Abstract
The burrowing polymitarcyid mayfly Ephoron shigae is distributed across Japan, Korea, northeast China and far east Russia. Some populations are bisexual, and others are unisexual, i.e. geographically parthenogenetic throughout Japan. In general, parthenogenetic organisms are often found in harsh environments, such as at high latitudes and altitudes, in xeric as opposed to mesic conditions, in isolated habitats such as islands and island-like areas, and at the peripheral regions of the taxon's range. In E. shigae, however, the distributions of bisexual and unisexual populations overlap broadly in their respective geographical ranges. In the analysis of mitochondrial 16S rRNA and COI, we revealed that unisexual populations were of monophyletic origin and recently differentiated somewhere in western Japan. In the nuclear DNA EFI-α analysis, parthenogenetic strains had two genotypes, i.e. the heterozygous genotype of E1/E3 and the homozygous genotype of E1/E1 or E3/E3, while specimens of bisexual lineage had 20 genotypes. These results are consistent with an automixis mode of reproduction for the parthenogenetic strains, and also support the monophyletic origin of the parthenogenetic strains. Furthermore, there would be no gene flow between the specimens of the bisexual lineage and those of the parthenogenetic strain.
Highlights
Most metazoans perpetuate generations via sexual reproduction [1]
The results of analysis of molecular variance (AMOVA) and Mantel testing indicated that geographical distance has primarily restricted gene flow among bisexual populations of E. shigae
With respect to the relationship between their pairwise genetic and geographical distances, gene flow between eastern and western Japanese populations is predominantly prevented by physical barriers such as the Japan Alps and Mount Fuji, which are the highest mountains in Japan that formed during the Pliocene [41,42]
Summary
Most metazoans perpetuate generations via sexual reproduction [1]. It is a paradox that bisexual reproduction, which is a sexually reproductive system relying on a male and a female, is the general mode of reproduction for metazoans, especially when considering that bisexual reproduction has a twofold cost compared with unisexual organisms with the ability to generate all female offspring It is well documented that parthenogenetic reproduction is most often found in harsh environmental habitats, such as at higher latitudes and altitudes, mountainous regions, some isolated conditions such as islands or in island-like habitats, in xeric as opposed to mesic environments, and in environments variously classified as periphery, extreme, stressful, transient or disturbed [1,7,8,9,10,11]
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