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Homoploid Hybridization between Native Salix cavaleriei and Exotic Salix matsudana (Salicaceae)

本地种云南柳与外来种旱柳 (杨柳科) 的同倍体自然杂交



全 文 :本地种云南柳与外来种旱柳 (杨柳科) 的同倍体自然杂交∗
吴  杰1ꎬ2ꎬ3ꎬ4ꎬ 王东超1ꎬ2ꎬ3ꎬ 杨永平1ꎬ2ꎬ3∗∗ꎬ 陈家辉1ꎬ2ꎬ3∗∗
(1 中国科学院昆明植物研究所东亚植物多样性与生物地理学重点实验室ꎬ 昆明  650201ꎻ 2 中国科学院
昆明植物研究所西南野生生物种质资源库ꎬ 昆明  650201ꎻ 3 中国科学院昆明植物研究所
青藏高原研究所昆明部ꎬ 昆明  650201ꎻ 4 中国科学院大学ꎬ 北京  100049)
摘要: 对分布于云南的旱柳 (Salix matsudana) 和云南柳 (Salix cavaleriei) 之间的一个自然杂交种进行了研
究ꎮ 野外观察表明疑似杂交种异蕊柳 (Salix × heteromera) 形态上介于旱柳和云南柳之间ꎬ 并得到了基于叶
形态特征的主成份分析的印证ꎮ 核基因 ITS 序列数据表明这三个种存在 ITS 序列的种内和个体内的多态
性ꎬ 且疑似杂交种的 ITS序列的基因型总是疑似亲本的嵌合体ꎮ 因此可以判定异蕊柳是旱柳和云南柳的自
然杂交后代ꎮ 流式细胞分析表明这三个种均为四倍体ꎬ 因而ꎬ 本杂交事件为同倍体杂交ꎮ 基于四个叶绿体
序列片段的数据表明本自然杂交事件是不对称的ꎬ 云南柳是异蕊柳的母本ꎮ 常见外来种旱柳与稀有本地种
云南柳的杂交可能导致稀有种云南柳的濒危甚至灭绝ꎮ 研究表明柳属植物的引种应非常谨慎ꎮ
关键词: 柳属ꎻ 同倍体杂交ꎻ 不对称杂交ꎻ 分子证据
中图分类号: Q 943              文献标志码: A            文章编号: 2095-0845(2015)01-001-10
Homoploid Hybridization between Native Salix cavaleriei
and Exotic Salix matsudana (Salicaceae)
WU Jie1ꎬ2ꎬ3ꎬ4ꎬ WANG Dong ̄chao1ꎬ2ꎬ3ꎬ YANG Yong ̄ping1ꎬ2ꎬ3∗∗ꎬ CHEN Jia ̄hui1ꎬ2ꎬ3∗∗
(1 Key Laboratory for Plant Diversity and Biogeography of East Asiaꎬ Kunming Institute of Botanyꎬ Chinese Academy of Sciencesꎬ
Kunming 650201ꎬ Chinaꎻ 2 Germplasm Bank of Wild Species in Southwest Chinaꎬ Kunming Institute of Botanyꎬ
Chinese Academy of Sciencesꎬ Kunming 650201ꎬ Chinaꎻ 3 Institute of Tibetan Plateau Research at Kunmingꎬ
Kunming Institute of Botanyꎬ Chinese Academy of Sciencesꎬ Kunming 650201ꎬ Chinaꎻ
4 University of Chinese Academy of Sciencesꎬ Beijing 100049ꎬ China)
Abstract: Natural hybridization between Salix matsudana and Salix cavaleriei was investigated based on populations
from Yunnanꎬ China. Field observations revealed that the putative hybridꎬ S􀆰 × heteromera had intermediate morpho ̄
logies between S􀆰 matsudana and S. cavaleriei. This was further confirmed by principal component analysis. Sequence
data of nuclear rDNA internal transcribed spacer region showed both intraspecific and intragenomic polymorphisms in
all the three speciesꎬ and S􀆰 × heteromera showed a strong additive pattern between its suspected progenitors at all
nucleotide sites of the genotypes identified. Thereforeꎬ S􀆰 × heteromera was confirmed to be a natural hybrid between
S􀆰 cavaleriei and S􀆰 matsudana. Flow cytometry analysis indicated that all the three species are tetraploidꎬ and the hy ̄
bridization was homoploid. Sequence data from four chloroplast datasets indicated that the hybridization was asym ̄
metricꎬ with S􀆰 cavaleriei as the maternal parent. The hybridization between the exotic common species S􀆰 matsudana
and native rare species S􀆰 cavaleriei might increase the risk of endangerment and even extinctionꎬ indicating that the
植 物 分 类 与 资 源 学 报  2015ꎬ 37 (1): 1~10
Plant Diversity and Resources                                    DOI: 10.7677 / ynzwyj201514052

∗∗
Funding: The National Natural Science Foundation of China (NSFC 31270271 to J􀆰 H. Chen)ꎬ the Project of Knowledge Innovation Program
of the Chinese Academy of Sciences (Grant No.: KSCX2 ̄EW ̄J ̄24)ꎬ Yunnan Natural Science Foundation (2010CD109 to J􀆰 H.
