J. Jpn. Bot. 89: 65–75 (2014)

Geographic Structure of a True Pantropical Plant with
Sea-drifted Seeds, Ipomoea pes-caprae (Convolvulaceae),
Revealed by Nuclear Markers
Matin Miryeganeha, Koji Takayamab, Yoichi Tateishic
and Tadashi Kajitaa,*aDepartment of Biology, Graduate School of Science, Chiba University,
1-33, Yayoi, Inage, Chiba, 263-8522 JAPAN;b
The University Museum, The University of Tokyo,
7-3-1, Hongo, Bunkyo-ku, Tokyo, 113-0033 JAPAN;
c
Faculty of Education, University of the Ryukyus,
1, Senbaru, Nishihara, Okinawa, 903-0129 JAPAN*Corresponding author: tkaji@faculty.chiba-u.jp(Accepted on July 6, 2013)“Pantropical plants with sea-drifted seeds” exhibit extremely wide distribution in thetropics and subtropics of the world, and the wide distribution range were attributed tolong distance seed dispersal by ocean currents. We performed phylogeographic analyses
on Ipomoea pes-caprae (Convolvulaceae) that is one of the ‘true-pantropical’ species,
using two low copy nuclear genes, HSP-90 and Waxy. Using 272 samples collectedfrom 34 populations throughout the global distribution range, seven and five haplotypes
were recognized in HSP-90 and Waxy respectively, and the geographic distributions of
haplotypes showed clear geographic structure. Several haplotypes were distributed in
AEP (Atlantic – East Pacific) or IWP (Indo – West Pacific) regions almost specifically,
while some haplotypes were very widespread across AEP or IWP regions. The presenceof the same haplotypes over populations isolated by the ocean suggests that long distancedispersal by ocean currents is responsible for gene flow within the wide distribution
range. Both the East Pacific and African Continent acted as geographic barriers thathave prevented seed dispersal. Presence of more haplotypes in Africa or Indian Oceanic
regions may suggest that the origin of the species was in these regions and that the species
expanded its distribution toward the New World.
Key words: Long distance dispersal, phylogeography, sea-dispersal.

“Pantropical plants with sea-drifted seeds”
are a group of plants consisting of littoral species
which are distributed in an extremely wide
distribution range across tropical and subtropicalareas worldwide (Takayama et al. 2006, 2008,
Vatanparast et al. 2011). Members of this plantgroup are characterized by their sea-drifted seeds

that can be easily conveyed over long distances
resulting in a vast distribution range overlittoral areas of the tropics. A few species fromdivergent families are known in this plant group,
and we categorized them into two groups basedon the number of component species. One ofthem is “true-pantropical”: Ipomoea pes-caprae

—65—

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Fig. 1. Geographical distributions of the 34 populations used in this study. Each circle corresponds to one
population that includes eight individuals. Populations of Ipomoea pes-caprae subsp. pes-caprae are shownby open circles, and subsp. brasiliensis by filled circles. Short vertical bars roughly suggests the positions ofboundaries between IWP (Indo–West Pacific) and AEP (Atlantic–East Pacific) regions. Population numbers
correspond to those in Table 1.

(L.) R. Br. (Convolvulaceae) and Canavalia
rosea (Sw.) DC. (Fabaceae) are in this category.
The other one is “sub-pantropical” species inwhich two or more closely related species are
distributed in specific ranges but pantropicalin total. Members of this category are somepairs or groups of plants: Hibiscus tiliaceus L.
and H. pernambucensis Arruda (Malvaceae);
Vigna marina (Burm.) Merr. and V. luteola(Jacq.) Benth. (Fabaceae); Entada phaseoloides(L.) Merr., E. rheedei Spreng. and E. gigas(L.) Fawc. & Rendle; and Rhizophora species(Rhizophoraceae).
One of the most successful studies ofPantropical plants with sea-drifted seeds bymolecular markers were performed for Hibiscus
tiliaceus and H. pernambucensis. Takayama
et al. (2006 and 2008) used chloroplast andnuclear markers to study the history and levelsof population differentiation, and revealed that,1) substantial gene flow was retained across the
Pacific to Indian Ocean, 2) American continents
played a strong role as barriers and 3) historicalmigration and introgression occurred over the
Atlantic. This research implied that the East
Pacific and Atlantic oceans were so big barriers
that the same species could not distribute acrossthe barriers. However, it has not been studied
yet whether those barriers were also effectively
working within species and promoting

