relationship of 14 Kobresia species that are most
closely related to the five included in this study. The
molecular phylogeny constructed with RAPD data
corresponds well to previous classification based on
morphology. While several studies have been conducted on Kobresia , there is still much to learn about
the biology of these plants. DENG et al. (2001a, 2001b)
found that Kobresia reproduce both sexually and
asexually. Asexual reproduction predominates over
sexual reproduction throughout the life cycle and
occurs by a short rhizome mechanism. Clonal growth
is common in many plants from alpine regions as
vegetative reproduction is a typical adaptation to
extreme environments (KLIMES et al. 1997). However,
asexual reproduction may decrease genetic diversity
(MCLELLAN et al. 1997; BAUERT et al. 1998). In
Kobresia species, genetic diversity may be further
decreased by spatial separation between populations
due to their vast range. Populations that are separated
by great distances may have decreased levels of genetic
diversity due to a lack of gene flow (WRIGHT 1943).
Hereditas 143 (2006)
Populations with relatively low genetic diversity may
not be able to adapt to less desirable or changing
conditions (FISCHER et al. 2000). It is necessary to
identify the degree of genetic diversity in Kobresia to
understand how they are able to thrive in a harsh
environment that is subject to disturbance. In the
present study the genetic diversity, genetic structure
and mating-system of five dominate Kobresia species
was estimated using RAPD markers. This study was
designed to estimate the genetic diversity of the five
species and analyze the mechanisms that maintain
genetic diversity. Such information may prove useful
in understanding the mechanisms for adaptation and
evolution of the Kobresia genus in the Qinghai-Tibet
plateau, and is relevant in the development of
sustainable methods for livestock grazing.
DNA markers are considered to be the most
suitable means for estimating genetic diversity because
of their abundant polymorphism and the fact that they
are independent of environmental conditions (GEPTS
1993). Random amplified polymorphic DNA (RAPD)
is a polymerase chain reaction (PCR) based marker
method that can provide abundant polymorphism,
and can be used to estimate population genetic
parameters (WILLIAMS et al. 1990; LYNCH and
MILLIGAN 1994). Due to these advantages, RAPD
has been widely used to investigate genetic diversity
(HARALD et al. 2001; BIRMETA et al. 2002; CHENG
et al. 2004) and genetic structure (BARBARA and
Fig. 1. UPGMA dendrogram based upon RAPD sata showing relationship amonf 14 Kobresia species.
Genetic diversity of five Kobresia species
Hereditas 143 (2006)
GLIDDON 1999; HANGELBROEK et al. 2002) of clonal
plants and grasses.
35
RAPD analysis
A total of 250 primers supplied by Sangon Biotech
Company, were used for a two step primer screening
procedure (SU et al. 2003). In the first step, one sample
of each species was randomly selected to identify
primers that produced at least three clear bands. In the
second step, a different individual from each population was randomly selected for RAPD analysis with
primers selected in the first step. Following the twostep primer screening, 14 primers with clear and
reproducible bands were chosen for each species.
A Biometra UNOII PCR was used for all DNA
amplifications. Amplification was carried out in 10 ml
with 50 mmol l1 Tris-HCL(pH /8.3), 10% Ficoll,
1 mmol l 1 Tartrazine, 500 mg ml1 BSA, 2 mmol l 1
MgCl2, 200 mmol l1 each of dATP, dGTP, dCTP and
dTTP, 1 mmol l 1 of primer, 0.5 units Taq DNA
polymerase and 5 10 ng template DNA. The thermal
cycles started with 948C for 5 min; then 40 cycles of
948C for 1 min, 378C for 1 min, 728C for 1 min and
ended with 728C for 7 min. The amplification products
were separated on 1.5% agarose gel with 0.5 mg ml 1
ethidium bromide in 0.5 /TBE buffer at 100 V for
3.5 4h and photographed under ultraviolet light.