Chen)ꎬ and the Youth Innovation Promotion Associationꎬ CAS
Author for correspondencesꎻ E ̄mail: yangyp@mail􀆰 kib􀆰 ac􀆰 cnꎻ chenjh@mail􀆰 kib􀆰 ac􀆰 cn
Received date: 2014-03-27ꎬ Accepted date: 2014-06-10
作者简介: 吴杰 (1988-) 女ꎬ 硕士研究生ꎬ 主要从事植物分类学及系统学研究ꎮ
introduction of Salix species should be made very cautiously.
Key words: Salixꎻ Homoploid hybridizationꎻ Asymmetryꎻ Molecular evidence
  Natural hybridization is prevalent and plays an
important role in the evolution of plantsꎻ the possible
outcomes of hybridization include breakdown of iso ̄
lating barriersꎻ introgressionꎻ increased genetic di ̄
versityꎻ and the origin of adaptationsꎬ ecotypesꎬ and
species ( Coyne and Orrꎬ 2004ꎻ Soltis and Soltisꎬ
2009ꎻ Abbott et al.ꎬ 2013). Natural hybrids usually
show mosaic morphological characters of their paren ̄
tal speciesꎻ thereforeꎬ the intermediate of parental
characters in morphology can be used to identify nat ̄
ural hybrids. Howeverꎬ this may not always be trueꎬ
because not all morphological characters have genet ̄
ic basis. Furtherꎬ when there is introgressionꎬ a hy ̄
brid may be similar to one of its parental species and
therefore difficult to identify (Rieseberg et al.ꎬ 1993ꎻ
Rieseberg and Wendelꎬ 1993). Besidesꎬ some mor ̄
phological intermediates might form via convergent
evolution (Rieseberg and Wendelꎬ 1993ꎻ Rieseberg
et al.ꎬ 1999). Molecular studies have shown that in ̄
terspecific hybridization is more prevalent than that
indicated by morphological and cytogenetic evi ̄
denceꎬ as reviewed by Rieseberg (1997) and Ar ̄
nold ( 1997 ). Many natural hybrid species have
been confirmed by molecular investigationsꎬ and nu ̄
merous historical hybridizations have also been re ̄
vealed (e􀆰 g.ꎬ Hardig et al.ꎬ 2000ꎻ Kaplan and Fe ̄
hrerꎬ 2007ꎻ Zha et al.ꎬ 2008).
The genus Salix L.ꎬ collectively known as wil ̄
lowsꎬ is a well ̄known taxonomically difficult plant
taxon that consists of some 460-520 species world ̄
wideꎬ which are mainly distributed in the north tem ̄
perate areas. Willows have high economic valueꎻ
species of this genus can be used in ornamentalsꎬ
fuelꎬ and medicinesꎬ and are good sources of energy
biomass as well (Fang et al.ꎬ 1999ꎻ Skvortsovꎬ 1999ꎻ
Argusꎬ 2010). Salix is taxonomically difficult be ̄
cause of common natural hybridizationꎬ simple flow ̄
ers that seldom present stable reproductive traitsꎬ di ̄
oecismꎬ and large phenotypic variation (Rechingerꎬ
1992ꎻ Skvortsovꎬ 1999ꎻ Argusꎬ 2010). As reviewed
by Argus (2010)ꎬ there are about 120 Salix hybrids
that have been recognized in the North American flo ̄
ra (113 native Salix species are recorded in North
American flora)ꎬ and about half of these are rela ̄
tively common. Indigenous species hybridize not only
with each otherꎬ but also with introduced willows
species. For exampleꎬ the introduced Old World
species Salix alba L. is documented to form natural
hybrids with indigenous species Salix lucida Muhlen ̄
berg and Salix nigra Marshall in New World (Ar ̄
gusꎬ 2010). Despite the prevalent natural hybridiza ̄
tion in Salixꎬ most Salix hybrids were identified by
morphological evidenceꎬ which might not be reliable
as mentioned aboveꎬ and seldom have been con ̄
firmed by molecular evidence. A morphological and
molecular study by Hardig et al. ( 2000) revealed
that about one ̄third plants originally identified as
Salix eriocephala were possible introgressants. An a ̄
symmetrical natural hybridization of Populusꎬ a
closely related genus of Salixꎬ was identified by
Hamzeh et al. (2007).
