Botanical Studies (2006) 47: 83-88.

*

Corresponding author: E-mail: sfhuang@nhcue.edu.tw;

Fax: 886-3-525-7118.

Migration of

Trochodendron aralioides

(Trochodendraceae)

in Taiwan and its adjacent areas

Shing-Fan HUANG

1,

* and Tsan-Piao LIN

2

1

Department of Applied Science, National Hsinchu University of Education, Hsinchu 300, Taiwan

2

Institute of Plant Biology, National Taiwan University, Taipei 106, Taiwan

(Received March 23, 2005; Accepted October 3, 2005)

ABSTRACT.

A migratory history of Trochodendron aralioides was postulated by integrating the published

fossil records and the data sets of chloroplast (cp) DNA and allozyme. The populations from Taiwan, the

Ryukyus, and Japan were investigated. The haplotype network of Trochodendron was constructed by the

computer program TCS by taking Tetracentron as outgroup to direct the haplotype network. The paper

shows that Japan¡¦s populations were clearly distinguished from those of the Ryukyus and Taiwan, with

Japan¡¦s being more primitive. Nine substitutions were found between Tetracentron and Japan¡¦s population

of Trochodendron. Based on fossil evidence, taking 50 My (million years ago) as divergence time between

Tetracentron and Trochodendron, the date of separation between Japan and Taiwan¡¦s populations can be dated

to 5.5 My, and the standard error is ¡Ó 2.8 My. Given a likely temperate origin and no extant or fossil plants

discovered in eastern China, the migratory route that led Trochodendron to move into Taiwan was likely

from Japan via the Ryukyus. However, the ancient populations in the Ryukyus should have vanished at least

once as the Ryukyus were submerged around 1.0 My. As a result, the populations now in the Ryukyu Islands

were derived from Taiwan, which is exemplified by the shared haplotype of cpDNA between Taiwan and the

Ryukyus and less heterozygosity in the Ryukyus compared to Taiwan based on allozyme data.

Keywords: Migration; petG-trnP; Taiwan; Trochodendron aralioides.

INTRODUCTION

Historical biogeography is the study of taxa in

space and time including their origin, migration, and

diversification (Myers and Giller, 1988). Explaining

how geological events and/or fluctuations in climate

have shaped the distribution of extant taxa is one of the

main aims of historical biogeography. However, when

the hypotheses of historical events are uncertain, the

distribution pattern of the genetic polymorphisms of the

focused taxa may provide another line of evidence with

which to test their soundness.

Taiwan is a continental island adjacent to mainland

China, the Ryukyus, and Japan. Migration of plants

through oceanic barriers to Taiwan may be due to former

landbridges or to long distance dispersal. Landbridges

are the direct connection between landmasses that make

the migration of taxa possible. Three hypotheses on

landbridges between Taiwan, the Ryukyus and Japan since

the late Miocene have been proposed (Ota, 1998). They

can be summarized as follows: Hypothesis I, proposed

by Kizaki and Oshiro (1977), postulated that a landbridge

connecting southeastern China, Taiwan, and the Ryukyus

was formed in the early Pleistocene about 1.5 My

(million years ago) and might have lasted to the middle

Pleistocene about 1 My. Hypothesis II, developed by Ujiie

and his colleague (Ujiie, 1990; Ujiie et al., 1991; Ujiie

and Nakamur, 1996), postulated (1) that one landbridge

connected the Asian continent, Taiwan, the central and

northern Ryukyus, and Japan in the late Miocene and

that the connection between Taiwan, the Ryukyus, and

Japan was broken during the Pliocene; (2) that another

landbridge connected southeastern China, Taiwan, and

the Ryukyus (excluding the islands of southern Ryukyus)

during the late Pleistocene about 20000 years ago; (3)

that another landbridge might have connected Taiwan

and the islands of the southern Ryukyus about 3800 years

ago. Hypothesis III was proposed by Kimura (2000). It

postulated (1) that one large landbridge connected the

Asian continent, Taiwan, the Ryukyus, and Japan during

the period of 1.6-1.3 My; (2) that the other landbridge

connected southeastern China and Taiwan, and extended

in one direction to eastern China and in the other direc-

tion to the Ryukyus during the period of 1.3-1.0 My; (3)

that the other landbridge connected the Asian continent,

Taiwan, and the Ryukyus and might have extended to

Japan about 0.2 My. After 0.2 My, only the southern

Ryukyus connected with Taiwan.

