Possibility of Undeveloped Egg Absorption during Embryogenesis: A Unique Phenomenon Observed in the Ovoviviparous Mayfly Cloeon dipterum

Oviparous, ovoviviparous and viviparous reproduction are interesting subjects for understanding animals' evolutionary pathways and adaptation to their life history and habitat conditions. In this study, we examined the reproductive mode of the ovoviviparous mayfly Cloeon dipterum, particularly comparing embryogenesis between hand-pairing and unmated females' common oviduct. Our study suggested that the high developmental rate of C. dipterum observed in a recent study could be ascribed to their absorption of unfertilized eggs. The developmental rates of hand-paired females were almost 100%, while their egg-bearing numbers were lower than those of virgin females. Thus, such reduced egg numbers suggest the maternal absorption of unfertilized eggs. This trait is thought to have evolved with the ovoviviparous characteristics of C. dipterum. We identified the basis of the irregularity of this species exhibiting such a high (i.e., 100%) developmental rate in our previous recent study.

Genealogical Position of Japanese Populations of the Globally Distributed Mayfly Cloeon dipterum and Related Species (Ephemeroptera, Baetidae): A Molecular Phylogeographic Analysis

In the present study, we add genetic data of the mayfly Cloeon dipterum collected from the Japanese Islands to the established molecular phylogenetic knowledge in the mitochondrial COI gene of Cloeon mayflies. Cloeon dipterum is a typical cosmopolitan species that includes six intraspecific haplotype groups. The present phylogenetic analysis revealed that haplotypes of the Japanese C. dipterum constitute a seventh group together with a haplotype from Korea. This East Asian group forms a sister group with previously known European and North American haplotype groups (i.e., the clade CT1 to CT3). The present phylogenetic analysis further revealed the occurrence of two described species (C. dipterum and Cloeon ryogokuensis) and possibly three species (Cloeon sp. 1 to 3) in Japan. Consideration is given to the degree of genetic differentiation, divergence time, and differentiation process among these seven genetic groups.

Concordance between molecular biogeography of Dipteromimus tipuliformis and geological history in the local fine scale (Ephemeroptera, Dipteromimidae)

Species distribution area is determined by both biotic and abiotic factors. In particular, significant geological events influence the biodiversity and the genetic structures of the organisms inhabiting the area. The establishment of physical barriers (e.g., mountains, rivers), drives species differentiation by their interference with biological dispersal or gene flow. The Japanese Islands have a high degree of biodiversity. This study focused on the Kii Peninsula, which stands out as a region of exhibiting particularly high biodiversity, and also exceptionally high endemism. The Kii Peninsula has experienced active mountain formation ever since the Quaternary period. In this study, we investigate the influence of geological events on the establishment of genetic diversity. We focused on the mayfly, Dipteromimus tipuliformis. Phylogenetic analyses were performed utilizing the mitochondrial DNA 16S rRNA and COI regions, and the nuclear DNA histone H3, PEPCK and 28S rRNA regions. As a result, it was shown that this mayfly exhibits a genetic structure that strongly reflects the geological history of the Kii Peninsula, and detected their dispersal process across the Median Tectonic Line. This is a unique and significant study, in that it clearly shows the relationship between the phylogenetic evolution of this mayfly and the corresponding geological history in surprisingly geographic fine scale.