population differentiation or speciation.
Because none of the sub-pantropical species
are distributed over those barriers, studying
true-pantropical species only can answer to thequestion.
In this study, we used two low copy nuclear
genes to reveal geographic structure of a truepantropical species with sea-drifted seeds,
Ipomoea pes-caprae. The species, commonly
called beach morning glory, has pink, large
flowers and sea-drifted seeds, and often forms
large creeping mats on beaches and coastal
dunes throughout tropical and subtropicalregions of the world (St. John 1970, Devall1992). The species comprises two subspecies,
ssp. pes-caprae and ssp. brasiliensis (L.) Ooststr,
which differ in the shape of the leaves and thedimensions of calyx and corolla, as well as indistribution ranges: ssp. pes-caprae is limited to
the northern part of Indian Ocean (from Arabia
to Indonesia) and ssp. brasiliensis is pantropical(St. John 1970, Fosberg and Sachet 1977,
Fig. 1). A preliminary study using chloroplast
sequences of intergenic spacer (IGS) for morethan 4 kbp showed no molecular differences
neither between the two subspecies nor among
populations of ssp. brasiliensis (Tadashi Kajita
unpublished). As no variation was obtainedin cpDNA markers, more neutral molecularmarkers with higher substitution rates areApril 2014

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required. In this study, we employed two low
copy nuclear genes, HSP-90 and Waxy, tostudy the geographic structure of the species byinvestigating the distribution of haplotypes. The
goal of the study is to reveal global geographic
structure in a true-pantropical species, Ipomoea
pes-caprae, and understand how the structure
was formed.
Materials and Methods
Population sampling and DNA extraction
Leaf samples of 272 individuals from34 populations of Ipomoea pes-caprae
were collected over the distribution range ofthe species (Table 1, Fig. 1). Five of these
populations (40 individuals; Nos. 1–5 of Table 1
and Fig. 1) were for Ipomoea pes-caprae subsp.
pes-caprae which were collected from northernpart of Indian Oceanic region. The rest of themwere from I. pes-caprae subsp. brasiliensis (Nos.
6–34). Leaf fragment samples were collected infields and kept and dried in silica-gel. Voucher
specimens of these samples were deposited in
the Herbarium of the University of the Ryukyus(URO). Total DNA was isolated from the silicagel dried leaf samples using the cetyltrimethyl
ammonium bromide (CTAB) extraction method
as was described in a previous study (Doyle &Doyle 1987), with the addition of purifying the
DNA solution with Glass Milk powder using
GeneClean III kit (Bio 101, Vista, CA).
Locus selection, Polymerase Chain Reaction(PCR) and Sequencing
Two low copy genes, heat shock protein
90 (HSP-90) (Steele et al. 2008) and Waxy
gene encoding granule-bound starch synthase
I (GBSSI) (Miller et al. 1999) used forphylogenetic studies of Ipomoea were selected
as genetic markers. We first confirmed whether
clear sequence data could be obtained for thesemarkers by direct PCR sequencing method
using the primers published in the previous
studies. The primers used for HSP-90 were Steel
15-F(ACG GAC AAG AGC AAG CTC GAT67G) and Steel 15-R(TTG TAG TCT TCC TTG
TTC TCA G) (Steele et al. 2008), and for Waxy
F(GAT ACC CAA GAG TGG AAC CC) andR(GTT CCA TAC GCA TAG CAT G) (Miller etal. 1999). We obtained clear single bands in PCR
and clear sequence data by direct sequencing forboth genes. We then designed internal specificprimers for these two loci: for HSP-90 (F: TAACGT ATT ATG TAT TTG CAC TGA T, R: TTT
AAT TAC AGA ACC AGA ATT ACC T) and for
Waxy (F: GCT CAT TTG GAA TTT TAT GC,
R: AAA TCA GCA CCA GCA GTA AT) basedon conserved regions in the alignment including
I. pes-caprae as well as all possible relevant
sequences from closely related Ipomoea species
obtained from GenBank. The partial sequence of
these genes included both coding and noncoding
regions (Table 2). Direct PCR sequencing using
the same primers as sequencing primer was
performed as was described in Vatanparast et al.
(2011), using the ABI 3500 automated sequencer(Applied Biosystems, Foster City, CA).
All of the samples for the two markers
produced clear sequence profiles. Although
double peaks would be expected for possible
heterozygotes in each marker, all the overlapping
peaks detected in this study were from only one
site. This allowed us to extract the two allelicnucleotide sequences when we detected a double
peak. All the obtained sequences were visually
determined and aligned by SEQSCAPE V2.5
software (Applied Biosystems). All sequence
data were deposited in GenBank with accession
nos. KF296518-24 and KF296558-62.
Nucleotide diversity and neutrality tests
Sequences were aligned and edited using
BioEdit v. 5.0.9.1 (Hall 1999). Coding regions
and open reading frames were identified bycomparison with the Ipomoea batatas EST datafrom GenBank. For each locus and for each pairof populations, nucleotide diversity in termsof π (Nei and Li 1979) at total sites (πt), silent
sites (πs) and nonsynonymous sites (πa) were
estimated using DnaSP v. 5.10.01 (Librado