China is rich in Salix speciesꎬ with about 275ꎬ
having been recorded (Fang et al.ꎬ 1999)ꎻ some of
the species described in Flora of China might be nat ̄
ural hybrids. In our previous studyꎬ we found that
Salix heteromera Handel ̄Mazzettiꎬ a tree willow dis ̄
tributed in limited areas of Yunnan provinceꎬ Chinaꎬ
always and almost only coexists with other two tree
willowsꎬ the invasive Salix matsudana Koidzumi and
the indigenous S􀆰 cavaleriei H. Léveillé under natural
conditions (Fig􀆰 1). Moreoverꎬ S􀆰 × heteromera is mor ̄
phologically ( e􀆰 g.ꎬ leaf morphologyꎬ stamen num ̄
berꎬ ovary stipe) intermediate between S􀆰 matsudana
and S􀆰 cavaleriei ( Table 1ꎬ Fig􀆰 2). Thereforeꎬ we
suspected that S􀆰 × heteromera might be a natural hy ̄
brid between S􀆰 matsudana and S􀆰 cavaleriei.
In this studyꎬ we used morphological and molec ̄
ular methods to elucidate whether Salix × heteromera
2                                  植 物 分 类 与 资 源 学 报                            第 37卷
Fig􀆰 1  Distribution of Salix × heteromeraꎬ S􀆰 cavalerieiꎬ and
S􀆰 matsudana in and around Yunnan provinceꎬ China
is a natural hybrid between S􀆰 matsudana and S􀆰 cava ̄
lerieiꎬ the direction of the possible hybridization and
its impact on natural populations of these species.
1  Materials and methods
1􀆰 1  Plant material
We selected five samples of the putative hybrid
Salix × heteromera and three samples each of its sus ̄
pected parents S􀆰 cavaleriei and S􀆰 matsudana from
Lashihaiꎬ Lijiangꎬ Yunnanꎬ China (26°53′51″ Nꎬ
100° 7′ 56″ E ) for sequencing of nuclear rDNA
(nrDNA) internal transcribed spacer (ITS) and cp ̄
DNA rbcLꎬ matKꎬ trnD ̄Tꎬ and atpB ̄rbcL regions.
One sample each of the above species was used for
Table 1  Morphological comparison of the putative Salix × heteromera with the suspected parents S􀆰 cavaleriei and S􀆰 matsudana
  Taxa
Characters
Leaf ( length × width) / cm Number of stamens Gynophore
  Flowering time
  S􀆰 cavaleriei 4-11 × 2-4 6-8 long   March to the end of June
  S􀆰 × heteromera 5-7 × 1􀆰 2-1􀆰 4 2-5 short   March to April
  S􀆰 matsudana 1􀆰 5-3 × 0􀆰 6-0􀆰 8 2 none   March to April
Fig􀆰 2  Leaf morphologyof S􀆰 cavalerieiꎬ S􀆰 × heteromera and S􀆰 matsudana
31期      WU Jie et al.: Homoploid Hybridization between Native Salix cavaleriei and Exotic Salix matsudana 􀆺     
flow cytometry analysis. In allꎬ 107 specimens from
four localities were used for morphological analysisꎬ
i􀆰 e.ꎬ Lashihaiꎬ Heilongtan (26°52′54″ Nꎬ 100°14′
1″ E)ꎬ Suhe (26°47′29″ Nꎬ 99°48′58″ E)ꎬ all in
Lijiangꎬ and Xizhou (25°51′14″ Nꎬ 100°8′ E) in
Daliꎬ Yunnan provinceꎬ China (see Table 2 for de ̄
tails) . All voucher specimens are deposited at the
Herbarium of Kunming Institute of Botanyꎬ Chinese
Academy of Sciences (KUN).
