Distribution and parthenogenetic fecundity of obligate and facultative parthenogenetic strains of the mayfly Ephoron shigae.

The polymitarcyid mayfly Ephoron shigae (Takahashi, 1924) generally propagates via sexual reproduction, but a parthenogenetic-dominant monophyletic strain derived from western Japanese sexual-dominant strains is widely distributed in Japan. This species may provide a case study on distribution patterns of genetic divergence in sexual-dominant and parthenogenetic-dominant strains of aquatic insects. In this study, we sought to characterize the distribution of parthenogenetic-dominant and sexual-dominant strains of E. shigae in eastern Japan and to assess whether there was sex ratio bias among local populations with and without the parthenogenetic strain, as well as the parthenogenetic reproductive success of both strains. We found that in eastern Japan, the parthenogenetic-dominant strain with the cytochrome c subunit I haplotype C1 is more widespread than sexual-dominant strains and that there are sites where both parthenogenetic-dominant and sexual-dominant strains coexist. There was no sex ratio bias at sites where the parthenogenetic-dominant strain was not observed, but at sites with both reproductive strains, the sex ratio was skewed toward females (64–99%), though the degree of bias depended on the relative frequency of the 2 strains. We found no evidence that females of the parthenogenetic-dominant strain mated after emergence; thus, in eastern Japan, females of the parthenogenetic-dominant strain may not mate even when males are present, and they pass through generations by parthenogenesis. The success rates of unfertilized oocytes of the parthenogenetic-dominant strain varied, with developmental success rates of 70% or higher. Oocyte batches with extremely low success rates (<10%) were also observed at 2 of the sites with both strains. Embryo developmental success of unfertilized oocytes in sexual-dominant strains was generally low (<30%), but exceptionally high rates of >90% were observed. At sites with both strains, the degree of developmental success of unfertilized oocytes in the parthenogenetic-dominant strain may be related to its relative frequency. Examining the causes of the distribution of these strains, and how they may change in the future, will provide new perspectives on reproductive modes and geographic parthenogenesis. [ABSTRACT FROM AUTHOR]