MATERIAL AND METHODS
Plant material
In July and August of 2003, tissue from K. humilis, K.
royleana, K. kansuensis, K. tibetica and K. setchwanensis was collected from the eastern region of the
Qinghai-Tibet plateau in China with altitudes ranging
from 2750 and 3900 m above sea level (Table 1). A
total of 161 K. humilis, 63 K. royleana , 86 K.
kansuensis, 88 K. tibetica and 86 K. setchwanensi s
clones were collected. The size, altitude and habitats of
the sampled populations and their approximate locations are shown in Table 1.
DNA preparation
Leaf tissue was harvested and stored in silica gel in
zip-lock plastic bags at room temperature. Total DNA
was extracted from the dried leaves according to the
2/CTAB method (LI and GE 2001). DNA extracted
was dissolved in 200 ml of 0.1 /TE buffer and was
quantified in 0.8% agarose gels.
Table 1. The population locations, habitats and altitude of K. humilis, K. royleana, K. kansuensis, K. tibetica and
K. setchwanensis from the Qinghai-Tibet plateau in China.
Species
Site
Population ID
Size
Habitats
Altitude(m)
Hezuo, Gansu
Tianzhu, Gansu
Tianzhu, Gansu
Hongyuan, Sichuan
Hongyuan, Sichuan
Ruoergai, Sichuan
Maqu, Gansu
Haibei,Qinghai
A1
A2
A3
A4
A5
A6
A7
A8
20
20
20
20
22
18
21
20
plain land
shady slope
sunny slope
plain land
shady slope
plain land
sunny slope
sunny slope
2850
2800
2800
3530
3600
3450
3820
3250
Tianzhu,Gansu
Maqu,Gansu
Lajishan,Qinghai
X1
X2
X3
21
21
21
bank shrub
sunny slope shrub
shady slope shrub
2750
3860
3630
Maqu, Gansu
Gahai, Gansu
Hongyuan,Sichuan
Ruoergai, Sichuan
G1
G2
G3
G4
21
22
22
21
shady slope
plateau swamp
plateau swamp
plateau swamp
3820
3500
3550
3450
Maqu, Gansu
Hongyuan, Sichuan
Ruoergai, Sichuan
Haibei, Qinghai
Z1
Z2
Z3
Z4
22
22
22
22
plateau
plateau
plateau
plateau
3360
3550
3450
3230
Luqu, Gansu
Maqu, Gansu
Hongyuan, Sichuan
Ruoergai, Sichuan
S1
S2
S3
S4
22
20
22
22
plain land
sunny slope
shady slope
sunny slope
K. humilis
K. royleana
K. kansuensis
K. tibetica
swamp
swamp
swamp
swamp
K. setchwanensis
3140
3380
3600
3460
36
Q.-F. Zhao et al.
Molecular weights were estimated using a 100 3000 bp DNA ladder.
Data analysis
RAPD bands were scored as present (1) or absent (0)
for each DNA sample. In order to evaluate genetic
variation with RAPDs, the percentage of polymorphic
bands (PPB) was calculated for each primer, also Nei’s
gene diversity (h) and Shannon’s index of diversity (I)
was calculated with POPGENE. In addition, the
coefficient for gene differentiation (Gst) and gene
flow (Nm) were calculated by POPGENE on the basis
of gene frequencies. We then analyzed the distribution
of variance by analysis of molecular variance with
software AMOVA.
RESULTS
Genetic diversity
Out of 250 oligonucleotide primers initially screened,
14 primers were chosen that together generated 179194 loci, of which 138-168 were polymorphic. Three
different estimators of genetic diversity were applied
and compared: percentage polymorphic loci (PPB%),
Nei’s gene diversity (h) and Shannon’s information
index (I) (Table 2).
All eight populations of K. humilis showed high
levels of genetic diversity. Among the eight populations, 14 RAPD primers amplified 194 scorable bands,
of which 168 bands (PPB 86.60%) were polymorphic.