1􀆰 2  Morphological Analysis
Principal components analysis ( PCA) of leaf
morphology was based on five leaf charactersꎬ i􀆰 e.ꎬ
maximum blade length (L)ꎬ maximum blade width
(W)ꎬ petiole length (PL)ꎬ blade length from the
base to the point of maximum width ( BL)ꎬ and
blade length ̄width ratio (LWR). At least five ma ̄
ture leaves of each specimen were measuredꎬ and
the average values were used in PCA.
The PCA included data standardized for each
trait and was performed using a correlation matrix
without rotationꎻ factor axes that described less than
11% of the overall variation in leaf morphology were
excluded from the analysis. The results were ana ̄
lyzed using software SPSS 11􀆰 5 (SPSSꎬ 2002).
1􀆰 3  Molecular analysis
1􀆰 3􀆰 1  DNA extractionꎬ PCR amplificationꎬ and se ̄
quencing
Total DNAs were isolated using the cetyltrimeth ̄
ylammonium bromide (CTAB) method of Saghai ̄Ma ̄
roof et al. (1984) as modified by Doyle and Doyle
(1987). The nrDNA ITS regions were amplified by
polymerase chain reaction ( PCR) using primers
“ITS ̄a” and “ITS ̄d” (Leskinen and Alstrom ̄Rapa ̄
portꎬ 1999). The direction of hybridization was de ̄
termine by amplifying partial sequences of the chlo ̄
roplast trnD ̄Tꎬ atpB ̄rbcL intergenic regionꎬ and
rbcL ̄matK gene by using the following primes:
“trnDGUCF” and “trnTGGU” for trnD ̄T (Demesure et
al.ꎬ 1995)ꎬ “atpB ̄1” and “ rbcL ̄1” for atpB ̄rbcL
(Chiang et al.ꎬ 1998)ꎬ “ 1F” and “ 1024R” for
rbcL (Lledo et al.ꎬ 1998)ꎬ and “3F_KIM f” and
“1R_KIM r” for matK (Janzenꎬ 2009).
PCR was performed using a PTC ̄100TM pro ̄
grammable thermal cycler (MJ Researchꎬ Inc.) in a
total volume of 25 μL containing 15 μL Power Taq
PCR MasterMix ( BioTeke Corporation )ꎬ 8􀆰 5 μL
ddH2Oꎬ 1 μL of each primerꎬ and 1􀆰 5 μL DNA
template. The PCR conditions included an initial de ̄
naturation for 3 min at 94 ℃ꎬ followed by 35 cycles
of 30 s at 94 ℃ for template denaturationꎬ 30 s at
50 ℃ for primer annealingꎬ 1 min at 72 ℃ for exten ̄
sionꎬ and a final extension period of 10 min at 72 ℃ .
The PCR products were purified using the PCR
Products Purification Kit (Biotype Corporation)ꎬ
Table 2  Samples used for sequencing and GenBank accession numbers
Taxon Voucher∗
GenBank accession numbers
ITS rbcL matK atpB ̄rbcL trnD ̄T
Salix cavaleriei
C518
C519
C1038∗∗
KF209139 ̄146
KF209147 ̄155
KF209128 ̄138
KF209231
KF209232
KF209230
KF209254
KF209255
KF209253
KF209243
KF209244
KF209242
KF209265
KF209266
KF209264
S􀆰 × heteromera
C1030
C1047
C1048∗∗
C1056
C1058
KF209156 ̄165
KF209166 ̄174
KF209175 ̄184
KF209185 ̄193
KF209194 ̄203
KF209233
KF209235
KF209236
KF209237
KF209238
KF209256
KF209257
KF209258
KF209259
KF209260
KF209245
KF209246
KF209247
KF209248
KF209249
KF209267
KF209268
KF209269
KF209270
KF209271
S􀆰 matsudana
C523
C1034
C1042∗∗
C1099
KF209222 ̄229
KF209204 ̄212

KF209213 ̄221
KF209241
KF209239
KF209240

KF209263
KF209261
KF209262

KF209252
KF209250
KF209251

KF209274
KF209272
KF209273

∗ All specimens collected by Jiahui Chen in Lashihaiꎬ Lijiangꎬ Yunnanꎬ Chinaꎬ and deposited in Herbarium of Kunming Institute of Botanyꎬ
Chinese Academy of Sciences (KUN)ꎻ ∗∗ Samples used for flow cytometry
4                                  植 物 分 类 与 资 源 学 报                            第 37卷
following the manufacturer’s instructions. The puri ̄
fied PCR products of nrDNA ITS were ligated to the
pUC18 plasmid vectorꎬ and the recombinant plas ̄
mids were cloned into competent Escherichia coli
DH5α cells ( Biotype Corporation ). The bacteria
that contained recombinant plasmid were sequenced
directlyꎬ and at least 8 different clones of each sam ̄
ple were used. Sequences were assembled by Ge ̄
neious (Drummond et al.ꎬ 2011) and aligned with
Muscle (Edgarꎬ 2004)ꎬ followed by manual correc ̄
tion using Geneious 5􀆰 4 (Drummond et al.ꎬ 2011).
