The current hybridization between the endemic Morus boninensis
Koidz. and the invasive Morus australis Poir. in the Ogasawara Islands
Wita YULIANTI1,2, *, Saeko KATOH1, Tao FUJIWARA1, Hidetoshi KATO1& Noriaki MURAKAMI1
1. Makino Herbarium, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji
City, Tokyo 192-0397, Japan
2. Visiting researcher, Tokyo Metropolitan University*

Author for correspondence: E-mail witaunand@gmail.com

Abstract:
Impacts of invasive alien plant species on its closely related endemic ones inoceanic islands are often problematic because hybridization may occur between them.
This study focused on endemic Morus boninensis (tetraploid of 2n=56) and its related
invasive species M. australis (diploid of 2n=28) in the Ogasawara Islands. Aim of thisstudy is to estimate proportion of the hybrids between the two species in wild seedlingof native habitats of M. boninensis and to evaluate the current genetic diversity of M.
boninensis. Observations and collections of plant samples were conducted in two wild
seedling populations, two mature tree populations of M. boninensis and 12 populationsof M. australis in the Ogasawara Islands. The 14 microsatellite markers were used for
population genetic analyses. Based on the results of UPGMA analysis, the Principal
Coordinate Analysis (PCoA), and the Bayesian Clustering, clear genetic differentiation
was detected between the two species in the Ogasawara Islands. Gene flows between
them unlikely or rarely occur maybe due to the difference in ploidy level. Hybrids were
not found in the seedling populations, either. Limited effects of M. australis to theendangered M. boninensis through hybridization were detected in this study. It was also
shown that M. boninensis still maintains high genetic diversity (HE = 0.53; AR = 3.27)
despite that it is endangered species. It may be able to survive if the number of theindividuals of the endangered species could be increased.