eCOlOgy

84

Botanical Studies, Vol. 47, 2006

In summary, Taiwan might have been connected with

the Asian continent several times in different geological

periods. The landbridges connecting Taiwan and Japan

through the Ryukyus were proposed in three different time

periods. (1) Ujiie and his colleague think they occurred

from the late Miocene perhaps to the early Pliocene,

about 5 My. (2) Kimura believes they occurred in the

Pleistocene about 1.6-1.3. (3) It occurred about 0.2 My

proposed by Kimura. The landbridges connecting Taiwan

and the Ryukyus were proposed in four different time

periods. (1) Kizaki and Oshiro placed it in the Pleistocene

1.5-1.0 My, and Kimura 1.6-1.0 My. (2) It occurred during

0.2-0.04 My proposed by Kimura, but this landbridge

only connected Taiwan and the southern Ryukyus. (3)

It occurred about 0.02 My proposed by Ujiie and his

colleagues, but this landbridge excluded the southern

Ryukyus. (4) It occurred about 3800 years ago proposed

by Ujiie and his colleague, but this landbridge only

connected Taiwan and the southern Ryukyus.

A calibration of divergence time may be performed

from a molecular substitution rate because genetic

mutation accumulates over time (Li, 1997). Thus,

molecular phylogeny provides us information about the

timing of historical events and reduces our reliance on

the timing of geological events and/or fossil records.

However, molecular phylogeny provides only weak

information about taxa in space, as extinction events

are not reflected in the gene tree with extant taxa. As a

result, to reconstruct a historical biogeography of a certain

taxon requires the incorporation of fossils and gene trees

(Manchester and Tiffney, 2001; Tiffney and Manchester,

2001). Fortunately, fossil records related to Trochodendron

have been reviewed recently (Pigg et al., 2001). Thus,

integrating fossil data and gene tree information to better

understand the phytogeography of Trochodendron is now

possible.

Trochodendron is characterized by a tree habit with ves-

selless wood, alternate leaves in a pseudowhorled arrange-

ment, flowers without sepals and petals, stamens in three

to four whorls, and many fused carpels with free stigmas.

The flowers are dichogamous, self-incompatible, and ob-

ligatorily xenogamous (Chaw, 1992). Phylogenetically, it

is closely related to Tetracentron based on DNA markers,

including 5.8S nuclear ribosomal (nr) DNA, trnL intron

chloroplast (cp) DNA, and rbcL-atpB intergenic spacer

cpDNA (Wu et al., 1999; Wu, 2001). These two genera

are grouped as the family Trochodendraceae or as separate

families and were considered as primitive in Hamamelidae

(Lu et al., 1993). Trochodendron aralioides is the only

extant member of Trochodendron and is distributed in Ja-

pan, the Ryukyus, and Taiwan (Wu et al., 2001). Allozyme

data suggest that Japan¡¦s populations are differentiated

from those in the southern Ryukyus and Taiwan (Wu et al.,

2001), as indicated by cpDNA data (Huang et al., 2004).

Huang et al. (2004) investigated the genetic structure of

all sampled populations and inferred a possible refuge in

the north-central part of the west of the Central Mountain

Range in Taiwan during the last glaciation.

In this study, we used Tetracentron as outgroup to

direct the gene genealogy of Trochodendron aralioides

and infer the migratory events involving it in Taiwan and

its adjacent areas by integrating the data of fossil records

and the genetic information of allozymes and cpDNA.