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Table 1. Samples of Ipomoea pes-caprae used in this study
No. Subspecies/region/locality
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

subsp. pes-caprae
Indian Ocean
FujairaWattala, Negambo
Hambantoa
Maharashutra
Kmala Beach, Phuket
subsp. brasiliensis
IWP
Tegula river mouth
Mjimwema
Sambava
Anse A La Manche, Mahe I.
Southwest of Changi Airport
Pantai Tratae, Java
Fucarne I., Port Headland, West Australia
Tak Doan, Puerto Princesa, Palawan I.
Takanabe, Miyazaki Pref.
Ogasawara I., Tokyo Pref.
N. of Ellis Beach, Queensland
Crowdy Head, Harrington, New South Wales:
Haashini-Lavengatonga
NW seashore, Upolu I.
Oahu I., Hawaii
Anaho, Nuku Hiva, Marquesus I.
Tongaliki, Easter I.
AEP
Playa Majahual, Sinaloa
Loca Blanca, Oaxaca
Esmeralda
Vera Cruz
Cuango, Colon
Indian River, Florida
Praia do Crispin, Para
Barra de Sirinhaem, Pernanbuco
Recreio, Rio de Janeiro
Joal-Fadiout
Labadi Beach
Musul, Luanda

and Rozas 2009). To confirm that the variation
obtained for the two markers were neutral,
departures from neutral expectation were tested
using Tajima’s D (Tajima 1989), Fu and Li’s
D*(Fu and Li 1993) by DnaSP (Librado andRozas 2009).
Phylogenetic and network analyses
Phylogenetic analyses were performed using
the Wagner maximum parsimony (MP) method

CountryN

LatitudeLongitude

UAE
Sri Lanka
Sri Lanka
India
Thailand

8
8
8
8
8

25.4936.997
6.135

56.36179.87381.1357.807

98.299South Africa
Tanzania
Madagascar
Seychelles
Singapore
Indonesia
Australia
Philippines
Japan
Japan
Australia
Australia
Tonga
Samoa
USA
French Polynesia
Chile

8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8

−29.23−6.836−14.295−4.96131.48739.32350.18955.592103.981106.381118.584
118.587
131.534
142.179145.635
152.748−175.229−171.694−157.711−139.049−109.175

MexicoMexicoEcuadorPanamaPanamaUSA
BrazilBrazilBrazilSenegalGhana
Angola

8
8
8
8
8
8
8
8
8
8
8
8

22.79715.709
0.958
8.915
9.392
27.872−0.647−8.618−22.99614.2565.583−8.8561.317−6.989−20.3079.461
32.16126.609−16.717−31.849−21.12−13.863
21.367−9.951−27.122−105.959−96.718−79.709−79.568−79.871−82.851−47.562−35.06−43.325−16.904−0.10613.199in PAUP* version 4.0 beta10 (Swofford 2002)
as was described in Vatanparast et al. (2011).
Bootstrap values were calculated based on
10,000 replicates searched by branch and boundalgorithm. We used sequence data of HSP-90 of
Ipomoea nil and Waxy of Ipomoea argillicola
obtained from GenBank as outgroups, according
to the phylogenetic tree including I. pes-caprae
and those species (Miller et al. 1999). To show
the relationships among haplotypes, a statisticalApril 2014

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Table 2. Nucleotide variations of HSP-90 and Waxy genes of Ipomoea pes-caprae

