Expression levels of hormone receptor and vitellogenin mRNAs in livers of Thai Medaka, oryzias minutillus, inhabiting the suburbs of Bangkok, Thailand

Abstract

The aim of this study was to examine the expression levels of steroid hormone receptors and vitellogenin in liver of Thai Medaka, Oryzias minutillus from natural environment. Adult Thai Medaka were collected from the suburbs of Bangkok, Thailand. Fish were identified as either to male or female by fin morphology and the sex was further confirmed by histological examination of the gonads. Fish whose sex was initially undeterminable (sex-undeterminable individual) but upon histological examination were determined to contain testicular or ovarian tissues were sorted into group 1 and 2, respectively. Some individuals had both testicular and oocyte tissue in the gonads and were classified into the intersex group. The level of Androgen Receptor (AR) in liver was higher in males and the group 1 than in females and the group 2. Estrogen Receptor (ER) expression in the liver was lower in males, the group 1 and intersex than in females and the group 2. ER levels in the group 1 and the intersex group were higher than those in males. Vitellogenin (Vtg) expression was measurable in the livers of females, in both sex-undeterminable groups and in the intersex group. Taking these results into consideration, the expression levels of AR and ER in liver supports the role of androgen and estrogen regulation as a biological marker of endocrine disruption in wild fish. AR and ER expression in the liver may be indicative of sexual dimorphism in normal male and female adult Thai medaka. In the sex-undeterminable and intersex individuals, the high levels of Vtg expression may indicate that the fish were exposed to feminizing stresses, possibly estrogenic chemicals. © 2011 Academic Journals Inc.