Effects of molinate on survival and development of Bombina orientalis (Boulenger) embryos.

Molinate, a thiocarbamate chemical is a slightly to moderately toxic herbicide in EPA (Environmental Protection Agency) toxicity class III, and is a registered as a General Use Pesticide (GUP). Bombina orientalis is one of the most common amphibians in the world and comprise a large proportion of their total number in Korea. B. orientalis spawns in the rice fields at spring when the massive application of agricultural chemicals occurs. In the present study, we examined the effects of molinate on embryonic survival and developmental abnormality in B. orientalis embryos. The difference in survival rate between vehicle control and molinate treated embryos was not observed until the blastula stage. The first statistically significant decrease in embryonic survival was observed at mouth open stage following exposure to 100 microM molinate (46.8% vs. 81.1% in control). When the embryos develop to tadpole stage survival was significantly decreased at 50 microM molinate (35.9% vs. 68.9% in control), suggesting that the lowest observed effective dose (LOED) for systemic toxicity in B. orientalis embryos is 50 microM. In survived embryos molinate exposure produced several types of severe developmental abnormalities in order of frequency with bent trunk, neurula with yolk plug, bent tail, tail dysplasia, ventral blister, eye dysplasia, thick-set body and cephalic dysplasia. This suggests that molinate targets multiple events in embryonic and larval development in this frog species. Together this suggested that molinate was detrimental for survival and development following zygotic transcription after midblastula transition in B. orientalis embryos.