HSP-90length
(bp)
785

exon

intron

H

Hd

πt

πs

πa

Tajima’s D Fu & Li D*

1–41, 481–785 40–481 7
0.77 0.0026 0.0098 0.0012
0.590
1.041
1–46, 126–301, 47–125,
Waxy
574
5
0.58 0.0016 0.0065 0.0011
0.862
1.555
405–574
302–404
Exon and intron suggest presumed positions in the alignment.
Nucleotide diversities are shown as of all sites (πt), of synonymous sites (πs) and of non-synonymous sites (πa). See
text for detail. H. Number of haplotypes. Hd. Haplotype diversity.

parsimony network was inferred for each locus
using TCS1.06 (Clement et al. 2000).
Results
Nucleotide variation and neutrality tests
Two nuclear loci were sequenced for 272
individuals representing 34 populations of
the species. The aligned length of nucleotide
sequence was 785 bp for HSP-90 and 574 forWaxy (Table 2). The level of polymorphism wascompared between the two loci by nucleotide
diversity, and they are in similar level, but HSP90 was a little more polymorphic (Hd = 0.77
and πt = 0.0026) than Waxy (Hd = 0.58 and πt= 0.0016) (Table 2). The diversity at noncoding
and synonymous sites was higher than thatat nonsynonymous sites. In neutrality tests,
both genes showed no significant departurefrom neutrality in Tajima’s D and Fu and Li’s
D*(Table 2).
Phylogenetic relationships
The total number of haplotypes recognizedin this study was seven (A1–A7) for HSP-90
and five (B1–B5) for Waxy (Table 3, Fig. 2).
Phylogenetic analyses of haplotypes including
outgroups inferred only one most parsimonious
tree (tree length = 6, consistency index (CI) =1.0 and retention index (RI) = 1.0) for HSP-90,
and two trees (tree length = 9, CI = 0.89, RI =0.67) for Waxy (Fig. 2). In the HSP-90 tree, twoclades with bootstrap of 63 % were recognized,
one of which consisted of A4 (orange in Fig. 2a)and A5 (blue), and the other of A6 (green) andA7 (red). In the majority rule consensus tree ofWaxy (Fig. 2d), B2 (green) and B4 (blue) formed
a clade with bootstrap value 69, which is sisterto B3 (yellow) with bootstrap support less than
50 %.
Haplotype Network
In the haplotype network of HSP-90 (Fig.
2b), A7 (red) that is the dominant haplotype of
ssp. pes-caprae was connected with one step toA6 (green). Other minor haplotypes, A1 (purple),
A3 (yellow) and A5 (blue), which appearedin less than 10 samples, were connected with
one step to major haplotypes A4 (orange) or
A2 (black), which appeared in more than 10
samples. In the haplotype network of Waxy (Fig.
2e), minor haplotype B3 (yellow in Fig. 2d) wasconnected with one step to a major haplotype B1(orange). A major haplotype B5 (black) was also
connected with one step to B1 (orange).
Geographic distribution of haplotypes
The geographic distribution of haplotypes
were shown on maps (Fig. 2). In the HSP90 map (Fig. 2c), three major haplotypes, A2(black in Fig. 2c), A4 (orange) and A6 (green),
had wide distributions across more than one
geographic region. A6 (green) was appeared inboth ssp. pes-caprae and ssp. brasiliensis. A2(black) was the only haplotype that appeared in
the American Continents, and was also in the
Pacific, Atlantic and Indian Oceanic regions.
Both A4 (orange) and A6 (green) appeared
widely from Africa to the Pacific regions. A7(red) was the dominant haplotype of ssp. pescaprae, and only appeared in the northern part

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Fig. 2. Phylogenetic trees, network and geographic distribution of haplotypes obtained from HSP-90 (a–c) and
Waxy (d–f) of Ipomoea pes-caprae. a, d. Maximum parsimony trees showing phylogenetic relationships
among haplotypes (see details in text). In figures for each gene, each color designates the same haplotype. b,
e. Haplotype network. Each haplotype is shown as a circle the size of which is proportional to the number ofindividuals possessing the haplotype. Mutational steps connecting haplotypes are represented by small open
circles. e, f. Geographic distribution of haplotypes. Each circle is a pie chart for a population that includes eightindividuals.