Nei’s gene diversity (h) was 0.2622 and Shannon’s
information index (I) was 0.3983 at the species level.
The mean percentage polymorphic loci PPB% was
62.65%, the mean Nei’s gene diversity (h) was 0.2126
and the mean Shannon’s information index (I) was
0.3185 at the population level.
All four populations of K. kansuensis also showed
high levels genetic diversity. Among the four populations, 14 RAPD primers amplified 179 bands and 142
bands (PPB 79.33%) were found to be polymorphic.
Nei’s gene diversity (h) was 0.2645 and Shannon’s
information index (I) was 0.3969 at the species level.
The mean percentage polymorphic loci PPB% was
63.27%, the mean Nei’s gene diversity (h) was 0.2266
and the mean Shannon’s information index (I) was
0.3369 at the population level.
The three populations of K. royleana showed high
levels of genetic diversity, and it was generally higher
than K. humilis and K. kansuensis. Among the three
populations, a total of 187 bands were scored by 14
primers, and 153 bands (PPB 81.82%) were polymorphic. Nei’s gene diversity (h) was 0.2738 and
Shannon’s information index (I) was 0.4099 at the
species level. The mean percentage polymorphic loci
Hereditas 143 (2006)
PPB% was 70.23%, the mean Nei’s gene diversity (h)
was 0.2446 and the mean Shannon’s information index
(I) was 0.3662 at the population level.
The four populations of K. tibetica showed the
highest genetic diversity among the five taxa. Among
the four populations, 14 RAPD primers amplified 187
bands and 164 bands (PPB 87.70%) were polymorphic. Nei’s gene diversity (h) was 0.3106 and
Shannon’s information index (I) was 0.4628 at the
species level. The mean percentage polymorphic loci
PPB% was 71.13%, the mean Nei’s gene diversity (h)
was 0.2521 and the mean Shannon’s information index
(I) was 0.3772 at the population level.
The four populations of K. setchwanensis showed
the lowest genetic diversity among the five species.
Among the four populations, 14 RAPD primers
amplified 180 bands and 138 bands (PPB 76.67%)
were found to be polymorphic. Nei’s gene diversity (h)
was 0.2527 and Shannon’s information index (I) was
0.3805 at the species level. The mean percentage
polymorphic loci PPB% was 58.06%, the mean Nei’s
gene diversity (h) was 0.1997 and the mean Shannon’s
information index (I) was 0.2998 at the population
level.
Genetic diversity and altitude
There is no observed correlation between genetic
diversity and altitude within any population from the
five species (P /0.217-0.768 /0.05, r / /0.7830.873 B/1, respectively) (Table 3).
Partitioning of RAPD variation
In the Kobresia species, AMOVA showed that a large
proportion of genetic variation (77.45%-89.06%) resided among individuals within populations, whereas a
small part (10.94%-22.55%) resided among populations (Table 4). The coefficient of gene differentiation
(Gst /10.66%-21.01%) calculated by POPGENE
showed similar results. K. royleana and K. kansuensis
contained little genetic differentiation among populations. K. setchwanensis had a much larger genetic
differentiation than all the other taxa sampled. In the
five species, the Nm (estimate of gene flow) calculated
by POPGENE are all higher than 1 (1.8801-4.1906).
The Nm of K. royleana is the largest, 4.1906. The Nm
of K. setchwanensis is the least, 1.8801 (Table 5).