Average sequence divergence ( i􀆰 e.ꎬ pairwise dis ̄
tance) was estimated using Kimura’s (1980) two ̄
parameter method in Mega 5􀆰 2 ( Tamura et al.ꎬ
2011).
1􀆰 3􀆰 2  Determination of ploidy using flow cytometry
DNA ploidy level of the putative hybrid Salix ×
heteromera and its suspected progenitors S􀆰 cavaleriei
and S􀆰 matsudana was determined by comparing
their DNA contents with those of taxa of known
ploidy levelꎻ we used S􀆰 gracilistyla Miquel as a ref ̄
erence sampleꎬ which has been reported to be dip ̄
loid with 2n = 2x = 38 (Rudykaꎬ 1990). To avoid
the risk of error due to instrument driftꎬ we simulta ̄
neously chopped the test samples and reference sam ̄
ple. Leaves (50 mg silica gel dryed leaf) of refer ̄
ence and test plants were placed in a plastic petri
dish containing 1 000 μL WPB (0􀆰 2 mol􀅰L-1 Tris
HClꎬ 4 mmol􀅰L-1 MgCl2􀅰6H2Oꎬ 2 mmol􀅰L
-1 ED ̄
TA Na2􀅰2H2Oꎬ 86 mmol􀅰L
-1 NaClꎬ 10 mmol􀅰L-1
sodium metabisulfiteꎬ 1% PVP ̄10􀆰 1% Triton X ̄
100ꎬ pH 7􀆰 5) for 30 min. Nextꎬ they were chopped
using a razor bladeꎬ passed each sample through a
30 ̄μm filterꎬ and added 150 μL of staining solution
(500 μg􀅰mL-1 RNase Aꎬ 1􀆰 12 mg􀅰mL-1 PI) for 15
min in dark. Each sample was run for 2-3 min on
Partec CyFlow Space flow cytometer. The peak of the
reference sample was adjusted to be located approxi ̄
mately at channel 100ꎬ so that the relative ploidy of
the unknown samples could be determined by com ̄
paring the peak positions of reference sample and the
test sample by using the following ratio (Doležel et
al.ꎬ 2007):
Sample ploidy = Reference ploidy ×          
          mean position of the sample peak
mean position of the reference peak
2  Resutls
2􀆰 1  Morphological analysis
The result of PCA of leaf morphology (Fig􀆰 3)
indicated that the putative hybrid Salix × heteromera
is an intermediate between and separate from its sus ̄
pected parents S􀆰 cavaleriei and S􀆰 matsudana along
the first factorꎬ which accounted for 84% of the vari ̄
ance observed.
2􀆰 2  Genotypes of ITS
The complete ITS regions of Salix × heteromeraꎬ
S􀆰 cavalerieiꎬ and S􀆰 matsudana were sequencedꎻ the
length varied from 593 to 599 base pairs (bp)ꎬ and
the aligned length was 603 bp (Fig􀆰 4). Both intra ̄
specific and intra ̄individual polymorphism were de ̄
tected in all the three species sequenced except for a
specimen of S􀆰 cavaleriei ( c518)ꎬ which had only
one ITS repeat type. In allꎬ 11 ITS DNA variations
(6 in ITS1 and 5 in ITS2 regionꎻ 2 are indels and
the other 9 are point mutations) were recognized in
the three species. The putative hybrid S􀆰 × hetero ̄
mera showed nucleotide additivity of its suspected
parents S􀆰 cavaleriei and S􀆰 matsudana at all variation
sites of the 11 genotypes. Furtherꎬ it showed the
most average sequence divergence that equaled the
sum average sequence divergence of its suspected
parents (see Table 3 and Fig􀆰 4 for details) .