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Key words
Conservation, Genetic diversity, Microsatellite markers
Introduction
Oceanic islands often have an exceptional rank in global biodiversity conservation
due to their high degree of endemism (Hobohm, 2000; Whittaker & Fernández-Palacios,2007). Oceanic islands are volcanic islands in the ocean such as the Hawaiian,
Galapagos and Canary Islands, which have never been connected to large landmasses.
They usually have unique land biota. For example, 524 endemic vascular plant species
were found in the Canary Islands (Fernández-Palacios & Whittaker, 2008).
Consequently, oceanic islands contribute to a disproportionately high degree to global
biodiversity despite their relatively low percentage of surface area and limited numberof species growing in the islands (Cowie & Holland, 2006).
The Ogasawara Islands in Japan are one of the oceanic islands and have many
endemic plant species, which should be carefully conserved. The islands locate about
1,000 km south of the main Island of Japan, and consist of several island groups,
Chichijima, Hahajima and others with total terrestrial area of 63.58 km2 (Government ofJapan, 2010). The Ogasawara Islands have been noted for their distinctive flora.
Kobayashi & Ono (1987) reported 112 endemic land plant taxa in the islands, andamong which 40% of vascular plant species, including 70% of the tree species, areendemic (Toyoda, 2003). In a recent report, the number of endemic plant taxa reached
161 of a total number of 441 vascular plant species and 32% (144 taxa) of them wererecognized as endangered (Government of Japan, 2010). As oceanic islands, ecosystemof the Ogasawara Islands has already suffered from various negative effects by humanimpacts, biological invasions and others. To control the effects, several studies forestablishing relevant management plans have been conducted in the Ogasawara Islands(Hata et al., 2006; Osawa, 2019), but more information are still needed to conserve theendemic species in the islands.
Morus boninensis is one of the endemic plant species in the Ogasawara Islands
that need careful conservation. It is dioecious tree species whose pollen and seeds are
dispersed by wind and birds, respectively and tetraploid (Tani et al., 2003). Its
population size has been reduced by over-exploitation before World War II
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(Government of Japan, 2010) since the Japanese government permitted free logging toencourage immigration to the islands during the last quarter of the 19th century andearly 20th century (Tani et al., 2003). Due to this, it is currently categorized as
endangered species (Environment Agency of Japan, 2000; Government of Japan, 2010;
Tani et al., 2006). Its distribution range is now limited to several small areas ofChichijima, Hahajima and Otoutojima (Toyoda, 2003, 2014). Therefore, the current
conservation status of M. boninensis needs to be investigated for establishing better
conservation strategies. Especially, the main concerns about the conservation of M.
boninensis at this moment are i) effects of M. australis, an Invasive Alien Species (IAS)
closely related to M. boninensis and ii) an expected decline of genetic diversity due tothe reduction of its population size.
IAS closely related to native species such as M. australis are expected to havestrong impacts on the native species by hybridization (Mooney & Cleland, 2001).
Hybridization between IAS and native species not only helps IAS to locally adapt totheir new environments by incorporating beneficial alleles from the native species(Prentis et al., 2008) but also make species boundary between them obscure and replacepure strains of native species with admixed ones (Levin et al., 1996). Morus australis,the IAS closely related to endemic M. boninensis, was introduced to the Ogasawara
Islands for sericulture of silkworm Bombyx mori (Awasthi et al., 2004; National
Institute for Environmental Studies, Japan, 2017; Tokyo Prefecture Ogasawara Island
Branch Office, 1938). Because it recently expanded even into conserved areas in theislands where M. boninensis is distributed, even after the cultivation of M. australis has
been stopped. It is the reason why M. australis has been recognized as IAS (National
Institute for Environmental Studies, Japan, 2017).
The hybridization between M. boninensis and M. australis in the Ogasawara
Islands have been reported (Tani et al., 2003, 2006). They suggested that the
hybridization between the two Morus species potentially leads to loss of purity ofendemic M. boninensis and the hybrids should be removed from the seedling pools. It isbecause the previous study focused only on artificially germinated seeds collected froma single female tree of M. boninensis and did not check any naturally growing seedlings
or mature trees in wild conditions (Tani et al., 2003, 2006). Thus, it is very urgent toconfirm hybrids in wild seedlings and mature trees in the Ogasawara Islands to clarify
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the present conservation status of M. boninensis.
In addition, evaluation of levels of genetic diversity in endemic M. boninensis isnecessary.

Genetic

diversity

reduction

leads

to

maladaptation tochanging

environmental conditions and are susceptible to extinct even after small alterations in
the environments (Dostálek et al., 2010; Isagi et al., 2020). Endemic species in islands
are usually illustrated by small and isolated populations (Dostálek et al., 2010), andoceanic insular species often possess lower genetic diversity than their continental
relatives (Frankham, 1997). Moreover, population size reduction is expected to cause
decreasing of the genetic diversity. Given that oceanic endemic species, M. boninensis
have experienced rapid declines in their individual number and population size, thespecies is expected to currently exhibit very low level of genetic diversity. Therefore,
investigation of the level of genetic diversity in M. boninensis is urgently necessary
from conservation aspects. In general, a proper evaluation of genetic diversity in target
species is challenging due to a lack of appropriate closely related taxon to compare. In
the previous study, Tani et al. (2006) analyzed M. boninensis populations using genetic
markers without any congeners. However, in our previous study (Yulianti et al., 2021),
it was revealed that M. australis in Ogasawara possesses similar amount of genetic
diversity to the populations in its native range. This result enables us to describe andevaluate the levels of genetic diversity in M. boninensis by comparing with those of M.
australis in the Ogasawara Islands and other native populations of the invasive species.
In this study, for better understanding of conservation status of endemic Morus,
population genetic analysis was conducted for both of endemic M. boninensis andinvasive M. australis growing in the Ogasawara Islands. Specifically, the following
questions were addressed here: 1) What actual impact does hybridization with invasive
M. australis have on native M. boninensis? 2) What level of the current genetic
diversity is observed in M. boninensis in the Ogasawara Islands?