DISCUSSION
In the present study, three different estimators of
genetic diversity were applied and compared: percentage polymorphic loci (PPB%), Nei’s gene diversity (h)
and Shannon’s information index (I). The percentage
polymorphic loci (PPB%) was easily inferred and was
Species
K. humilis
K. royleana
K. kansuensis
K. setchwanensis
Samplesize
Numberpolymorphic
Percentage polymorphic loci PPB(%)
Observed number
of alleles na
A1
A2
A3
A4
A5
A6
A7
A8
Mean
Species
SD
X1
X2
X3
Mean
Species
SD
G1
G2
G3
G4
Mean
Species
SD
Z1
Z2
Z3
Z4
Mean
Species
SD
S1
S2
S3
S4
Mean
Species
SD
20
20
20
20
22
18
21
20
20.13
161
117
123
112
128
125
119
116
113
119.13
168
60.31%
63.40%
57.73%
65.98%
64.43%
61.34%
59.79%
58.25%
62.65%
86.60%
21
21
21
21.00
63
130
128
136
131.33
153
69.52%
68.45%
72.73%
70.23%
81.82%
21
22
22
21
21.50
86
112
116
111
114
113.25
142
62.57%
64.80%
62.01%
63.69%
63.27%
79.33%
22
22
22
22
22.00
88
136
135
138
123
133.00
164
72.73%
72.19%
73.80%
65.78%
71.13%
87.70%
22
20
22
22
21.50
86
102
101
101
114
104.50
138
56.67%
56.11%
56.11%
63.33%
58.06%
76.67%
1.6031
1.6340
1.5773
1.6598
1.6443
1.6134
1.5979
1.5825
1.6615
1.8660
0.3416
1.6952
1.6845
1.7273
1.7023
1.8182
0.3867
1.6257
1.6480
1.6201
1.6369
1.6327
1.7933
0.4061
1.7273
1.7219
1.7380
1.6578
1.7113
1.8770
0.3293
1.5667
1.5611
1.5611
1.6333
1.5806
1.7667
0.4241
Effective number Nei’sgene
Shannon’s
of alleles ne
diversity h information
index I
1.3685
1.3625
1.3276
1.3924
1.3823
1.3832
1.3438
1.3375
1.3622
1.4418
0.3563
1.4036
1.4307
1.4219
1.4117
1.4711
0.3691
1.3795
1.3885
1.3985
1.4072
1.3934
1.4567
0.3789
1.4618
1.4072
1.4381
1.4106
1.4294
1.5322
0.3395
1.3463
1.3448
1.2983
1.3716
1.3403
1.4272
0.3598
0.2141
0.2131
0.1933
0.2313
0.2232
0.2234
0.2024
0.2003
0.2126
0.2622
0.1824
0.2375
0.2478
0.2485
0.2446
0.2738
0.1889
0.2202
0.2238
0.2272
0.2351
0.2266
0.2645
0.1920
0.2661
0.2446
0.2587
0.2390
0.2521
0.3106
0.1699
0.2003
0.1999
0.1802
0.2184
0.1997
0.2527
0.1890
0.3194
0.3208
0.2917
0.3468
0.3341
0.3326
0.3052
0.3021
0.3185
0.3983
0.2492
0.3569
0.3683
0.3735
0.3662
0.4099
0.2595
0.3289
0.3344
0.3360
0.3483
0.3369
0.3969
0.2633
0.3948
0.3701
0.3882
0.3555
0.3772
0.4628
0.2318
0.2984
0.2982
0.2746
0.3281
0.2998
0.3805
0.2648
Genetic diversity of five Kobresia species
K. tibetica
ID
Hereditas 143 (2006)
Table 2. RAPD diversity in the study species and populations.
37
38
Q.-F. Zhao et al.
Hereditas 143 (2006)
Table 3. The analysis of correlation between genetic
diversity and altitude using SPSS in five species based
on Shannon information index (I).
Species
K. humilis
K. royleana
K. kansuensis
K. tibetica
K. setchwanensis
Total
P
r
0.369
0.324
0.217
0.660
0.768
0.369
0.369
0.873
/0.783
0.340
/0.232
0.196
Table 5. Gene diversity of species (Ht); gene diversity
within populations (Hs); coefficient of gene differentiation (Gst); estimate of gene flow (Nm), Nm /0.5(1Gst)/Gst.
Species
utilized extensively in the study (LI and GE 2001; XIE
et al. 2004). However QIAN and GE (2001) consider
Nei’s gene diversity (h) and Shannon’s information
index (I) to be superior to percentage polymorphic loci
(PPB%) as the latter lacks the ability to describe
frequency differences of polymorphic bands.