MATeRIAlS AND MeTHODS

Sampling

A total of 24 populations were sampled, including 20

from Taiwan and two each from the Ryukyus and Japan

as described before (Huang et al., 2004: Table 1), and

each population was represented by four individuals at

least 50 meters apart. An outgroup, Tetracentron sinense,

was collected from one tree in the botanical garden of

the Kunming Institute of Botany, Chinese Academy of

Science, which originated from Kaoligongshan, Yunnan.

Fresh leaves were collected from each individual tree, and

they were either desiccated with silica gel and stored in

a freezer (-30¢XC) permanently after complete dryness, or

they were stored in the freezer (-70¢XC) directly.

DNA sequencing

The DNAs were extracted from the sample leaves

using the protocol of Murray and Thompson (1980). The

DNA extracting solution was then used to amplify the

markers for detecting the variation by polymerase chain

reaction (PCR). One marker was used in this study, i.e.

the intergenic spacer of petG-trnP. The primers for petG-

trnP were 5¡¦-GGT CTA ATT CCT ATA ACT TTG GC-3¡¦

forward and 5¡¦-GGG ATG TGG CGC AGC TTG G-3¡¦

in reverse. The initial denaturing temperature was 95¢XC

for 3 min and then 30 s for each thermal cycle. Thirty

four thermal cycles were given for amplification with the

annealing temperature of 55¢XC for 30 s, and the extension

temperature of 72¢XC for 45 s. The last extension time was

set for 10 min after the completion of 34 thermal cycles.

The PCR products were then purified with the commercial

kit and then sequenced with a sequencing machine

ABI3100 using Big Dye terminator.

Sequence analysis

The sequence of Tetracentron sinense (GenBank

accession number AY835400) was aligned with 95

sequences of Trochodendron aralioides (GenBank

accession number, AY294754-AY294848) by eye. The

computer program TCS (Templeton et al., 1992) was

used to reconstruct the gene genealogy by taking gaps as

missing data.

Fossil records of

Trochodendron

A possible distribution pattern in the past and divergent

history of Trochodendron was described by reviewing the

papers of Jin and Shang (1998), Manchester (1999), and

Pigg et al. (2001).

HUANG and LIN ¡X Migration of

Trochodendron aralioides

85

Molecular dating

Tests of the existence of a constant substitution rate

have been developed, such as the relative rate test (Sarich

and Wilson, 1973) and likelihood ratio test (Goldman and

Yang, 1994) although estimation of constant rate has been

relaxed for different taxa (ref. Near and Sanderson, 2004).

These tests either require three taxa or a phylogenetic

tree for testing or assessing. In this paper, two taxa,

Trochodendron and Tetracentron, were dealt with so

that a test for the existence of a local molecular clock

is out of question. In consequence, such a clock was

assumed to exist in the Tetracentron and Trochodendron

group. The equation K=2RT can then be applied, where

K is the predicted average substitution per site between

two homologous sequences, R is the substitution rate,

and T is the divergence time (Li, 1997). Using the sim-

plest substitution model (Jukes and Cantor, 1969), K=

(-3/4)ln(1-4D/3), where D is the observed average

substitution per site between two homologous sequences,

and the variance of K is D(1-D)/[L(1-4D/3)

2

], where L is

the total length of base pairs (Li, 1997). In practice, D is

represented by N/L, where N is the observed number of

substitution, and L is the total length of base pairs. The

substitution rate R can be calculated by applying known

D and T. D is obtained through observation, and T may

be estimated from fossil records. Once R is estimated, the

divergence time between any two isolated groups can be

calculated when the D between them is observed.