Materials and methods
Plant materials of two populations of M. boninensis (CMB, HMB), one of M.
australis (CHK) and two wild seedling populations (HSE, HUS) were collected during
2019-2020, and they were analyzed together with materials of M. australis from 12
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populations which were used in previous study (Yulianti et al., 2021) (Table 1 and Fig.
1). All samples except in seedling populations were sampled from mature trees. Bothpopulation of M. boninensis on Chichijima (CMB) and on Hahajima (HMB) were foundneighboring to M. australis (CHK) and to seedling (HSE) population, respectively.
Individuals of the seedling populations containing some juvenile trees were collectedwithout identifying the taxa. Fresh-young leaves in the apical stems were collected fromeach individual in order to obtain total DNAs with the best quality. The collected leaves
were stored in plastic bags with silica gel to keep them dry until DNA extraction.
Voucher specimens were also collected during the sample collection and deposited in
the Makino Herbarium of Tokyo Metropolitan University (MAK) (Table 1).

Fig.1 Map of sampling localities for M. boninensis (black circle), M.
australis (open square) and wild seedling populations (gray diamond) in
the Ogasawara Islands. Population IDs are shown in Table 1.

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Extraction of total DNAs and amplification of microsatellite markers wereconducted using the same methods as reported by Yulianti et al. (2021). Total of 14
microsatellite primers developed for M. boninensis (Tani et al., 2005) and for M. indica(Aggarwal et al., 2004; Krishnan et al., 2014) were used for population genetic analyses.
The amplified DNA was examined by using the ABI 3130xl genetic analyzer. The
obtained data were processed by using the GeneMapper software version 4.0 supplied
by ABI to determine the genotype of each sample. Since M. boninensis is a tetraploid
species, the genotype determination for the species was based on the dosage of eachallele that implied partial heterozygotes, following Tani et al. (2005) and Tsuda et al.
(2017). For example, when only one allele is found then written as AAAA, when fouralleles are found then written as ABCD, when two alleles are found then written as
AABB, and when three alleles are found then the one allele with the highest peak written
double, such as AABC.
To show the genetic similarity among populations of M. boninensis, M. australis
and seedling, UPGMA dendrogram based on Nei’s genetic distances, DA (Nei et al.,1983) was drawn by using Populations 1.2.30 (Langella, 1999) and FigTree v.1.4.4(Rambaut, 2018). The Principal Coordinate Analysis (PCoA) based on pairwise FST
values was conducted by using GenAlEx 6.503 (Peakall & Smouse, 2012).
The Bayesian clustering analysis was conducted to evaluate genetic structure andto detect hybrids by using the software STRUCTURE version 2.3.4 (Falush et al., 2003,
2007; Pritchard et al., 2000). All investigated population were tested together by using
the setting number of burn-in 100000 and MCMC 100000 which K = 1-19 by applied
10 runs for each K value. The ∆K value was adopted through Evanno’s method (Evanno
et al., 2005) by using STRUCTURE HARVESTER (Earl & vonHoldt, 2012). Results
were visualized with software CLUMPP 1.1.2 (Jakobsson & Rosenberg, 2007) andDistruct 1.1 (Rosenberg, 2004).
The allele frequencies of M. boninensis, M. australis and seedlings were obtainedby SPAGeDi software (Hardy & Vekemans, 2002). Prior to this analysis, the putative
admixed individuals in each population were excluded based on the result of
STRUCTURE analysis mentioned above. Number of alleles per locus (NA), expected
heterozygosity (HE), allelic richness (AR), and fixation index (FIS) were also calculated
as genetic parameters by SPAGeDi software.
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Table 1 Sampling localities and genetic parameters of M. boninensis, M. australis and seedlingin the Ogasawara Islands.
IslandLocation
M. boninensis
Chichijima Higashi-Kaigan
Hahajima
SekimonPop ID Lat. (°N) Long. (°E) Voucher ID