2􀆰 3  Chloroplast haplotypes
In allꎬ 11 haplotypes ( 4ꎬ 1ꎬ 2ꎬ 4 for atpB ̄
rbcLꎬ matKꎬ rbcLꎬ trnD ̄Tꎬ respectively) were de ̄
tected in the chloroplast regions sequencedꎻ 10 of
the haplotypes of the putative hybrid Salix × hetero ̄
mera were identical to those of S􀆰 cavalerieiꎬ and one
haplotype (a deletion in the atpB ̄rbcL region) was
exclusive to S􀆰 × heteromera ( see Table 4 for de ̄
tails) . Thereforeꎬ the hybrid S􀆰 × heteromera had
S􀆰 cavaleriei as the plastid donor parentꎬ and the hy ̄
bridization was unidirectionalꎬ i􀆰 e.ꎬ asymmetrical.
51期      WU Jie et al.: Homoploid Hybridization between Native Salix cavaleriei and Exotic Salix matsudana 􀆺     
2􀆰 4  Ploidy level
Flow cytometry analysis of intact leaf nuclei in ̄
dicated that all the three species were tetraploid
(Fig􀆰 5). The diploid standard sample (Salix graci ̄
listyla) nuclei produced a single peak that appeared
at channel 100ꎬ with average coefficient of variation
(CV) of 9􀆰 24%ꎬ and the peak mean channel of the
three test samples were around 200 ( S􀆰 cavaleriei:
X = 198􀆰 22ꎬ CV = 4􀆰 50%ꎻ S􀆰 × heteromera: X =
208􀆰 22ꎬ CV = 4􀆰 19%ꎻ S􀆰 matsudana: X = 209􀆰 57ꎬ
CV= 5􀆰 22%). Thusꎬ the three species were conclu ̄
ded to be tetraploid with the chromosome number of
2n= 4x= 76. Our result is consistent with the repor ̄
ted ploidy of S􀆰 matsudana (Sudaꎬ 1958ꎬ 1963).
Fig􀆰 3  Plot of leaf characters according to the first and second factor
scores derived from PCA (factor 1 described 84% and factor 2
described an additional 11% of the overall variation)
Fig􀆰 4  Schematic diagram of sequence alignments of the ITS region in Salix × heteromeraꎬ S􀆰 cavalerieiꎬ and S􀆰 matsudana. Sequence names
are presented as “species name_voucher_clone number” (cav=S􀆰 cavalerieiꎬ het =Salix × heteromeraꎬ mat=S􀆰 matsudana)
Fig􀆰 5  Estimation of nuclear DNA content by using flow cytometry. (A) Simultaneous analysis of nuclei isolated from standard diploid species
(Salix gracilistyla) and from S􀆰 cavalerieiꎻ (B) Simultaneous analysis of nuclei isolated from standard diploid species (S􀆰 gracilistyla) and
from S􀆰 × heteromeraꎻ (C) Simultaneous analysis of nuclei isolated from standard diploid species (S􀆰 gracilistyla) and from S􀆰 matsudana
6                                  植 物 分 类 与 资 源 学 报                            第 37卷
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3  Discussion
The biparental inherited nrDNA ITS variation of
the putative hybrid Salix × heteromera and its suspec ̄
ted parents S􀆰 cavaleriei and S􀆰 matsudana were inves ̄
tigated. Our results showed that concerted evolution is
not completed in these three species. Both intraspecif ̄
ic and intra ̄individual polymorphisms were detected
for all the three species. Multiple nrDNA repeats were
common at interspecificꎬ intraspecificꎬ and intraindi ̄
vidual levelsꎬ arising both from organismal processes
such as hybridization and polyploidization and by ge ̄
nomic processes such as gene and chromosome seg ̄
ment duplication and various forms of homologous and
non ̄homologous recombination (Alvarez and Wendelꎬ
2003). The putative hybrid S􀆰 × heteromera showed
both intraspecific and intraindividual polymorphism of
nrDNA ITS regionꎬ and all the variant nucleotide
sites were perfectly additive ( i􀆰 e.ꎬ chimera) of its
suspected parents S􀆰 matsudana and S􀆰 cavaleriei (Ta ̄
ble 3ꎬ Fig􀆰 4). Stochastic genomic processes men ̄
tioned above are not likely to produce such an addi ̄
tive nucleotide pattern. Moreoverꎬ the ITS sequence
diversity of S􀆰 × heteromera (0􀆰 007) was considerably
higher than those of S􀆰 matsudana ( 0􀆰 004 ) and
S􀆰 cavaleriei (0􀆰 003). Furtherꎬ S􀆰 × heteromera occurs
only where both S􀆰 cavaleriei and S􀆰 matsudana are
presentꎻ it is a morphological intermediate between
S􀆰 cavaleriei and S􀆰 matsudana as indicated by the
features of leafꎬ stamenꎬ ovary stipeꎬ and PCA anal ̄
ysis of morphological traits. Taken togetherꎬ these
findings suggest that S􀆰 × heteromera is a natural hy ̄
brid between S􀆰 cavaleriei and S􀆰 matsudana.