ReSUlTS AND DISCUSSION

gene genealogy of

Trochodendron

A total of 482 base pairs and 96 sequences between

genes petG and trnP were aligned for Trochodendron and

Tetracentron. Thirteen variable sites were detected (Table

1). A cpDNA gene genealogy was reconstructed (Figure

1). There are nine substitutions between Tetracentron,

denoted O in Figure 1, and the closest Trochodendron

haplotype of Aishu, Japan (denoted H in Figure 1). From

haplotype H, haplotype I in Japan and haplotype A in

Taiwan and the Ryukyus are derived. From haplotype A,

haplotypes C and D in Taiwan are derived.

Fossil records of

Trochodendron

Fossils related to Trochodendron, namely Norden-

skioldia Heer and Trochodendroides E.W. Berry, can be

traced back to the late Cretaceous in the higher latitudes

in the Northern Hemisphere, including North America,

Siberia, Japan and northeastern China (Lu et al., 1993).

Nordenskioldia became widespread in the Paleocene in

higher latitudes in North America, Europe, and Asia (Man-

chester, 1999; Pigg et al., 2001). The immediate ancestor

of Trochodendron, the fruit of which is similar to extant

Table 1. Haplotypes of Trochodendron and Teracentron, based on variation of sequence between genes petG and trnP of

chloroplast DNA and their distribution.

Haplotype

Variable site

Distribution

0000122222333

0669756779059

3384258292082

O (Tetracentron)

TCCCGTTTTTGGG

Yunnan (China)

A ( Trochodendron)

CATTGCTGGCATA

Taiwan, the Ryukyus

C (Trochodendron)

CATTGCCGGCATA

Taiwan

D (Trochodendron)

CATTGCCGGCATA

Taiwan

H (Trochodendron)

CATTGTTGGCATA

Aishu (Japan)

I (Trochodendron)

CATTCTTGGCATA

Chomonkyo (Japan)

Figure 1. The gene genealogy of Trochodendron aralioides (A,

C, D, H, I) and Tetracentron sinensis (O) based on a sequence

between genes petG and trnP of chloroplast pDNA reconstructed

by TCS, Version 1.03 by taking gaps as missing data. Sampling

individuals are in parentheses. Haplotypes H and I are restricted

to Japan; haplotype A includes populations from Taiwan and

the Ryukyus; and haplotypes C and D are restricted to central-

north Taiwan. O is the outgroup. One arrow repres ents one

substitution.

86

Botanical Studies, Vol. 47, 2006

Trochodendron and the leaves of which are intermediate

between Trochodendron and Tetracentron (Pigg et al.,

2001), appeared in the early Middle Eocene (49-50 My)

in western North America. Plants of Trochodendron were

also recorded in the Oligocene in Liaoning, northeastern

China (Jin and Shang, 1998), and in the Miocene in west-

ern North America, Kamchatka, and Japan (Manchester,

1999). Manchester (1999) suggested that the migration

of this genus might have occurred through Beringia in an

intercontinental exchange of flora because the occurrence

of the fossil Trochodendron in Europe has been excluded

(Pigg et al., 2001).

Molecular dating: colonization of Taiwan¡¦s

Trochodendron

population

The fossil pollen records of Trochodendron in Taiwan

were found from the peat of the Quaternary between

38000 and 4500 years before the present (BP) (Chung and

Huang, 1972a, b). Since the Trochodendron pollen was

recovered near the bottom of the peat, it was estimated to

have inhabited northern Taiwan for at least 30000 years.

This suggestion seems too recent to give any insight

into the migratory history of Trochodendron. Instead, a

molecular dating is used to estimate the possible timing

o f Trochodendron¡¦s first move into Taiwan. Because

the direct ancestor of Trochodendron, the leaves of

which were intermediate between Trochodendron and

Tetracentron, may be traced to the early Middle Eocene

about 49-50 My (Pigg et al.), we may use 50 My as the

divergence time between Trochodendron and Tetracentron.