N

NA

HE

AR

F IS

CMB
HMB

27.060226.6841142.2273
142.1614

M. australis
Chichijima Mikazuki
Yoakeyama
Hatsuneura
Fukiagedani
Tatsumidoro
Higashi-Kaigan
Kitafukurozawa
Siguredamu

MAK467151
MAK467152
Mean

17
4.43
8
3.71
12.50 4.07

0.53
0.52
0.53

3.32
3.21
3.27

0.06
-0.03
0.02

CMK
CYO
CHT
CFK
CTT
CHK
CCP
CSG

27.0986
27.0856
27.077027.0755
27.0640
27.0649
27.060227.0599142.1869
142.2172142.2177
142.2087
142.2199
142.2276
142.2031
142.2068

MAK467119
MAK467124
MAK467125
MAK467120
MAK467123
MAK467153
MAK467122
MAK467121

21
12
11
23
15
15
12
16

5.86
5.64
4.71
5.64
5.07
4.21
4.86
4.57

0.66
0.68
0.59
0.61
0.60
0.58
0.60
0.59

4.55
4.89
4.30
4.28
4.24
3.69
4.39
3.93

0.08
0.14
0.21
0.04
0.02
0.14
0.11
0.06

KitakoSekimonKuwanokiyama
Nakanotaira
Minamizaki

HKT
HSK
HKW
HNK
HMN

26.6963
26.674426.6638
26.632126.6114142.1436
142.1562
142.1520
142.1736
142.1762

MAK467128
MAK467129
MAK467130
MAK467127
MAK467126
Mean

25
33
13
14
22
17.85

5.93
6.21
5.93
5.14
5.36
5.32

0.63
0.63
0.65
0.61
0.58
0.62

4.26
4.40
4.98
4.33
4.07
4.33

0.08
0.14
0.11
0.06
0.11
0.10

SekimonUchusawa

HSE
HUS

26.6846
26.6649

142.1614
142.1566

MAK467154
MAK467155
Mean

52
7.21
8
4.00
30.00 5.61

0.65
0.60
0.63

4.56
4.00
4.28

0.04
-0.05
-0.01

Hahajima

Seedling
HahajimaN, number of individuals; NA, number of alleles; HE, expected heterozygosity; AR, allelicrichness; FIS, fixation index.

Results
All the used microsatellite loci showed polymorphisms for both the species. A
total of 200 alleles were obtained from the 14 loci. The highest number of alleles was
obtained from Mos0340-2 and MulSSR 258 (22 alleles), followed by MESTSSR 31 and
Mos0031 (19 alleles), MulSSR 338 (18 alleles), MulSSR 313 (16 alleles), MulSSR 147
and MulSTR 2 (15 alleles), Mos0157-2 (13 alleles), MESTSSR 48, MulSTR 4 (11alleles), MESTSSR 126 (eight alleles), and MUL3SSR6 (six alleles). The fewest
number of alleles was detected by MESTSSR 73, which showed only five alleles.
The genetic similarity among all the investigated population was revealed in the
UPGMA dendrogram based on Nei’s genetic distances, DA (Fig. 2). Two clusters wererevealed in the dendrogram. The first cluster comprises of all the population of M.
boninensis. The second cluster comprises of all the population of M. australis andseedling/juvenile populations. The result of the Principal Coordinate Analysis (PCoA) is

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shown in Fig. 3. The first axis showed 68.86 % of the total variation. This axis clearly
distinguished the two species, M. boninensis and M. australis. The seedling population
formed a group together with M. australis (Fig. 3). The second axis showed only 7.96 %of the total variation. There was no obvious groping found along this axis either
between the two species or between populations in the island groups of the Ogasawara
Islands.

Fig.2 The UPGMA dendrogram among all populations of M. boninensis, M.
australis and seedling in the Ogasawara Islands based on Nei’s DA genetic
distance (Nei et al., 1983). Underline represents seedling populations.