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Table 3. Haplotype tables for HSP-90 and Waxy genes of Ipomoea pes-caprae.
Polymorphic sites are designated by positions from the 5’ end in the alignment.
Dot (•) indicates that the character state is same as the one in the first line.
HSP-90Waxy

Haplotype 92 255 272 295 510 683 778
A1
T
G
C
A G
T
A2
.
T
.
T
.
.
.
A3
G
T
.
T
.
.
.
A4
.
T
.
T
.
A
.
A5
.
T
.
T
T
A
.
A6
.
T
.
T
.
.
C
A7
.
T
C
T
.
.
C
40 61 327 382 540
B1
C
G G
A G
B2
T
C
.
G
.
B3
T
.
.
.
.
B4
.
C
.
G
A
B5
.
.
A
.
.

of the Indian Oceanic region. Other minor
haplotypes, A1 (purple), A3 (yellow), and A5(blue) appeared in the West Africa, in East
Africa, and in East Africa and West Australia,
respectively.
In the Waxy map (Fig. 2f), three major
haplotypes, B1 (orange), B2 (green) andB5 (black), appeared across more than one
geographic region. B1 (orange) was shared byboth ssp. pes-caprae and ssp. brasiliensis. B1(orange) appeared from the Atlantic America,
West and East Africa to the Northern Part ofIndian Ocean (as ssp. pes-caprae). B2 (green)
appeared from the East Africa to South Pacific.
B5 (black) appeared from the South Pacific,
West and East side of American Continents
and West Africa. Other minor haplotypes,
B3 (yellow) and B4 (blue) appeared in the
Eastern part of the Indian Oceanic region, andin Oceania to the South Pacific. The number of
haplotypes appeared in geographical regions
were summarized in Table 4.
Discussion
Studying the 272 samples of Ipomoea pescaprae collected from 34 populations around theglobe by two low copy nuclear genes HSP-90
and Waxy, seven and five haplotypes respectively
were detected. The level of polymorphism in the

nuclear genes was much higher than cpDNA in
which no variation was found in the analyses
using more than 4,000 bp from a wide range ofdistribution (Tadashi Kajita, unpublished). The
levels of polymorphism shown by nucleotide
diversity of the markers (πt in Table 2) were
0.0026 for HSP-90 and 0.0016 for Waxy which
was not very different from the variations of
nuclear markers studied in another sea-dispersed
plants Bruguiera gymnorhiza in which averageπt was 0.008 (Urashi et al. 2013). The variationsof the two genes analyzed were not significantly
departed from neutral, hence the two markers
would be used to analyze the demographic
history of I. pes-caprae by using distribution
maps, haplotype trees and networks.
AEP-IWP like geographic structure with small
migration over boundaries
The resultant distribution maps of haplotypesof both nuclear genes suggest that Ipomoea pescaprae has a clear geographic structure: a few
haplotypes (A2 and A1 of HSP-90; B5 and B1 ofWaxy) appeared predominantly in the American
Continents and West Africa, and the others
appeared also predominantly from East Africa
to the South Pacific across Indian Oceanic andPacific regions (Fig. 2). The distribution patternis well consistent with AEP (Atlantic and East

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Table 4. Summary of number of haplotypes of HSP-90 and Waxy in Ipomoea pes-caprae by regions. The columnof “pes-caprae” designates the number of haplotypes obtained from I. pes-caprae subsp. pes-caprae and othercolumns from subsp. brasiliensis
pes-caprae

AfricaAsia-Oceania

Pacific Island

America

HSP-90

2

6

3

3

1

Waxy

1

3

3

3

2

Total

3

9

6

6

3

EA

W IO

E IO

W Pacific

East Pacific

W Atlantic

HSP-90

2

5

3

3

1

1

Waxy

2

2

3

3

1

2

Total

4

7

6

6

2

3

EA. East Atlantic. W IO. West Indian Ocean. E IO. East Indian Ocean

Pacific) and IWP (Indo-West Pacific) regions.
The AEP and IWP regions are biogeographic
regions and the distribution pattern following
them is a common feature of many sea-dispersed
plants including mangroves (Triest 2008). The
AEP region is over the area from the American
Continents, Atlantic Ocean and East Africa, andthe IWP region is the other area of the globe
from East Africa to the South Pacific. Many
sea-dispersed plant species are exclusively
distributed in either region, and the boundaries ofthe distribution are at the African continent andthe East Pacific Ocean. These land and oceanic
barriers are likely to be very strong to prevent
migration between the AEP and IWP regions formany sea-dispersed plants.
In our study, a true-pantropical species,
Ipomoea pes-caprae, has genetic structure
consistent with the biogeographic region AEP
and IWP. This implies that the barriers have beenstrong enough to shape the genetic structure of
the species. However, the barriers would nothave been quite strong enough to prevent thegene flow between the regions, because there
are a few common haplotypes distributed inboth AEP and IWP regions (A2 of HSP-90,
B1 of Waxy in two population of West Africa,
and B5 of Waxy in a population of the South
Pacific). These results will suggest that the seadispersal ability of Ipomoea pes-caprae would
be sufficient to override the barriers and tokeep gene flow over the barriers, even though