Ordinarily, a PPB around 50% is regarded as high
genetic diversity (MA et al. 2000). Compared to other
plants (QIAN et al. 2000; ZHU et al. 2002; XIE et al.
2004) utilizing RAPD, the five Kobresia species also
showed high genetic diversity both at the species level
(percentage polymorphic loci, PPB 76.67%-87.70%;
Nei’s gene diversity (h) 0.2527-0.3106; Shannon’s
information index (I) 0.3805-0.4628) and at the
population level (mean percentage polymorphic loci,
PPB 58.06%-71.13%; mean Nei’s gene diversity (h)
0.1997-0.2521; mean Shannon’s information index (I)
0.2998-0.3772).
Asexual reproduction of Kobresia plants by short
rhizomes predominate over sexual reproduction
throughout the life cycle (ZHOU 2001). The input of
genets from seedlings may be a possible explanation
for the relatively high genetic diversity for these clonal
plants (HARALD et al. 2001; TORIMARU et al. 2003).
Initial seedling recruitment and repeated seedling
recruitment are important mechanisms in maintaining
genetic diversity (ERIKSSON 1989, 1993).
K. humilis
SD
K. royleana
SD
K. kansuensis
SD
K. tibetica
SD
K. setchwanensis
SD
Ht
Hs
Gst
Nm
0.2622
0.0333
0.2738
0.0357
0.2646
0.0368
0.3106
0.0289
0.2528
0.0358
0.2126
0.0235
0.2446
0.0296
0.2266
0.0296
0.2521
0.0233
0.1997
0.0249
0.1891
2.1445
0.1066
4.1906
0.1438
2.9766
0.1884
2.1534
0.2101
1.8801
Fruit production of the five Kobresia species has
been reported in the field showing that several
Kobresia plants can produce several hundred seeds
per m2 (DENG et al. 2001a, 2001b). Because of the
harsh plateau environment and hard seed vessel, only
about 2-4% of seeds germinate. Clonal growth is
therefore a more efficient means of reproduction
than sexual reproduction. Despite low success rates,
sexual reproduction has been observed in natural
populations. Studies suggest that even low rates of
seedling recruitment are sufficient in maintaining high
levels of genetic diversity (SOANE and WATKINSON
1979; WATKINSON and POWELL 1993). In the alpine
environment, vegetative growth is important in stabilizing the populations, whereas facultative sexual
reproduction plays an important role in maintaining
levels of genetic diversity and long-term adaptation
and evolutionary dynamics.
At higher altitudes Rutidosis leiolepis exhibits significant clonality and vegetative reproduction is becomes more important, consequently genetic variation
decreases with the higher levels of vegetative growth
(YOUNG et al. 2002). In R. leiolepis the increase in
Table 4. Analysis of molecular variance (AMOVA) in five species. No. of permutations: 1000; degree of freedom
(df); sum of squares (SSD); expected mean squares (MSD); probability of null distribution (P(value)).
Species
K. humilis
K. royleana
K. kansuensis
K. tibetica
K. setchwanensis
Source of variance
Variance
Variance
Variance
Variance
Variance
Variance
Variance
Variance
Variance
Variance
among populations
within populations
among populations
within populations
among populations
within populations
among populations
within populations
among populations
within populations
df
7
153
2
60
3
82
3
84
3
82
SSD
MSD
775.72 110.82
3319.66 21.70
171.90 85.95
1440.76 24.01
292.44 97.48
1743.71 21.27
481.60 160.53
2074.14 24.69
410.23 136.75
1545.44 18.85
Variance component
Percentage (%)
P(value)
4.43
21.10
2.95
24.01
3.55
21.27
6.18
24.69
5.49
18.85
16.96%
83.04%
10.94%
89.06%
14.29%
85.71%
20.00%
80.00%
22.55%
77.45%
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
B/0.0010
Hereditas 143 (2006)
clonal reproduction is due to decreased effectiveness
of sexual reproduction which resulted from limited
pollinator availability and a shortened flowering
season at higher elevations. This reduction in sexual
reproduction would lead to a decrease in genotype
diversity. On the other hand, ANNA et al. (2001)
concluded that the number of ‘‘safe sites’’ for recruitment of seeds should increase with latitude. In this
study, there was no correlation between genetic
diversity and altitude (Table 3). This is likely due to
the fact that Kobresia are wind pollinated and do not
rely on insect pollinators with limited ranges. Pollinator availability is not a limiting factor for Kobresia .