Salix matsudana and S􀆰 cavaleriei also showed
intraspecific and intraindividual polymorphism in the
ITS region. Considering that both species are tetra ̄
ploidꎬ as shown by our flow cytometry analysisꎬ it is
possible that both species are of allopolyploid ( hy ̄
brid) origin and have merged and maintained both
ITS repeat types of their progenitors. Divergent re ̄
peats of ITS have been reported to be clearly main ̄
tained over tens of millions of years (Baumel et al.ꎬ
2001ꎻ Alvarez and Wendelꎬ 2003).
71期      WU Jie et al.: Homoploid Hybridization between Native Salix cavaleriei and Exotic Salix matsudana 􀆺     
Flow cytometry analysis revealed that Salix ×
heteromera is tetraploid and thus is a homoploid hy ̄
brid. In natureꎬ homoploid hybrid speciation might
be a rare phenomenonꎻ in thatꎬ parent species must
be closely related for the homoploid hybrid to be via ̄
bleꎬ or the differences in chromosome arrangement
might affect mitosis (Riesebergꎬ 1997ꎻ Coyne and
Orrꎬ 2004ꎻ Abbott and Riesebergꎬ 2012). The pa ̄
rental species S􀆰 cavaleriei and S􀆰 matsudana belong
to Salix subgenus Salixꎬ and sections Wilsonia K􀆰 S.
Hao ex C􀆰 F. Fang & A􀆰 K. Skvortsov and Salixꎬ re ̄
spectively. A molecular phylogenetic study showed
that these two sections belong to different clades
(Azuma et al.ꎬ 2000ꎻ Chen et al.ꎬ 2010)ꎬ indica ̄
ting that S􀆰 cavaleriei and S􀆰 matsudana are not
closely related species. This is consistent with their
morphological differences. For exampleꎬ they showed
differences in stamen numberꎬ which is an important
character in systematics of Salix ( Dingꎬ 1995aꎻ
Skvortsovꎬ 1999ꎻ Argusꎬ 2010): S􀆰 cavaleriei has 6
- 12 stamensꎬ and S􀆰 matsudana has 2 stamens.
They also differed ecologically: S􀆰 matsudana is
somewhat adapted to arid and semiarid environment
(Skvortsovꎬ 1999) and grows along rivers or depres ̄
sions amidst sand in basin and plain areasꎬ whereas
S􀆰 cavaleriei is a typical riparian speciesꎬ requiring
high moistureꎬ and only occurs around streams and
can even grow in water. Our field observations re ̄
vealed that many individuals of S􀆰 × heteromera are
big trees and apparently viableꎬ and the hybrids
share their habitat closely with S􀆰 matsudanaꎻ how ̄
everꎬ the coexistence of the hybrid and its parents
can also be observed at one site ( e􀆰 g.ꎬ along a
stream). Moreoverꎬ the flowering time of S􀆰 × hetero ̄
mera overlaps with that of its parents. These findings
indicate that S􀆰 × heteromera is not significantly di ̄
vergent in ecology from its progenitors. All ho ̄
moploid hybrid species that have been documented
thus far are ecologically divergent from their parental
species (Abbott and Riesebergꎬ 2012). In additionꎬ
the restricted distribution of S􀆰 × heteromera ( only
occurs when both its parents are present) suggests
that it is sterile or its progeny is sterile and / or invia ̄
ble and therefore lacks the ability to expand beyond
its distribution range (i􀆰 e.ꎬ it is not a true species).
