Biological and genetic characteristics of Glyptotendipes tokunagai (Diptera: Chironomidae) on the basis of successive rearing of forty-two generations over seven years under laboratory conditions.

Members of the nonbiting midge family Chironomidae have been used worldwide as water-quality indicators or toxicity test organisms. The purpose of this study was to establish the chironomid Glyptotendipes tokunagai Sasa as a new test species by conducting successive rearing under laboratory conditions. We monitored biological and genetic aspects of >42 successive generations over 7 yr, and also compared the development of the 39th generation with the fourth generation under five constant temperatures of 15, 20, 25, 30, and 35°C. We observed that the number of eggs in an egg mass and the adult body sizes decreased rapidly in the early generations, and thereafter tended to stabilize from the fifth generation to the 42nd generation. In all generations, the mean hatching rate was >75%. Males were predominant in the early generations, but the sex ratio increased to 0.5 (ranged 0.24-0.61) in later generations. The genetic divergence of the reared generations, analyzed by using the mitochondrial cytochrome c oxidase subunit I gene, decreased from 0.0049 to 0.0004 as the generations progressed. In comparison with the fourth generation, the mortality and developmental time of the 39th generation were generally greater, and the adult body sizes were generally smaller. The estimated low developmental threshold temperatures of eggs, male larvae to male adults, and female larvae to female adults were 9.6, 11.3, and 9.7°C, respectively. The optimal rearing temperature was determined to be 25°C. This is the first record of domesticated rearing of a wild chironomid species under laboratory conditions for >7 yr.