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Proc. Arthropod. Embryol. Soc. Jpn. 47, 33–35 (2012)Ⓒ 2012 Arthropodan Embryological Society of Japan
ISSN 1341–1527

The Origin and Distribution Expansion of Unisexual Populations in the
Geographically Parthenogenetic Mayfly, Ephoron shigae(Insecta: Ephemeroptera: Polymitarcyidae)*Kazuki SEKINÉ1) and Koji TOJO2)
Division of Insect Sciences, National Institute of Agrobiological Sciences, 1–2, Owashi, Tsukuba, Ibaraki 305–8634, Japan
Department of Biology, Faculty of Science, Shinshu University, 3–1–1, Asahi, Matsumoto, Nagano 390–8621, Japan
E-mail: ktojo@shinshu-u.ac.jp (KT)1)
2)

The burrowing polymitarcyid mayfly Ephoron shigae(Takahashi) is distributed widely in Japan, and it is also
reported to be present in Korea and Far East Russia. Some
populations are bisexual, while others are unisexual(solely females). As such E. shigae is a geographicallyparthenogenetic mayfly (Watanabe and Ishiwata, 1997;
Tojo et al., 2006). In general, parthenogenetic populations
are found in harsher environments such as at high latitudes
and altitudes, in xeric as opposed to mesic conditions(Suomalainen et al., 1987), in isolated habitats such as
islands and island-like areas, and in the peripheral regionsof the taxonʼs range (Cuellar, 1977). In E. shigae, however,the distributions of bisexual and unisexual populations
overlap broadly in their respective geographic ranges
through Japan (Honshu, Shikoku, and Kyushu). E. shigae
therefore, provides a uniquely well-suited model to study
the differentiation of unisexual and bisexual populations,the establishment of parthenogenesis, and the dispersal ofparthenogenetic individuals.
In a previous study on E. shigae, obligatory
parthenogenesis (i. e., parthenogenesis is the normal
mode of reproduction) appeared in unisexual populations(Tojo et al., 2006). Furthermore, all the individuals that
reproduced by parthenogenesis were diploid females(2n=12; sex chromosome type was XX), indicating
thelytokous parthenogenesis (Sekiné and Tojo, 2007,2010). The restoration mode into diploidy inparthenogenetic eggs is automictic in that the female
pronucleus fuses into the second polar body following thecompletion of meiosis.
In this study, we have examined whether theunisexual populations are of polyphyletic or monophyletic
origin, as inferred from partially sequenced mitochondrial
16S rRNA (373 bp) and COI (658 bp) regions (Fig. 1).
Although various haplotypes [16S rRNA: 29 haplotypes,
COI: 58 haplotypes] were observed among the bisexual
populations, only one haplotype each (16S rRNA: one
haplotype, named “16S_s1”; COI: one haplotype, named“COI_s1”) was found among the unisexual populations
through Japan widely. The “16S_s1” and “COI_s1”
haplotypes were allocated to each of the western Japaneseclades (group I; Fig. 1). Therefore, the unisexual populations
were of monophyletic origin and recently differentiatedsomewhere within western Japan. Furthermore, thedistribution range of parthenogenetic strains has expanded
rapidly across Japan, especially eastern Japan.
Parthenogenetic individuals that enter a bisexual
population or a new locality are fully able to reproduce
without a mate. Conversely, the efforts of males that matewith a parthenogenetic female are wasted. That is, theparthenogenetic individuals that enter a bisexual
population possess the superior reproductive strategy.
Furthermore, in order for the range of a bisexual population
to expand, at least one pair of male and female must
disperse simultaneously. In the case of a parthenogenetic
strain, however, even if only one individual relocates, she
is able to reproduce, and so establish a population.
Therefore, parthenogenetic strains naturally have greater
dispersal and establishment potential than bisexualstrains.
The results of our genetic analysis indicated that theparthenogenetic strains are monophyletic in origin, andoriginated in western Japan, from where they must have
expanded into eastern Japan.
In the Abukuma-gawa River (Fukushima Prefecture)and the Ara-kawa River (Saitama Prefecture) bisexual
populations of eastern Japan, female individuals having the“s1-haplotype”, which is common to parthenogeneticstrains, were observed. Within these populations, the sex
ratio in the Ara-kawa population was significantly biased
toward females (♀:♂=102:58, binomial test p<0.05) as of2006. The Ara-kawa River has a mixed unisexual/bisexual
population that is considered to have resulted from theinvasion of the originally bisexual population by aparthenogenetic strain. In the Abukuma-gawa population(locality: Fukushima City), Watanabe et al. (1998) reported* Abstract of paper read at the 47th Annual Meeting of the Arthropodan Embryological Society of Japan, June 10-11, 2011 (Biwako, Shiga).

Fig. 1

NJ dendrograms based on mitochondrial DNA regions of Ephoron shigae; the 16S rRNA (1A: 373 bp) and COI (1B: 658 bp) regions. Specimens in group I were taken from bisexual populations of western Japan. Specimens ingroup II were taken from bisexual populations of eastern Japan, and specimens in group III, from bisexual populations of the Korean Peninsula (Republic of Korea). All specimens from parthenogenetic unisexual populations
across Japan were found to possess the same respective haplotypes in each gene region (the “16S_s1” and “COI_s1” haplotypes). Furthermore, both the “16S_s1” and the “COI_s1” haplotypes were determined to be of thesame clade for all of the western Japanese populations. Bootstrap probabilities (1,000 times) of more than 95% are indicated under the branches.

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35Fig. 2Sex ratios and composition of haplotypes of the mayfly Ephoron shigae at four study sites along the Abukuma-gawa River. Although research intosex ratios was conducted in both 2009 and 2010, similar results were obtained. Only female individuals were used in the haplotype analysis basedon the COI mitochondrial DNA region. The haplotype categorized as “COI_s1” and other haplotypes are displayed separately as distinct groups.
The “COI_s1” haplotype is common to all of the specimens from the parthenogenetic populations, and the other haplotypes were referred tocollectively, as they together represent the haplotypes of the bisexual populations of eastern Japan.

that the sex ratio was not significantly biased toward
females and that many male individuals were observed in1991 and 1992. However, in our recent experience at thesame site, almost all individuals were female: ♀:♂=100:0
in 2009 (p<0.01), and ♀:♂=105:1 (p<0.01) as per our2010 observations (Fig. 2). Also, of great interest, the ratioof males tended to increase upstream, even within thesame river system [Fig. 2; e. g., in the uppermost
population located in Nakajima village (ca. 60 km upstream
from Fukushima City), ♀:♂=50:74 in 2009, and 61:64 in
the 2010 observations].
Furthermore, an analysis based on the mitochondrial
region of COI revealed the “COI_s1” haplotype to besignificantly more frequent than non “COI_s1” females inthe downstream populations (Fig. 2). Whereas in theupstream populations, the opposite pattern was observed.
Therefore, in the Abukuma-gawa River, parthenogenetic
individuals must have entered and established themselves
first within the downstream population in or around
Fukushima City, which has since come to be almost
completely composed of parthenogenetic females in lessthan 20 years. The increasing parthenogenetic bias in thesex ratio of the local populations within this river system isincreasing progressively starting downstream and rapidly
moving upstream. This situation represents a very
important example of the replacement phase of thetransition from a bisexual population to a fullyparthenogenetic female-only population. This process,which may have occurred or presently be happening inother parts of Japan, would explain the formation of otherparthenogenetic populations across Japan.
References

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