Facultative parthenogenesis in the burrowing mayfly, Ephoron eophilum (Ephemeroptera: Polymitarcyidae) with an extremely short alate stage

Facultative parthenogenesis is important for mayflies with short alate stages because females are able to reproduce without mating. We studied facultative parthenogenesis in Ephoron eophilum, a mayfly with an extremely short alate stage. We examined the survival rates of embryos from unfertilized eggs, in addition to investigating the number of chromosomes in parthenogenetic offspring and the mode of inheritance by nuclear genetic analyses using Exon-Primed Intron-Crossing markers. The survival rate of thelytokous embryos was 0-70.2% (16.7 ± 26.7%, mean ± S.D.). Sixteen chromosomes were present throughout most of the mitotic metaphase in parthenogenetic offspring, which was similar to the number recorded in diploid females. All parthenogenetic offspring were homozygous in nuclear genetic analyses, despite the presence of heterozygous mothers. These results indicate that E. eophilum has the ability to reproduce via facultative parthenogenesis, producing mostly diploid thelytokous offspring. The restoration of ploidy level occurs by automixis via terminal fusion or gamete duplication, and causes rapid reduction of heterozygosity. However, despite this, significant deviation from Hardy-Weinberg equilibrium (HWE) was not observed in the studied populations. This is because facultative parthenogenesis in these circumstances normally has little influence on population genetic structuring, even though parthenogenetic embryos exhibit a high survival rate. The lack of influence of parthenogenesis on the population structure of the natural population strongly suggests that parthenogenesis rarely occurs under natural circumstances.