Effects of methyl testosterone exposure on sexual differentiation in medaka, Oryzias latipes

Studies were conducted to characterize effects of a known androgen on sexua l differentiation and development of medaka, Oryzias latipes (d-rR strain), at two life stages. Embryos were injected with graded doses of methyl test osterone (MT) prior to epiboly. The occurrence of sex-reversal, and the gon adosomatic index (GSI) were evaluated in adults. Primary germ cells were co unted and gonad Volumes calculated for larvae to determine if sex-reversal could be detected at an early life stage. Sex-reversal of genetic females t o phenotypic males was observed at both life stages. The GSI for phenotypic females was greater than for phenotypic males, while the GSI in XX males w as similar to XY males. MT appeared to reduce the GSI of XX females exposed to MT but not sex-reversed. Our results indicate that embryonic exposure t o androgens influences sexual development in medaka. Utilizing the d-rR str ain of medaka allows detection of an effect as early as 2 weeks after chemi cal exposure making this a useful tool to screen chemicals for effects on s exual differentiation. Published by Elsevier Science Ltd. All rights reserv ed.