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Fig.3Result of the PCoA among all
populations of M. boninensis, M.
australis and seedling in the Ogasawara
Islands based on FST values.

a)b)Fig. 4 Result of Bayesian clustering (STRUCTURE) analysis of M. boninensis, M. australis
and seedling. a) delta K as number of clusters, b) Bar plot at K = 2. Each bar represents an
individual of Morus in this study, and the heights of the bars of each color are proportional tothe posterior means of the estimated admixture proportions. Locations of population are
shown in Fig. 1 and Table 1.

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The result of the STRUCTURE analysis for M. boninensis and M. australis isshown in Fig. 4. The ∆K value was maximal at K = 2. Based on this maximal K value,
two clusters corresponding to the two species, M. boninensis and M. australis wereobserved. No hybrid individual was detected in seedling populations. However, eightindividuals admixed between two clusters were found in the populations of M.
boninensis (HMB) and M. australis (HKW). Two individuals found in HMB exhibitedthe genetic admixture in close to 2:1 ratio (0.62:0.38 and 0.68:0.32) between the M.
boninensis (dark gray) and the M. australis (light gray) clusters.
A relatively high level of genetic diversity was observed for all the population of
M. boninensis and M. australis even after excluding all the putative hybrids. The mean
genetic diversity for M. boninensis (HE = 0.53, AR = 3.27) was almost the same as thoseof M. australis (HE = 0.62, AR = 4.33) and those of seedling populations (HE = 0.63, AR =4.28). The values of heterozygosity (HE) for M. boninensis were 0.52 and 0.53, while in
M. australis, it ranges from 0.58 to 0.68, and for seedling populations, 0.60 and 0.65,
respectively. The allelic richness (AR) for M. boninensis were 3.21 and 3.32, it ranged
from 3.69 to 4.98 in M. australis, and 4.00 and 4.56 in seedling populations (Table 1).

Discussion
Genetic research on hybridization between native species and IAS are necessary
specially to conserve endangered native species. As mentioned in the introduction part,
IAS is hypothesized to have the strong impacts on native closely related species byhybridization (Mooney & Cleland, 2001). Such interspecific hybridization with native
species not only enables IAS to assimilate the invaded area by introduction of the
genotype adapted to the area (Prentis et al., 2008) but also replace pure strains of thenative species with admixed ones, namely ‘genetic contamination’.
In this study, extent of hybridization between an endangered species, M.
boninensis and its closely related IAS, M. australis in natural condition of the
Ogasawara Islands was investigated. Tani et al. (2003) investigated the extent of thehybridization between the two species by examining a proportion of hybrids in embryosof seeds collected from a mother tree of M. boninensis. They detected hybrids in some