Nine substitutions between Tetracentron and Japan¡¦s

Trochodendron and one substitution between Japan and

Taiwan¡¦s populations were detected based on petG-trnP

cpDNA (Figure 1). The divergence time between Taiwan

and Japan, thus, would be about 5.5 My (¡Ü 50*10

6

*ln(1

-4*(1/482)/3))/ln(1-4*(9/482)/3)), and the standard error

would be ¡Ó 2.8 My (¡Ü 50*10

6

*(((1/482)*(1-1/482)/(482*

(1-4*(1/482)/3)

2

))/((-3/4)*ln(1-4*(9/482)/3)). The average

of this dating is consistent with the landbridge connecting

the Asian continents and Japan, the Ryukyus and Taiwan

during the late Miocene and Pliocene proposed by Ujiie

and his colleague, and the timing corresponds to the

incipient emergence of Taiwan.

How did

Trochodendron

migrate into Taiwan.

Considering that Taiwan is the southern limit of the

distribution of the extant Trochodendron, and the fossil

records were only reported in north-eastern China, Japan,

Kamchaka, and North America; the populations of Troch-

odendron in Taiwan should have migrated from the north

instead of from south-western China as suggested by Lu

et al. (1993). This was proposed on the basis of the higher

diversity of the extant Hamamelidae in southwestern

China but was not supported by the fossil records of

Trochodendron. There are two possible migratory routes

for the temperate plants moving into Taiwan from the

North: (1) from the North via continental China to Taiwan

and (2) from Japan via the Ryukyus to Taiwan.

The extant species is only distributed in Japan, the

Ryukyus, and Taiwan while the fossils on the Asian conti-

nent are only recorded in Liaoning and northeastern China

(Jin and Shang, 1998). No fossils have ever been reported

in eastern China. Thus, the first route via continental

China to Taiwan seems unlikely. The likelier route is

thus from Japan via the Ryukyus to Taiwan, which traces

the distribution pattern of the extant species. Moreover,

about 5% of the flora of Taiwan is restricted to Japan, the

Ryukyus and Taiwan (Hsieh, 2002), which implies that

some of these species may have experienced the same

historical events as Trochodendron.

Ryukyu¡¦s extant populations were derivatives

of Taiwan

As the migratory route of Trochodendron is from

Japan via the Ryukyus to Taiwan, the population from

the Ryukyus should be older than Taiwan¡¦s. However,

the Ryukyus¡¦ haplotype of cpDNA is the same as Taiwan¡¦

s and different from Japan¡¦s (Table 1; Huang et al.,

2004). As mentioned by Kimura (2000), the Ryukyus

were submerged around 1 My. As a result, the ancient

populations in the Ryukyus were completely wiped out

at least once. As the haplotype of cpDNA of Amami of

the central Ryukyus is also the same as Taiwan¡¦s (Huang

et al., 2004), a direct connection between them must ex-

ist. The connection between Amami and Taiwan proper

existed until 25000 years ago (Kimura, 2000) during the

last glaciation, or around the 20000 years ago proposed

by Ujiie and his colleague. According to the allozyme

data (Huang et al., 2004: Table 5), the genetic diversity

in Iriomote Island of the southern Ryukyus is the small-

est (H

0

=0.076) in contrast with 0.133 in Erkeshan, 0.131

in Yuanyang Lake, and 0.127 in Taipingshan of northern

Taiwan. The appearance of a population with higher

heterozygosity is either due to being in a distribution

center or being in an area to which individuals have

migrated from two or more distribution centers. The

case of being in a distribution center is more likely as

the populations are located in northern Taiwan, where

a geographical barrier occurs in the north. Higher

heterozygosity in Taiwan¡¦s population suggests a coloni-

zation event from Taiwan to the Ryukyus.

Acknowledgements. The first author would like to give

his cordial thanks to Drs. Xin Tian and Peng Sheng,

Kunming Institute of Botany, Chinese Academic of

Science, for providing materials of Tetracentron.

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88

Botanical Studies, Vol. 47, 2006

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