While there is no correlation between altitude and
genetic diversity, the type of habitat does have an effect
on genetic diversity. It has been shown that population
genetic diversity correlates with habitat (SAGNARD
et al. 2002; XU et al. 2003; LIU et al. 2004;). For
Kobresia , seed germination and seedling survival may
vary with habitat, accounting for the observed correlation in genetic diversity. In K. humilis, three different
estimators of genetic diversity (PPB, h, I) were higher
in plain land (A4, A6, A1) and shady mountain slope
(A5, A2) than in the sunny mountain slope (A7, A8,
A3) (Table 2). The plain land habitat and the shady
mountain slope have higher moisture levels than the
sunny mountain slope. The increased moisture content
may allow for increased germination and survival of
seedlings.
In K. setchwanensis, the three estimators of genetic
diversity (PPB, h, I) were higher for the sunny
mountain slope (S4, S2) and plain land (S1) habitat
than the shady mountain slope (S3) (Table 2).
This may be due to the higher temperature of the
sunny mountain slope and the plain land habitat.
K. setchwanensis prefers higher temperatures and this
may allow for increased seedling survival and therefore
increased genetic diversity in the sunny mountain
slope and plain land habitat (ZHOU 2001).
The population genetic structure of a species is
affected by many evolutionary factors including mating-system, gene flow and seed dispersal, as well as
natural selection (HAMRICK and GODT 1990). Genetic
differentiation can confer the mating-system of plants
(DE PACE and QUALSET 1995; TOMOSHI et al. 2000;
YOUNG et al. 2002;). For example, TOMOSHI et al.
(2000) predicted that high levels of self-fertilization are
dominant in Carex sociata by analysis of Fst-values.
DE PACE and QUALSET (1995) studied the mating
system of Dasypyrum villosum and estimated that the
outcrossing rate varies from 55 to 100%. In present
study, AMOVA shows that most of the genetic
variability resides among individuals within populations, whereas only a minor part resides among
Genetic diversity of five Kobresia species
39
populations. Additionally the Gst-values of the five
Kobresia species (0.1066-0.2101) are all lower than the
average
Gst-values
of perennial herbs (0.233) and monocotyl (0.231)
(HAMRICK and GODT 1990). Ordinarily, there is
51% genetic variability among populations of self
fertilized plants (HAMRICK 1987). The high levels of
genetic diversity among Kobresia populations suggest
that all of the species undergo sexual reproduction at
some level.
Gene-flow plays a critical role in differentiation
among populations. The gene flow (1.8801-4.1906) of
the five species was higher than one. Generally, when
Nm is greater that one, gene flow can resist genetic
drift in populations and prevent differentiation among
them (SLATKIN 1985). The strong winds of the
Qinghai-Tibet plateau might promote pollination
and gene flow. Also grazing herds may contribute to
seed dispersal and subsequent gene flow.
Although the five species of Kobresia undergo
significant asexual reproduction there is still enough
sexual reproduction to maintain relatively high levels
of genetic diversity. High levels of genetic diversity
may allow the Kobresia to continue to adapt to the
harsh conditions of the Qinghai-Tibet plateau.
Acknowledgements This work was supported by the
Foundation of Key Laboratory of Arid Agroecology of
Lanzhou University.
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