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of the artificially germinated embryos and concluded that the hybridization with M.
australis has a strong negative impact on the regeneration of native M. boninensis. In
their study, however, seedlings and mature trees were not investigated in the natural
condition and the seeds used were collected only from a single female tree. Thus, theimpact of the hybridization in wild still remains unclear. In this paper, naturally growing
Morus individuals were collected by especially focusing on seedlings and juveniles to
examine the impact of hybridization in natural conditions.
Of the 60 seedlings and juveniles from two populations, all individuals wereidentified as M. australis and no hybrid was suggested even though seedling population
at Sekimon (HSE) was adjacent to M. boninensis population (HMB) (Figs.1 & 4).
Considering high proportion (13%) of hybrids in the artificially germinating seeds of M.
boninensis was reported (Tani et al. 2003), the hybrid seedlings might not be able tosurvive and/or grow in wild conditions. Although there is no direct evidence, natural
selections against seedlings/juveniles of hybrids may act in the Morus species.
On the other hand, several individuals genetically admixed between the two
species, possibly F1 hybrids, were found in the populations of the mature trees (Fig. 4).
Two individuals found in HMB exhibited the proportions of genetic admixture between
the M. boninensis cluster (dark gray) and the M. australis cluster (light gray) as close to
2:1 ratio. Given that M. australis and M. boninensis are diploid and tetraploid species,
respectively, the proportion of genetic admixture in F1 hybrids is expected to be 1/3 oftheir genetic constitution from diploid M. australis and 2/3 from tetraploid M.
boninensis. Thus, these two individuals can be interpreted as triploid F1 hybrids
between them. In addition, the admixed individuals found in HKW population of M.
australis exhibited various patterns of genetic admixture differentiated from that intypical F1 hybrid mentioned above: Two showing similar proportions genetically close
to pure strain of M. boninensis and the other four showing more admixed from M.
australis than the expected ratio for the F1 hybrids.
These possible hybrid individuals other than F1 can be explained by following two
hypotheses. Firstly, the two hybrids genetically close to the pure strain of M. boninensis
could be artifacts resulting from the possession of shared alleles between the two
species. They could be interpreted as pure strain of M. boninensis. If this is true, theother four hybrids can be also considered as F1 hybrids with more shared alleles
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between M. australis and M. boninensis. Thus, the presence of the shared alleles might
have caused the pattern observed in HKW population. Secondly, the hybrids could be
those backcrossing with parental species. A few studies reported interploidal gene flowin angiosperms (e.g., Betula by Tsuda et al., 2017 & Wang et al., 2014; Arabidopsis by
Arnold et al., 2015). In general, however, triploid rarely produces viable gametes due tofailure in homologous chromosome pairing during meiosis. Thus, this hypothesis might
be unlikely. In future study, these hypotheses may need to be tested by incorporating
population genetic analysis with cytological analysis such as those using flow
cytometry.
In conclusion, our study showed no hybrid seedlings in wild condition and noevidence of the expected genetic contamination in M. boninensis populations despite of
the presence of some F1 hybrids. Thus, the degree of hybridization between native M.
boninensis and alien M. australis in the Ogasawara Islands is relatively more limited
than previously expected.
The level of genetic diversity within M. boninensis, an endangered endemic plant
species in the Ogasawara Islands, was observed almost same as that within its widelydistributed relative, M. australis. Reductions of genetic diversity in insular endemics
compared to continental counterparts (ex. related taxa in the mainland) have been
discussed in several previous studies (Frankham, 1997; García-Verdugo et al., 2015;
Stuessy et al., 2014). The island endemics are assumed to display low levels of genetic
diversity (Stuessy et al., 2014). However, the mean level of heterozygosity and allelicrichness of M. boninensis (HE = 0.53; AR = 3.27) was only slightly lower than those of M.
australis in the Ogasawara Islands (HE = 0.62; AR = 4.33) in this study (Table 1). Morus
australis is widely distributed from the Himalayas to Japanese archipelagos including the
Ryukyu and Izu Islands of Japan. Genetic diversities of M. australis in other areas ofJapan ranged from 0.51 to 0.73 (HE) and from 3.70 to 5.80 (AR) (Yulianti et al., 2021).
Frankham(1997)