population differentiation happened. Violations
to the regular AEP-IWP distribution patternwere also observed in other two groups of subpantropical species, Hibiscus tiliacues – H.
pernambucensis (Takayama et al. 2006, 2008)
and Rhizophora species (Takayama et al. 2013).
Gene flow is kept across the extremely wide
distribution by long distance dispersal of seadrifted seeds
In spite of the genetic structure between IWP
and AEP regions, most of the major haplotypesof Ipomoea pes-caprae were extremely widelydistributed, which would be caused by dispersal
by sea-drifted seeds. In the distribution maps
(Fig. 2), major haplotypes except for A7 of
HSP-90 were distributed over two or more
oceanic regions, with the widest distributionof A2 of HSP-90 in the Atlantic, Pacific andIndian Oceanic regions. These extremely wide
distributions of haplotypes overriding the two
major barriers for other sea-dispersed plants
suggests that this species has very effective long
distance dispersal by sea-drifted seeds. Indeed,
our preliminary study suggested that the seedsof Ipomoea pes-caprae could float on seawater
for two years, retaining viability (Yoichi Tateishi
unpublished data).
The relationship between the two subspecies
The results of this study may suggest thatthe origin of ssp. pes-caprae was from ssp.

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brasiliensis. A major haplotype A7 of HSP90 specifically appeared in ssp. pes-caprae
and it was monophyletic with A6 that is oneof the major haplotypes of ssp. brasiliensis
distributed in the Indian Oceanic and Pacificregion and partially appeared in ssp. pes-caprae(Figs. 2a, 2c). Waxy gene also showed that thetwo subspecies shared the same haplotype B1,
which may suggest the diversification of the two
subspecies might not be very old. Subspecies
pes-caprae might be diversified from ssp.
brasiliensis at the northern margin of the
distribution of ssp. brasiliensis in the northernpart of the Indian Oceanic region. This kind of
speciation at the edge of the wide distribution
range of pantropical plants with sea-drifted seeds
were also reported in Hibiscus tiliaceus and its
allied species (Takayama et al. 2006).
Implication on the route of population expansion
The present distributions of haplotypes andphylogenetic trees as well as networks suggests
the route of population expansion might befrom Africa or Indian Oceanic region to the
Pacific and Atlantic regions. The number of
haplotypes appeared in geographical regions
were summarized in Table 4 and showed thetendency that higher number of haplotype inAfrica and Indian Oceanic regions and lower
in the New World. In both genes, the New
World populations are almost fixed to a single
haplotype in each DNA region (A2 for HSP90 and B5 for Waxy in Fig. 2), which suggests
relatively newer origin of the new world
population from small founder populations
that have experienced severe bottleneck, or the
populations were under severe selection. In the
haplotype network of Waxy (Fig. 2e), the newworld haplotype B5 is closer to B1 (1 step) of
the Atlantic and Indian Oceanic regions than toB2 (4 steps) in the Pacific and Indian Oceanic
regions. This suggests an expansion from Africa
or Indian Oceanic region both westward andeastward.
We need to pay special attention to the73

similar distribution patterns of haplotypes (A1in HSP-90 and B5 in Waxy in Fig. 2c, f) acrossthe New World. Although our data suggest thatthe establishment of the New World populationis newer, the Isthmus of Panama was closed
about 3.1–3.5 million years ago (Knowlton etal. 1993, Marko 2002). Sea-drifted seeds could
not be dispersed through the Isthmus of Panama
after the closure. So, the presence of the same
haplotypes in both Pacific and Atlantic sides of
the American Continents would be establishedeither by migration of seeds 1) across the Pacific,
Indian and Atlantic Oceans or 2) through theIsthmus of Panama by other vectors like animalor human. We still don’t have enough data toanswer which case is more plausible, but further
analyses increasing the number of markers andsamples may answer this question. Gene flow
between the Pacific and Atlantic side populations
were also reported using microsatellite markers
in other sub-pantropical species, Hibiscus
pernambucensis (Takayama et al. 2008) andRhizophora mangle (Takayama et al. 2013).
Conclusion
This is the first phylogeographic study ona true pantropical plant with sea-drifted seeds,
Ipomoea pes-caprae. Our data using two nuclear
genes showed significant genetic differentiation
between the IWP and AEP regions, anddistribution of the same haplotypes throughout
the two regions. These results suggest that
dispersal by sea-drifted seeds was responsible forgene flow across the wide distribution range, andthat both the East Pacific and African Continent
acted as geographical barriers for the migration
by drifted seeds even for the true pantropicalplants. Presence of more haplotypes in Africa
and Indian Oceanic region may imply that theorigin of the species was in these regions, andthat the species expanded its distribution to the
New World.
This work was supported by the Japan
Society for the Promotion of Science (JSPS)