Howeverꎬ this needs to be further investigated.
Chloroplast DNA is usually maternally transmit ̄
ted in angiosperms ( Mogensenꎬ 1996 )ꎬ and se ̄
quencing can be used to determine hybrid origin
(Zhou et al.ꎬ 2008). Our results from the four chlo ̄
roplast sequence datasets indicated that the hybrid ̄
ization is unidirectionalꎬ i􀆰 e.ꎬ asymmetricꎬ with
Salix cavaleriei as the maternal parent. Hybridization
tends to be unidirectional at sites where one of the
parental species is rareꎬ because the pollens deliv ̄
ered to the rare species would consist mainly of pol ̄
len from the common species ( Riesebergꎬ 1995ꎻ
Zhou et al.ꎬ 2008). In the habitat of our putative
hybridꎬ S􀆰 cavaleriei is rare and S􀆰 matsudana is
more abundantꎬ and S􀆰 × heteromera shared its habi ̄
tat closely with S􀆰 matsudana. Under such circum ̄
stanceꎬ the rare species is usually the maternal par ̄
ent of the hybrid ( Riesebergꎬ 1995)ꎻ this would
have been the possible reason for the asymmetric hy ̄
bridization observed.
Hybridization is also often associated with habi ̄
tats that have been altered by anthropogenic disturb ̄
ance (Abbott and Riesebergꎬ 2012). This might be
the case in our current studyꎻ Salix matsudana is
not documented in Floras as native species in Yun ̄
nan province (Dingꎬ 1995bꎻ Fang et al.ꎬ 1999).
This species has long been used as an ornamental
plant and cultivated almost all over the temperate
zone in the world. It is widely cultivated around
farmlandsꎬ villagesꎬ and deforestation areas. The
cultivated plants might escape and naturalizeꎻ in ̄
deedꎬ natural population of S􀆰 matsudana is at pres ̄
ent quite common in north ̄central Yunnan. There ̄
foreꎬ crossing occurred between the previously allo ̄
patricꎬ common and widespread S􀆰 matsudana and
the rare and narrowly distributed S􀆰 cavaleriei. Hy ̄
bridization between common and rare species might
have severe consequences for the rare species: if fit ̄
ness of the hybrid is lower relative to either parental
8                                  植 物 分 类 与 资 源 学 报                            第 37卷
species or even sterile ( i􀆰 e.ꎬ outbreeding depres ̄
sion)ꎬ the growth rate of the rare species may de ̄
cline below that required for replacement (i􀆰 e.ꎬ de ̄
mographic swamping). Ifꎬ howeverꎬ the hybrid is
fertile or fitness decline is negligibleꎬ the hybrid
tends to backcross more frequently with the common
species and may displace the rare species (i􀆰 e.ꎬ ge ̄
netic assimilation) (Rieseberg and Wendelꎬ 1993ꎻ
Rhymer and Simberloffꎬ 1996ꎻ Ellstrand et al.ꎬ
1999ꎻ Wolf et al.ꎬ 2001). In our caseꎬ S􀆰 cavaleriei
did not seem to be seriously threatened by its hybrid ̄
ization with the exotic S􀆰 matsudana at presentꎬ be ̄
cause although S􀆰 cavaleriei is rare and highly re ̄
quires a moist habitatꎬ some of its distribution range
is not invaded by S􀆰 matsudana (e􀆰 g.ꎬ Tengchong of
Yunnan province and south Sichuan province ).
Howeverꎬ if S􀆰 matsudana continues to invade the
distribution range of S􀆰 cavaleriei by means of human
intervention and becomes numerically superior com ̄
pared to S􀆰 cavalerieiꎬ S􀆰 cavaleriei might become in ̄
creasingly endangered or even extinct through genet ̄
ic assimilation and / or outbreeding depressionꎬ re ̄
gardless of the fitness of hybrids. Thereforeꎬ our
study indicated that willows should be introduced for
purposes such as ornamentation and afforestation
with cautionꎬ since they may cross with indigenous
willow species and increase the risk of rare species
becoming extinct.
Acknowledgments: We thank Zhikun Wuꎬ Detuan Liu of
Kunming Institute of Botany for their kind assistance in field
work.
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01                                  植 物 分 类 与 资 源 学 报                            第 37卷