investigatedgenetic

diversity

reductions

bycomparingheterozygosities in populations of island endemic plants (HIS) to those of mainland
related species (HM) based on allozyme data, and the ratio (HIS/HM) was 0.54. In thisstudy, the ratio between endemic M. boninensis and wide-spread M. australis was 0.85.
It suggests that M. boninensis maintains high levels of genetic diversity and it does notseem to follow tendency of genetic diversity reducing in insular endemics.
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No reduction of genetic diversity in M. boninensis may be attributed to biologicalcharacteristics of this species. Stuessy et al. (2014) suggested biological characteristics
influencing levels of genetic variation in insular endemics. For example, the generation
time (Stuessy et al., 2014) regulates the development of genetic variation within
populations by influencing the rates of recombination. As another possible reason,
outcrossing of dioecious plant might be suitable for maintenance of higher levels ofgenetic diversity (Government of Japan, 2010; Paschoa et al., 2018). On the other hand,small population size was thought as negatively affecting to maintain genetic diversity.
Morus boninensis is dioecious and produces many seeds which are dispersed by birds,
suggesting this species is predominantly outbreeder. Additionally, the generation time of
M. boninensis seems very long. A stump over 2000 years inferred from annual rings wasreported in the Ogasawara Islands (Toyoda, 2003). These characteristics are suitable toaccumulate and maintain genetic diversity within the species. However, the population
size of M. boninensis rapidly declined because the trees had been heavily logged during
the last quarter of the 19th century and the start of 20th century with encouragement by
Japanese government (Tani et al., 2003). This reduction in population size is likely tolead to the loss of genetic diversity within M. boninensis in near future.
Tani et al. (2006) also reported that there was no observation of seedlingrecruitment since 1995. In this study any seedlings or juveniles of M. boninensis were
not detected even in adjacent site to mother (mature) population on Hahajima (Fig. 4).
This suggests that M. boninensis failed any regenerations in recent 20 years, despite
intensive efforts to the conservation of the species. In current condition, although M.
boninensis was maintained relatively high amount of genetic diversity compatible to M.
australis, M. boninensis will decline if no seedling recruitment will occur in future.
Acknowledgements
We would like to thank Dr. Adam L. Cronin and Dr. Katsuyuki Eguchi of Tokyo
Metropolitan University for their extensive suggestion on our study. We thank Dr. Yuhki
Murakami, Dr. Francesco Ballarin and Dr. Isaac Planas Sitja for their assistance during
the analyses. Thanks are also given to Dr. Suichiro Tagane, Dr. Kiyotaka Hori, Dr.
Noriyuki Fujii, Mr. Takeshi Kikuchi, Ms. Atsuko Maeda, Ms. Yasuko Ishida, Mr. Kenya
Ishida, Mr. Masafumi Goto, Mr. Izumi Takenaka and Mr. Wahyu Wiratul to help us
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during collection of materials. We thank Tsukuba Botanical Garden for giving the
support to conduct our SSR analyses. We also would like to thank Tokyo Metropolitan
Government who provided the scholarship for WY. This work is a part of the doctor
thesis of WY. This study was partly supported by JSPS KAKENHI Grant No.
17H04609 and the budget for the Strategic Research Project from Tokyo Metropolitan
University to NM.

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ABSTRACT IN JAPANESE
小笠原諸島における固有種オガサワラグワと侵略的外来種シマグワとの交雑の現状

ウィタ ユリアンティ、加藤 朗子、藤原 泰央、加藤 英寿、村上 哲明
東京都立大学理学研究科生命科学専攻（牧野標本館）

侵略的外来種が近縁な自生種に与える悪影響は、それらの間での交雑が起きる可能性があるため、特に固有種が多い海洋島で深刻である。本研究では、小笠原諸島の固有種であるオガサワラグワ（Morus boninensis, 4 倍体）と、それに近縁な侵略的外来種のシマ
グワ（M. australis,２倍体）に焦点を当てた。この研究の目的は、オガサワラグワの自生地の実生集団におけるシマグワとの雑種の割合を推定すること、ならびにオガサワラグ
ワの種内の遺伝的多様性を評価することである。本研究では、小笠原諸島におけるクワ属植物の 2 つの野生実生集団、オガサワラグワの 2 つの成熟個体集団、および 12 のシマ
グワの成熟個体集団で観察と植物サンプルの採集を行い、14 個のマイクロサテライト・
マーカーを用いて集団遺伝学的解析を行った。UPGMA 解析、主座標分析（PCoA）、お
よびベイズ・クラスタリング解析の結果、小笠原諸島産のこれら 2 種の間で明確な遺伝的分化が認められた。そして、おそらく倍数性レベルの違いによって、２種間の遺伝子
流動はほとんど起きてはいなかった。雑種は、実生集団でも全くみられなかった。シマグワの交雑を通じての固有種オガサワラグワへの悪影響は、このように限定的であることが分かった。さらにオガサワラグワは、絶滅危惧種であるにもかかわらず、種内に高い遺伝的多様性（He = 0.53; AR = 3.27）を維持していることも明らかになった。オガサワ
ラグワの個体数を増やすことができれば、この種を絶滅から救える可能性がある。

キーワード
遺伝的多様性、保全、マイクロサテライト・マーカー235− 235 −

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