74

植物研究雑誌

KAKENHI (nos. 22405005, 19370032,
16370043 and 11740467 to TK; nos. 14405015
and 12575011 to YT), and field works werepartially supported by a Grant-in-Aid by Yamada
Science Foundation 2005 and Fujiwara Natural
History Foundation 1999 (to TK). We thank Drs.
Y. Abe, Alessandro, G. Ameka, C. Baider, M.
Bovini, D. Darnaedi, H. C. de Lima, G. Flora,
J.A. Jiménez, M. Kato, Y. Kita, D. A. Madulid,
S. H. Meenakshisundaram, R. Nagahisma, S.
Nakata, E. Narvaez, D. Neil, T. Ohi-Toma, K.
Watanabe-Toma, E. Reynoso, J. P. Roux, A.
Santiágo, G. I. Seneviratne, A. Tidiane Ba, N.
Wakita, JBRJ (Brazil), CNPq (Brazil), MPEG
(Brazil), MINAE (Costa Rica), OTS (Costa
Rica), QCNE (Ecuador), M.S. Swaminathan
Research Foundation (India), University ofColumbo (Sri Lanka), STRI (Panama), MEXU
(Mexico), NOU (New Caledonia) and National
Museum (Philippines) for their support in fieldsampling.
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75

M. Miryeganeha，高山浩司 b，立石庸一 c，梶田 忠 a：
核マーカーによって示された汎熱帯海流散布植物グン
バイヒルガオ（ヒルガオ科）の地理的構造
汎熱帯海流散布植物とは，熱帯域を中心に地球を一周するほど広い分布域を持つ，海流散布植物の総称である．その構成要素は，グンバイヒルガオやナガミハマナ
タマメ等，1 種のみで汎熱帯域に広がる分布域を持つ汎熱帯種と，オオハマボウとアメリカハマボウ，あるいは
ハマアズキとナガバハマササゲ等，互いにごく近縁ない
くつかの種の分布域を合わせると汎熱帯域になる準汎熱帯種の，2 つのグループに分けられる．本研究では，
汎熱帯海流散布植物のうち，代表的な汎熱帯種であるグ
ンバイヒルガオ Ipomoea pes-caprae（ヒルガオ科）が，広大な分布域内でどのような遺伝構造を持つかを，2 つの核遺伝子マーカー（HSP-90 と Waxy）を用いて明らかにした．分布域を広く網羅するような地域から採集された 34 集団 272 個体で 2 つの遺伝子の塩基配列を決定し，系統解析，ネットワーク解析，ハプロタイプ分布の比較を行ったところ，IWP（インド洋─西太平洋地域）と AEP（大西洋─東太平洋地域）にそれぞれ特有のハ

プロタイプが分布する，明瞭な遺伝構造の存在が明らかになった．このことは，アフリカ大陸と東太平洋が，IWP と AEP の間の種子の移動を妨げていることを示唆している．一方，それぞれの地域内では，海で遠く隔てられた集団間でも同一のハプロタイプが見られることから，海流による長距離種子散布が，広大な分布域内における遺伝子流動の維持に貢献していることが示唆された．また，ハプロタイプの数を地域ごとに比較したところ，アフリカ大陸とインド洋地域で多く，新大陸ではほぼ１つに固定しており，アフリカ大陸かインド洋地域
から新大陸への移動が生じたことが示唆された．一方，新大陸東西の集団間でも共通のハプロタイプが分布していることから，パナマ地峡を越えるような遺伝子流動が起こった可能性も示唆された．
（a 千葉大学大学院理学研究科生物学コース，b

東京大学総合研究博物館，c
琉球大学教育学部）